View Filing Data

Print Document View Excel Document

Document and Entity Information - USD ($) $ in Billions	12 Months Ended
Document and Entity Information - USD ($) $ in Billions	Dec. 31, 2019	Feb. 14, 2020	Jan. 31, 2020
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Entity Registrant Name	ORCHARD THERAPEUTICS PLC
Current Fiscal Year End Date	--12-31
Entity Central Index Key	0001748907
Entity Filer Category	Large Accelerated Filer
Entity Well-known Seasoned Issuer	Yes
Entity Small Business	false
Entity Common Stock, Shares Outstanding		97,068,993
Entity Public Float			$ 0.9
Entity Shell Company	false
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Incorporation, State or Country Code	X0
Entity Address, Address Line One	108 Cannon Street
Entity Address, City or Town	London
Entity Address, Postal Zip Code	EC4N 6EU
City Area Code	44
Local Phone Number	(0) 203 808-8286
Document Annual Report	true
Document Transition Report	false
Entity File Number	001-38722
Entity Emerging Growth Company	false
Entity Tax Identification Number	00-0000000
Entity Address, Country	GB
Title of 12(b) Security	American Depositary Shares, each representing one ordinary share, nominal value £0.10 per share
Trading Symbol	ORTX
Security Exchange Name	NASDAQ
Documents Incorporated by Reference	Portions of the Registrant’s definitive proxy statement for its 2020 Annual General Meeting are incorporated by reference into Part III of this Annual Report on Form 10-K where indicated.

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 19,053	$ 335,844
Marketable securities	305,937
Trade receivables	1,442	2,103
Prepaid expenses and other current assets	8,530	6,985
Research and development tax credit receivable	14,934	10,585
Total current assets	349,896	355,517
Non-current assets:
Operating lease right-of-use-assets	19,415
Property and equipment, net	7,596	5,476
Research and development tax credit receivable, net of current portion	13,710
Restricted cash	4,264	3,837
Other long-term assets	4,400	1,212
Total non-current assets	49,385	10,525
Total assets	399,281	366,042
Current liabilities:
Accounts payable	11,984	18,125
Accrued expenses and other current liabilities	37,980	29,780
Operating lease liabilities	5,892
Total current liabilities	55,856	47,905
Long-term debt, net	24,699
Operating lease liabilities, net of current portion	15,320
Other long-term liabilities	4,213	6,799
Total liabilities	100,088	54,704
Commitments and contingencies (Note 14)
Shareholders’ equity:
Ordinary shares, £0.10 par value, authority to allot up to a maximum nominal value of £13,023,851.50 of shares at December 31, 2019 and 2018, respectively; 96,923,729 and 85,865,557 shares issued and outstanding at December 31, 2019 and 2018, respectively.	12,331	10,924
Additional paid-in capital	738,481	587,490
Accumulated other comprehensive income	2,042	3,163
Accumulated deficit	(453,661)	(290,239)
Total shareholders’ equity	299,193	311,338
Total liabilities and shareholders’ equity	$ 399,281	$ 366,042

Consolidated Balance Sheets (Parenthetical) - GBP (£)	Dec. 31, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Ordinary Shares, Par Value	£ 0.10	£ 0.10
Ordinary Shares, Authorized	£ 13,023,851.50	£ 13,023,851.50
Ordinary Shares, Issued	96,923,729	85,865,557
Ordinary Shares, Outstanding	96,923,729	85,865,557

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Product sales, net	$ 2,513	$ 2,076
Type of Revenue [Extensible List]	us-gaap:ProductMember	us-gaap:ProductMember	us-gaap:ProductMember
Costs and operating expenses
Cost of product sales	$ 805	$ 422
Type of Cost, Good or Service [Extensible List]	us-gaap:ProductMember	us-gaap:ProductMember	us-gaap:ProductMember
Research and development	$ 117,363	$ 205,319	$ 32,527
Selling, general and administrative	57,218	31,366	5,985
Total costs and operating expenses	175,386	237,107	38,512
Loss from operations	(172,873)	(235,031)	(38,512)
Other income (expense):
Interest income	7,362	1,116
Interest expense	(1,538)
Other income (expense), net	1,387	4,390	(1,179)
Total other income (expense), net	7,211	5,506	(1,179)
Net loss before income tax	(165,662)	(229,525)	(39,691)
Income tax benefit (expense)	2,240	(970)	(53)
Net loss attributable to ordinary shareholders	$ (163,422)	$ (230,495)	$ (39,744)
Net loss per share attributable to ordinary shareholders, basic and diluted	$ (1.75)	$ (10.22)	$ (4.48)
Weighted average number of ordinary shares outstanding, basic and diluted	93,240,355	22,559,389	8,872,768
Other comprehensive (loss) income
Foreign currency translation adjustment	$ (1,387)	$ (964)	$ 4,398
Unrealized gain (loss) on marketable debt securities	266
Total other comprehensive (loss) income	(1,121)	(964)	4,398
Total comprehensive loss	$ (164,543)	$ (231,459)	$ (35,346)

Consolidated Statements of Convertible Preferred Shares and Shareholders' Equity - USD ($) $ in Thousands	Total	Convertible Preferred Shares	Ordinary Shares	Additional Paid-in Capital	Accumulated Other Comprehensive Income (loss)	Accumulated Deficit
Temporary equity, Beginning balance, Shares at Dec. 31, 2016		11,204,199
Temporary equity, Beginning balance, Value at Dec. 31, 2016		$ 16,970
Stockholders Equity, Beginning Balance, Shares at Dec. 31, 2016			7,446,931
Stockholders Equity, Beginning Balance at Dec. 31, 2016	$ (16,524)		$ 957	$ 2,790	$ (271)	$ (20,000)
Temporary equity, Issuance of convertible preferred shares, net of issuance costs, Shares		14,693,207
Temporary equity, Issuance of convertible preferred shares, net of issuance costs, Value		$ 66,981
Temporary Equity Reclassification of convertible preferred shares, Shares		(25,897,406)
Temporary Equity Reclassification of convertible preferred shares, Value		$ (83,951)
Reclassification of convertible preferred shares, Shares		25,897,406
Reclassification of convertible preferred shares Value	83,951	$ 83,951
Issuance of convertible preferred shares, net of issuance costs, Shares		7,380,272
Issuance of convertible preferred shares, net of issuance costs, Value	50,118	$ 50,118
Share-based compensation expense	1,019			1,019
Ordinary shares committed to be issued as part of license agreements	1,534			1,534
Ordinary shares issued as part of license agreements	1,653		$ 188	1,465
Ordinary shares issued as part of license agreements, Shares			1,480,190
Foreign currency translation adjustment	4,398				4,398
Net loss	(39,744)					(39,744)
Stockholders Equity, Ending Balance, Shares at Dec. 31, 2017		33,277,678	8,927,121
Stockholders Equity, Ending Balance at Dec. 31, 2017	86,405	$ 134,069	$ 1,145	6,808	4,127	(59,744)
Issuance of convertible preferred shares, net of issuance costs, Shares		26,891,222
Issuance of convertible preferred shares, net of issuance costs, Value	242,744	$ 242,744
Share-based compensation expense	6,766			6,766
Exercise of share options	28		$ 2	26
Exercise of share options, Shares			14,545
Ordinary shares issued as part of license agreements	1,385		$ 83	1,302
Ordinary shares issued as part of license agreements, Shares			651,419
Effect of corporate reorganization,including conversion of preferred shares to ordinary shares		$ (376,813)	$ 7,647	369,166
Effect of corporate reorganization, including conversion of preferred shares to ordinary shares, Shares		(60,168,900)	60,168,900
Issuance of ADSs in initial (follow-on) public offering, net of issuance costs	205,469		$ 2,047	203,422
Issuance of ADSs in initial (follow-on)public offering, net of issuance costs, Shares			16,103,572
Foreign currency translation adjustment	(964)				(964)
Net loss	(230,495)					(230,495)
Stockholders Equity, Ending Balance, Shares at Dec. 31, 2018			85,865,557
Stockholders Equity, Ending Balance at Dec. 31, 2018	311,338		$ 10,924	587,490	3,163	(290,239)
Share-based compensation expense	19,424			19,424
Exercise of share options	$ 2,003		$ 158	1,845
Exercise of share options, Shares	1,209,335		1,209,335
Issuance of ESPP shares	$ 1,306		$ 16	1,290
Issuance of ESPP shares, Shares			123,569
Issuance of ADSs in initial (follow-on) public offering, net of issuance costs	129,665		$ 1,233	128,432
Issuance of ADSs in initial (follow-on)public offering, net of issuance costs, Shares			9,725,268
Foreign currency translation adjustment	(1,387)				(1,387)
Unrealized gain on marketable debt securities	266				266
Net loss	(163,422)					(163,422)
Stockholders Equity, Ending Balance, Shares at Dec. 31, 2019			96,923,729
Stockholders Equity, Ending Balance at Dec. 31, 2019	$ 299,193		$ 12,331	$ 738,481	$ 2,042	$ (453,661)

Consolidated Statements of Convertible Preferred Shares and Shareholders' Equity (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Statement Of Stockholders Equity [Abstract]
Issuance of ADSs in initial (follow-on)public offering, net of issuance costs	$ 605	$ 4,200

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities
Net loss attributable to ordinary shareholders	$ (163,422)	$ (230,495)	$ (39,744)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	1,675	1,199	302
Share-based compensation	19,424	6,766	1,019
Non-cash interest expense	311
Amortization of provision on loss contract	(3,855)	(6,300)
Non-cash consideration for licenses and milestones		94,776	3,126
Amortization of (discount) premium on marketable securities	(676)
Unrealized foreign currency gains	(1,859)
Changes in operating assets and liabilities:
Trade receivables	715	(927)
Research and development tax credit receivable	(17,564)	(10,079)	(152)
Prepaids and other assets	(5,151)	(5,867)	(3,753)
Operating leases, right-of-use-assets	3,064
Accounts payable	(6,413)	14,848	1,930
Accrued expenses and other current liabilities	11,434	31,663	4,672
Other long-term liabilities	(1,424)	6,880	113
Operating lease liabilities	(2,390)
Net cash used in operating activities	(166,131)	(97,536)	(32,487)
Cash flows from investing activities
Proceeds from sales and maturities of marketable securities	109,019
Purchases of marketable securities	(414,010)
Purchases of property and equipment	(4,367)	(4,032)	(1,559)
Net cash used in investing activities	(309,358)	(4,032)	(1,559)
Cash flows from financing activities
Issuance of debt from credit facility, net of issuance costs	24,466
Issuance of convertible preferred shares, net of issuance costs		149,367	115,696
Issuance of ADRs in public offerings	130,270	209,669
Payment of offering costs	(605)	(4,200)
Proceeds from employee equity plans	3,322	28
Net cash provided by financing activities	157,453	354,864	115,696
Effect of exchange rate changes on cash	1,672	(3,471)	4,709
Net increase (decrease) in cash and restricted cash	(316,364)	249,825	86,359
Cash and restricted cash —beginning of year	339,681	89,856	3,497
Cash, cash equivalents, and restricted cash —end of year	23,317	339,681	89,856
Supplemental disclosure of non-cash investing and financing activities
Settlement of tranche obligations			1,402
Property and equipment included in accrued expenses and accounts payable at period end	647		$ 1,247
Convertible preferred shares issued for licenses		$ 93,391
Supplemental disclosure of cash flow information
Cash paid for interest	1,227
Cash paid for taxes	$ 1,474

Nature of the Business and Basis of Presentation

12 Months Ended

Dec. 31, 2019

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Nature of the Business and Basis of Presentation

1. Nature of Business and Basis of Presentation

Orchard Therapeutics plc and its subsidiaries (the “Company”) is a global gene therapy leader dedicated to transforming the lives of people affected by rare diseases through the development of innovative, potentially curative gene therapies. The Company’s ex vivo autologous gene therapy approach utilizes genetically-modified blood stem cells and seeks to correct the underlying cause of disease in a single administration. The Company is advancing seven clinical-stage programs across multiple therapeutic areas, including inherited neurometabolic disorders, primary immune deficiencies and blood disorders, where the disease burden on children, families and caregivers is immense and current treatment options are limited or do not exist.

The Company is a public limited company incorporated pursuant to the laws of England and Wales. Orchard Therapeutics plc (formerly Orchard Rx Limited) was originally incorporated under the laws of England and Wales in August 2018 to become a holding company for Orchard Therapeutics Limited. Orchard Therapeutics Limited was originally incorporated under the laws of England and Wales in September 2015 as Newincco 1387 Limited and subsequently changed its name to Orchard Therapeutics Limited in November 2015.

Pursuant to a corporate reorganization, all the interests in Orchard Therapeutics Limited were exchanged for the same number and class of newly issued shares of Orchard Rx Limited and, as a result, Orchard Therapeutics Limited became a wholly owned subsidiary of Orchard Rx Limited. On October 29, 2018, Orchard Rx Limited re-registered as a public limited company and changed its name to Orchard Therapeutics plc and Orchard Therapeutics Limited changed its name to Orchard Therapeutics (Europe) Limited.

On November 1, 2018, the Company’s different classes of preferred shares and our ordinary shares were consolidated on a one-for-0.8003 basis. Following the share consolidation, each share was re-designated as an ordinary share on a one-for-one basis. Accordingly, all share and per share amounts for all periods presented in the consolidated financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect the reverse stock split.

As part of the corporate reorganization as described above, each ordinary share with a nominal value of £0.00001 was redenominated as an ordinary share with a nominal value of £0.10. Accordingly, equity accounts for all periods presented in the consolidated financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect the effects of the redenomination of the Company’s ordinary shares.

On November 2, 2018, the Company closed its initial public offering (IPO) of American Depositary Shares (“ADS”) in which the Company sold an aggregate of 16,103,572 ADSs representing the same number of ordinary shares at a public offering price of $14.00 per ADS. Net proceeds were $205.5 million, after deducting underwriting discounts and commissions of $15.8 million and offering expenses of $4.2 million paid by the Company.

In June 2019, the Company completed a follow-on public offering of ADSs in which the Company sold an aggregate of 9,725,268 ADSs representing the same number of ordinary shares at a public offering price of $14.25 per ADS. Net proceeds were $129.7 million, after deducting underwriting discounts and commissions of $8.3 million and offering expenses of $0.6 million paid by the Company.

Orchard Therapeutics plc is a continuation of Orchard Therapeutics Limited and its subsidiaries, and the corporate reorganization has been accounted for as a combination of entities under common control. The corporate reorganization has been given retrospective effect in these financial statements and such financial statements represent the financial statements of Orchard Therapeutics Limited for all periods prior to the corporate reorganization. In connection with the corporate reorganization, outstanding share option awards of Orchard Therapeutics Limited were exchanged for share awards and option grants of Orchard Therapeutics plc with identical terms and restrictions.

The Company is subject to risks and uncertainties common to development-stage companies in the biotechnology industry. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s technology will be obtained, that any products developed will obtain necessary government regulatory approval or that any products, if approved, will be commercially viable. The Company operates in an environment of rapid technological innovation and substantial competition from pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees, consultants and service providers. Even if the Company’s product development efforts are successful in gaining regulatory approval, it is uncertain when, if ever, the Company will realize significant revenue from product sales.

Through December 31, 2019, the Company funded its operations primarily with proceeds from the sale of convertible preferred shares and ADSs in the IPO and follow-on offering. The Company has incurred recurring losses since its inception, including net losses of $163.4 million, $230.5 million and $39.7 million, for the years ended December 31, 2019, 2018 and 2017, respectively. As of December 31, 2019, the Company had an accumulated deficit of $453.7 million. The Company expects to continue to generate operating losses for the foreseeable future. The Company expects that its cash, cash equivalents, and marketable securities on hand as of December 31, 2019 of $325.0 million, will be sufficient to fund its operations and capital expenditure requirements through at least the next twelve months. The Company will seek additional funding through private or public equity financings, debt financings, collaborations, strategic alliances and marketing, distribution or licensing arrangements. The Company may not be able to obtain financing on acceptable terms, or at all, and the Company may not be able to enter into collaborations or other arrangements. The terms of any financing may adversely affect the holdings or the rights of the Company's stockholders. The future viability of the Company is dependent on its ability to raise additional capital to finance its operations. If the Company is unable to obtain funding, the Company will be forced to delay, reduce or eliminate some or all of its research and development programs, product portfolio expansion or commercialization efforts, which could adversely affect its business prospects, or the Company may be unable to continue operations. Although management continues to pursue these plans, there is no assurance that the Company will be successful in obtaining sufficient funding on terms acceptable to the Company to fund continuing operations, if at all.

Basis of presentation

The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and include the accounts of the Company and its wholly owned subsidiaries, after elimination of all intercompany accounts and transactions.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2019

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Use of estimates

The preparation of consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenue and expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual for research and development expenses, the research and development tax credit receivable, the Strimvelis loss provision, the fair values of ordinary and convertible preferred shares, share-based compensation, operating lease assets and liabilities, and income taxes. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results could differ materially from those estimates.

Concentration of credit risk

The Company has no significant off-balance sheet risk, such as foreign currency contracts, options contracts, or other foreign hedging arrangements. Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash and other receivables. Periodically, the Company maintains deposits in accredited financial institutions in excess of federally insured limits. The Company deposits its cash in financial institutions that it believes have high credit quality and has not experienced any losses on such accounts and does not believe it is exposed to any unusual credit risk beyond the normal credit risk associated with commercial banking relationships or entities for which it has a receivable.

Foreign currency translation

The reporting currency of the Company is the U.S. dollar. The Company has determined the functional currency of the parent company, Orchard Therapeutics plc, is U.S. dollars because it predominantly raises capital and expends cash in U.S. dollars. The functional currency of our subsidiary operations is the applicable local currency. Transactions in foreign currencies are translated into the functional currency of the subsidiary in which they occur at the foreign exchange rate in effect on at the date of the transaction. Monetary assets and liabilities denominated in foreign currencies at the balance sheet date are translated into the functional currency of the relevant subsidiary at the foreign exchange rate in effect on the balance sheet date. Non-monetary assets and liabilities denominated in foreign currencies that differ from the functional currency are translated into the functional currency at the exchange rates prevailing at the date of the transaction. The Company recorded a foreign currency transaction gain of $1.5 million, and $4.4 million for the years ended December 31, 2019 and 2018, respectively, and a foreign currency transaction loss of $1.2 million for the year ended December 31, 2017, respectively, which is included in other income (expense) in the statements of operations and comprehensive loss.

The results of operations for subsidiaries, whose functional currency is not the U.S. dollar, are translated at an average rate for the period where this rate approximates the prevailing foreign exchange rates at the dates of the transactions and the balance sheet of these subsidiaries are translated at prevailing foreign exchange rates at the balance sheet date. Exchange differences arising from this translation of foreign operations are reported as a component of other comprehensive income (loss).

Cash and cash equivalents

The Company considers all highly liquid investments purchased with original maturities of 90 days or less at the date of acquisition to be cash equivalents.

Marketable securities

Marketable securities consist of investments with original maturities greater than ninety days. The Company has classified its investments with maturities beyond one year as short term, based on their highly liquid nature and because such marketable securities represent the investment of cash that is available for current operations. The Company considers its investment portfolio of investments as available-for-sale. Accordingly, these investments are recorded at fair value, which is based on quoted market prices or other observable inputs. Unrealized gains and losses are recorded as a component of other comprehensive income (loss). Realized gains and losses are determined on a specific identification basis and are included in other income (loss). Amortization and accretion of discounts and premiums is also recorded in other income (loss).

The Company evaluates its investments with unrealized losses for other-than-temporary impairment. When assessing investments for other-than-temporary declines in value, the Company considers such factors as, among other things, how significant the decline in value is as a percentage of the original cost, how long the market value of the investment has been less than its original cost, the Company’s ability and intent to retain the investment for a period of time sufficient to allow for any anticipated recovery in fair value and market conditions in general. If any adjustment to fair value reflects a decline in the value of the investment that the Company considers to be “other than temporary,” the Company reduces the investment to fair value through a charge to the statement of operations and comprehensive loss. No such adjustments were necessary during the periods presented.

Restricted Cash

Cash and cash equivalents that are restricted as to withdrawal or use under the terms of certain contractual agreements are recorded as restricted cash on our consolidated balance sheet. The Company’s lease for the Fremont manufacturing facility requires a letter of credit of $3.0 million at December 31, 2019. The Company is also contractually required to maintain a cash collateral account associated with corporate credit cards and other leases in the amount of $1.3 million at December 31, 2019. The Company includes the restricted cash balance in cash and cash equivalents when reconciling beginning-of-period and end-of-period total amounts shown on the consolidated statements of cash flows. The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported in the consolidated balance sheet that sum to the total of the amounts reported in the consolidated statement of cash flows (in thousands):

	December 31,
	2019		2018
Cash and cash equivalents	$	19,053	$	335,844
Restricted cash		4,264		3,837
Total cash, cash equivalents and restricted cash shown in the statement of cash flows	$	23,317	$	339,681

Property and equipment

Property and equipment are recorded at cost and depreciated or amortized using the straight-line method over the following estimated useful lives.

Property and equipment:		Estimated useful life
Lab equipment		5-10 years
Leasehold improvements		Shorter of lease term or estimated useful life
Furniture and fixtures		4 years
Office and computer equipment		3-5 years

As of December 31, 2019, and 2018, the Company’s property and equipment consisted of furniture and fixtures, office and computer equipment, lab equipment and leasehold improvements. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation are removed from the accounts, and any resulting gain or loss is included in the statement of operations and other comprehensive loss. Repairs and maintenance expenditures, which are not considered improvements and do not extend the useful life of property and equipment, are expensed as incurred.

Impairment of long-lived assets

Long-lived assets consist of property and equipment. Long-lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset group for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset group to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset group are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset group over its fair value, as determined in accordance with the related accounting literature. To date, the Company has not recorded any impairment losses on long-lived assets.

Fair value measurements

Certain assets and liabilities of the Company are carried at fair value under U.S. GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable:

•

Level 1—Quoted prices in active markets for identical assets or liabilities.

•

Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data.

•

Level 3—Unobservable inputs that are supported by little or no market activity that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques.

To the extent that the valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement.

The carrying values of the Company’s trade receivables, other receivables, accounts payable, accrued expenses and other current liabilities approximate their fair values due to the short-term nature of these assets and liabilities.

Segment information

Operating segments are defined as components of an enterprise for which separate discrete information is available for evaluation by the chief operating decision maker in deciding how to allocate resources and assess performance. The Company and the Company’s chief operating decision maker, the Company’s Chief Executive Officer, views the Company’s operations and manages its business as a single operating segment, which is focused on discovering, acquiring, developing and commercializing gene therapies for patients with rare disorders. The Company operates in three geographic regions: the United Kingdom, European Union, and United States. The Company had property and equipment of $2.6 million and $5.0 million located in the United Kingdom and United States, respectively, as of December 31, 2019. The Company had property and equipment of $1.7 million and $3.8 million located in the United Kingdom and United States, respectively, as of December 31, 2018. The Company had right-of-use assets of $3.7 million in the United Kingdom and European Union and $15.7 million in the United States as of December 31, 2019. The Company had product sales of $2.5 million and $2.1 million in 2019 and 2018, respectively, all of which originated in the European Union.

Research and development costs

Research and development costs are expensed as incurred. Research and development expenses consist of costs incurred in performing research and development activities, including salaries, share-based compensation and benefits, facilities costs, depreciation, third-party license fees, certain milestone payments, and external costs of outside vendors engaged to conduct clinical development activities and clinical trials, as well as costs to develop a manufacturing process, perform analytical testing and manufacture clinical trial materials. Non-refundable prepayments for goods or services that will be used or rendered for future research and development activities are recorded as prepaid expenses. Such amounts are recognized as an expense as the goods are delivered or the related services are performed, or until it is no longer expected that the goods will be delivered, or the services rendered. In addition, funding from research grants is recognized as an offset to research and development expense on the basis of costs incurred on the research program. Royalties to third parties associated with our research grants will be accrued when they become probable.

Research contract costs and accruals

The Company has entered into various research and development contracts. These agreements are cancelable, and related costs are recorded as research and development expenses as incurred. When billing terms under these contracts do not coincide with the timing of when the work is performed, the Company is required to make estimates of outstanding obligations as of period end to those third parties. Any accrual estimates are based on a number of factors, including the Company’s knowledge of the progress towards completion of the research and development activities, invoicing to date under the contracts, communication from the research institution or other companies of any actual costs incurred during the period that have not yet been invoiced, and the costs included in the contracts. Significant judgments and estimates may be made in determining the accrued balances at the end of any reporting period. Actual results could differ from the estimates made by the Company. The historical accrual estimates made by the Company have not been materially different from the actual costs.

Share-based compensation

The Company measures share-based awards granted to employees and directors based on the fair value on the date of the grant and recognizes compensation expense for those awards over the requisite service period, which is the vesting period of the respective award. Forfeitures are accounted for as they occur.

Prior to the adoption of Accounting Standards Update (“ASU”) No. 2018-07, Compensation—Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting (“ASU 2018-07”), which is discussed below under “Recently adopted accounting pronouncements,” the measurement date for non-employee awards was generally the date the services were completed, resulting in financial reporting period adjustments to share-based compensation during the vesting terms for changes in the fair value of the awards. At the end of each financial reporting period prior to completion of the service period, the fair value of the unvested awards was remeasured using the then-current fair value of the Company’s ordinary shares and updated assumption inputs in the Black-Scholes option-pricing model.

After adoption of ASU 2018-07, the measurement date for non-employee awards is the date of the grant. The compensation expense for non-employees is recognized, without changes in the fair value of the award, over the requisite service period, which is the vesting period of the respective award.

The Company classifies share-based compensation expense in its consolidated statement of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in which the award recipient’s service payments are classified.

Valuation of Stock Options

The fair value of each option is estimated on the date of grant using the Black-Scholes option pricing model. Assumptions used in the option pricing model include the following:

Expected volatility. The Company estimates its expected share price volatility based on the historical volatility of publicly traded peer companies and expects to continue to do so until it has adequate historical data regarding the volatility of its own traded share price.

Expected term. The expected term of the Company’s share options has been determined utilizing the “simplified method” for awards that qualify as “plain-vanilla” options.

Risk-free interest rate. The risk-free interest rate is determined by reference to the United States Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award.

Expected dividend. Expected dividend yield is based on the Company’s history of not paying cash dividends on ordinary shares. The Company does not expect to pay any cash dividends in the foreseeable future.

Fair value of ordinary shares. Options granted subsequent to the Company’s IPO are issued with an exercise price equal to the fair market value of the Company’s ADS at the date of grant.

Valuation of RSUs

We estimate the fair value of our performance-based restricted stock unit (“RSUs”) awards or components of RSU awards whose vesting is contingent upon market conditions, such as volume weighted-average price (“VWAP”), using the Monte-Carlo simulation model. The fair value of RSUs or components of RSU awards where vesting is contingent upon market conditions is amortized based upon the estimated derived service period. The fair value of RSUs or components of RSUs granted to our employees and directors is determined, where vesting is dependent on future services or regulatory or research and development milestones, based upon the quoted closing market price per share on the date of grant.

Comprehensive loss

Comprehensive loss includes net loss as well as other changes in shareholders’ equity (deficit) that result from transactions and economic events other than those with shareholders. For the years ended December 31, 2019, 2018 and 2017, the components of accumulated other comprehensive loss are detailed as follows (in thousands):

	Currency Translation Adjustment			Unrealized Gain (Loss) on Marketable Securities		Accumulated Other Comprehensive Income (Loss)
Balance at December 31, 2016	$	(271	)	$	—	$	(271	)
Other comprehensive (loss) income, net of tax		4,398			—		4,398
Balance at December 31, 2017	$	4,127		$	—	$	4,127
Other comprehensive (loss) income, net of tax		(964	)		—		(964	)
Balance at December 31, 2018	$	3,163		$	—	$	3,163
Other comprehensive (loss) income, net of tax		(1,387	)		266		(1,121	)
Balance at December 31, 2019	$	1,776		$	266	$	2,042

Leases

The Company determines if an arrangement is a lease at contract inception. Operating lease assets represent a right to use an underlying asset for the lease term and operating lease liabilities represent an obligation to make lease payments arising from the lease. Operating lease liabilities with a term greater than one year and their corresponding right-of-use assets are recognized on the balance sheet at the commencement date of the lease based on the present value of lease payments over the expected lease term. Certain adjustments to the right-of-use asset may be required for items such as initial direct costs paid or incentives received. The Company made an accounting policy election to not record a right-of-use asset or lease liability for leases with a term of one year or less. To date, the Company has not identified any material short-term leases, either individually or in the aggregate.

As the Company’s leases do not provide an implicit rate, the Company utilized the appropriate incremental borrowing rate, which is the rate incurred to borrow on a collateralized basis over a similar term an amount equal to the lease payments in a similar economic environment. The Company estimated the incremental borrowing rate based on the Company’s currently outstanding credit facility as inputs to the analysis to calculate a spread, adjusted for factors that reflect the profile of secured borrowing over the expected term of the lease.

The components of a lease should be split into three categories: lease components (e.g., land, building, etc.), non-lease components (e.g., common area maintenance, utilities, performance of manufacturing services, purchase of inventory, etc.), and non-components (e.g., property taxes, insurance, etc.). Then the fixed contract consideration (including any related to non-components) must be allocated based on fair values to the lease components and non-lease components. Although separation of lease and non-lease components is required, certain practical expedients are available to entities. Entities electing the practical expedient would not separate lease and non-lease components. Rather, they would account for each lease component and the related non-lease component together as a single component. The Company has elected not to apply the practical expedient and with respect to its lease of manufacturing space at a contract manufacturing organization, the Company has instead allocated the consideration between the lease and non-lease components of the contract. The Company calculated the fair value of the lease component using financial information readily available as part of its master services arrangement. The remainder of the consideration was allocated to the non-lease components.

Strimvelis loss provision

As part of the GSK transaction, the Company is required to maintain commercial availability of Strimvelis in the European Union until such time that an alternative gene therapy is available (Note 11). Strimvelis is not currently expected to generate sufficient cash flows to overcome the costs of maintaining the product and certain regulatory commitments; therefore, the Company initially recorded a liability associated with the loss contract of $18.4 million in 2018. The Company recognizes the amortization of the loss provision on a diminishing balance basis based on the actual net loss incurred associated with Strimvelis and the expected future net losses to be generated until such time as Strimvelis is no longer commercially available. The amortization of the provision is recorded as a credit to research and development expense. We have made an estimate of the expected future losses associated with Strimvelis and adjust this estimate as facts and circumstances change regarding the commercial availability and costs of maintaining and selling Strimvelis. The Company does not update the accrued loss provision for any subsequent adjustment of the future losses, however, the timing of recognizing the amortization of what was originally recorded is adjusted for the updated future losses. The following table below outlines the changes to the Strimvelis loss provision for the periods ended December 31, 2019 and 2018 (in thousands):

	Twelve Months Ended
	December 31,
	2019			2018
Balance at beginning of period	$	10,339		$	-
Provisions		—			18,351
Amortization of loss provision		(3,855	)		(6,366	)
Foreign currency translation		306			(1,646	)
Balance at end of period	$	6,790		$	10,339

As of December 31, 2019, $3.0 million of the Strimvelis loss provision was classified as current, and $3.8 million was classified as non-current. As of December 31, 2018, $4.2 million of the Strimvelis loss provision was classified as current, and $6.1 million was classified as non-current.

United Kingdom Research and development income tax credits

As a company that carries out research and development activities, the Company is able to submit tax credit claims from two U.K. research and development tax relief programs, the Small and Medium-sized Enterprises research and development tax credit (“SME”) program and the Research and Development Expenditure Credit (“RDEC”) program depending on eligibility. Qualifying expenditures largely comprise employment costs for research staff, consumables and certain internal overhead costs incurred as part of research projects for which the Company does not receive income.

Each reporting period, management evaluates which tax relief programs the Company is expected to be eligible for and records a reduction to research and development expense for the portion of the expense that it expects to qualify under the programs, that it plans to submit a claim for, and it has reasonable assurance that the amount will ultimately be realized. Based on criteria established by HM Revenue and Customs (“HMRC”), management of the Company expects a proportion of expenditures being carried in relation to its pipeline research, clinical trials management and manufacturing development activities to be eligible for the research and development tax relief programs for the years ended December 31, 2019, 2018 and 2017.

The RDEC and SME credits are not dependent on the Company generating future taxable income or on the ongoing tax status or tax position of the Company. The Company has assessed its research and development activities and expenditures to determine whether the nature of the activities and expenditures will qualify for credit under the tax relief programs and whether the claims will ultimately be realized based on the allowable reimbursable expense criteria established by the U.K. government which are subject to interpretation. At each period end, the Company estimates the reimbursement available to the Company based on available information at the time.

The Company recognizes credits from the research and development incentives when the relevant expenditure has been incurred and there is reasonable assurance that the reimbursement will be received. Such credits are accounted for as reductions in research and development expense.

The Company has recorded a U.K. research and development tax credit as an offset to research and development expense in the consolidated statements of operations and comprehensive loss of $17.6 million, $10.2 million and $0.7 million, for the years ended December 31, 2019, 2018 and 2017, respectively. During 2019, the Company recorded $4.1 million related to a change in estimate associated with our U.K research and development tax credit from prior years. The change in estimate was based on the results of a tax credit analysis associated with our qualified projects and research and development expenditures. This amount was recorded as an offset to research and development expense in 2019.

As of December 31, 2019, the Company’s tax incentive receivable from the U.K. was $28.6 million, of which $14.9 million was classified as current and $13.7 million was classified as non-current. As of December 31, 2018, the Company’s tax incentive receivable from the U.K. was $10.6 million, all of which was classified as current. The effects of foreign currency translation increased the receivable by $0.7 million in 2019 and decreased the receivable by $0.5 million in 2018. As of December 31, 2019, the Company has received amounts totaling $0.2 million associated with our 2016 tax incentive claim with HMRC.

Income taxes

The Company is primarily subject to corporation taxes in the United Kingdom and the United States. The calculation of the Company’s tax provision involves the application of both United Kingdom and United States tax law and requires judgement and estimates.

The Company accounts for income taxes in accordance with ASC Topic 740, Accounting for Income Taxes, which provides for deferred taxes using an asset and liability approach. The Company recognizes deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements or tax returns. The Company determines its deferred tax assets and liabilities based on differences between financial reporting and tax bases of assets and liabilities, which are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. Valuation allowances are provided if, based upon the weight of available evidence, it is more likely than not that some or all of the net deferred tax assets will not be realized.

The Company accounts for uncertainty in income taxes by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed as the amount of benefit to recognize in the consolidated financial statements. The amount of benefits that may be used is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate, as well as the related net interest and penalties.

Product sales

During the year, product sales include sales of Strimvelis, which is currently distributed exclusively at the San Raffaele Hospital in Milan, Italy. San Raffaele Hospital will purchase and pay for Strimvelis and submit a claim to the payer. The Company’s contracted sales with San Raffaele Hospital contain a single performance obligation and the Company recognizes revenue from product sales when the Company has satisfied its performance obligation by transferring control of Strimvelis to San Raffaele Hospital. Control of the product generally transfers upon the completion of the scheduled Strimvelis treatment. The Company’s product sales represent total gross product sales of Strimvelis. The Company evaluated the variable consideration under Accounting Standards Codification (ASC) 606, Revenue from Contracts with Customers, and there is currently no variable consideration included in the transaction price for Strimvelis. Costs to manufacture the product and those associated with administering the therapy are included in cost of product sales. As the product is sold in direct relation to a scheduled treatment, the Company estimates that there is minimal risk of product return, including the risk of product expiration.

Net income (loss) per share

The Company follows the two-class method when computing net income (loss) per share as the Company has issued shares that meet the definition of participating securities. The two-class method determines net income (loss) per share for each class of ordinary and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to ordinary shareholders for the period to be allocated between ordinary and participating securities based upon their respective rights to receive dividends as if all income for the period had been distributed.

Basic net income (loss) per share attributable to ordinary shareholders is computed by dividing the net income (loss) attributable to ordinary shareholders by the weighted average number of ordinary shares outstanding for the period. Diluted net income (loss) attributable to ordinary shareholders is computed by adjusting net income (loss) attributable to ordinary shareholders to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net income (loss) per share attributable to ordinary shareholders is computed by dividing the diluted net income (loss) attributable to ordinary shareholders by the weighted average number of ordinary shares outstanding for the period, including potential dilutive ordinary shares. For purpose of this calculation, outstanding options and convertible preferred shares are considered potential dilutive ordinary shares.

The Company’s convertible preferred shares contractually entitle the holders of such shares to participate in dividends but do not contractually require the holders of such shares to participate in losses of the Company. Accordingly, in periods in which the Company reports a net loss attributable to ordinary shareholders, such losses are not allocated to such participating securities. In periods in which the Company reports a net loss attributable to ordinary shareholders, diluted net loss per share attributable to ordinary shareholders is the same as basic net loss per share attributable to ordinary shareholders, since dilutive ordinary shares are not assumed to have been issued if their effect is anti-dilutive.

The Company reported a net loss attributable to ordinary shareholders for the years ended December 31, 2019, 2018, and 2017.

Recently adopted accounting pronouncements

In June 2018, the Financial Accounting Standards Board (“FASB”) issued ASU No. 2018-07 (“ASU 2018-07”). ASU 2018-07 expands the scope of Topic 718, Compensation—Stock Compensation, to include share-based payments issued to nonemployees for goods or services. Consequently, the accounting for share-based payments to nonemployees and employees will be substantially aligned. ASU 2018-07 supersedes Subtopic 505-50, Equity—Equity-Based Payments to Non-Employees. The amendments are effective for public companies for fiscal years beginning after December 15, 2018, including interim periods within that fiscal year. For all other companies, the amendments are effective for fiscal years beginning after December 15, 2019, and interim periods within fiscal years beginning after December 15, 2020. Early adoption is permitted, but no earlier than a company’s adoption date of Topic 606. ASU 2018-07 was adopted as of January 1, 2017 and did not have a material impact on the Company’s financial position, results of operations or cash flows.

In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) (“ASU 2016-02”), which sets out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e., lessees and lessors). ASU 2016-02 supersedes the lease guidance under FASB ASC Topic 840, Leases, resulting in the creation of FASB ASC Topic 842, Leases (“ASC 842”). The new standard requires that all lessees (i) recognize, on the balance sheet, liabilities to remit lease payments and right-of-use assets, representing the right to use the underlying asset for the lease term for both finance and operating leases, and (ii) disclose qualitative and quantitative information about its leasing arrangements.

In July 2018, the FASB issued ASU 2018-10, Codification Improvements to Topic 842, Leases (“ASU 2018-10”) and ASU 2018-11, Leases (Topic 842) Targeted Improvements (“ASU 2018-11”), which added an optional transition method under which financial statements may be prepared under the revised guidance for the year of adoption, but not for prior years. Under the latter method, entities will recognize a cumulative catch-up adjustment to the opening balance of retained earnings in the period of adoption.

As we are no longer an emerging growth company, ASC 842 became effective for us in 2019. The Company adopted ASC 842 using the modified retrospective approach with an effective date of January 1, 2019 for leases that existed on that date. Prior period results continue to be presented under ASC 840 based on the accounting standards originally in effect for such periods. This standard provides a number of optional practical expedients in transition. The Company applied the package of practical expedients to leases that commenced prior to the effective date, whereby it elected not to reassess the following: (i) whether any expired or existing contracts contain leases; (ii) the lease classification for any expired or existing leases; and (iii) initial direct costs for any existing leases. The Company elected the short-term lease recognition exemption for all leases that qualify, where a right-of-use asset or lease liability will not be recognized for short term leases that have terms of one year or less.

The operating lease right-of-use assets and corresponding liabilities relate to existing facility operating leases in London, UK, Boston, Massachusetts, and the San Francisco Bay Area, California, as well as an embedded operating lease for research and development space at a contract manufacturing organization. The most significant effects of adoption were the recognition of material new right-of-use assets and corresponding liabilities on its consolidated balance sheet related to its existing facility operating leases (see Note 7). The adoption of this standard has had a material impact on the Company’s financial position but did not significantly affect the Company’s results of operations or cash flows. The following table summarizes the financial impact on the Company’s consolidated balance sheet upon recording of the adoption entries for ASU 2016-02 on January 1, 2019 (in thousands):

	January 1, 2019 Prior to ASC 842 Adoption		ASC 842 Adjustment			January 1, 2019 as Adjusted
Consolidated balance sheet data (in thousands):
Operating lease right-of-use assets	$	—	$	22,348		$	22,348
Prepaid rent (1)		373		(373	)		—
Deferred rent (2)		181		(181	)		—
Deferred rent, non-current (3)		357		(357	)		—
Operating lease liabilities		—		3,746			3,746
Operating lease liabilities, net of current portion		—		18,748			18,748

(1)

Included in Prepaid expenses and other current assets in 2018 consolidated balance sheet

(2)

Included in Accrued expenses and other current liabilities in 2018 consolidated balance sheet

(3)

Included in Other long-term liabilities in 2018 consolidated balance sheet

In May 2017, the FASB issued ASU No. 2017-09, Compensation—Stock Compensation (Topic 718): Scope of Modification Accounting (“ASU 2017-09”), which clarifies when to account for a change to the terms or conditions of a share-based payment award as a modification. Under the new guidance, modification accounting is required only if the fair value, the vesting conditions or the classification of the award (as equity or liability) changes as a result of the change in terms or conditions. ASU 2017-09 is effective for all entities for annual periods, and interim periods within those annual periods, beginning after December 15, 2017. Early adoption is permitted, including adoption in any interim period, for 1) public business entities for reporting periods for which financial statements have not yet been issued and 2) all other entities for reporting periods for which financial statements have not yet been made available for issuance. The Company adopted ASU 2017-09 as of January 1, 2018. The adoption of ASU 2017-09 did not have a material impact on the Company’s financial position, results of operations or cash flows.

In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash (“ASU 2016-18”), which requires companies to include amounts generally described as restricted cash and restricted cash equivalents in cash and cash equivalents when reconciling beginning-of-period and end-of-period total amounts shown on the statement of cash flows. The Company adopted ASU 2016-18 for annual period beginning after December 15, 2017. Prior to the adoption of ASU 2016-18, the Company did not have material balances meeting the definition of restricted cash or restricted cash equivalents.

In August 2016, the FASB issued Accounting Standards Update No 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments (“ASU 2016-15”) to clarify guidance on the classification of certain cash receipts and payments in the statement of cash flows. The Company adopted this guidance as of January 1, 2018. The adoption of ASU 2016-15 did not have a material impact on the Company’s financial statements.

In October 2016, the FASB issued ASU No. 2016-16, Income Taxes (Topic 740): Intra-Entity Transfer of Assets Other than Inventory (“ASU 2016-16”), which requires the recognition of the income tax consequences of an intra-entity transfer (sales) of an asset, other than inventory, when the transfer occurs. The standard is effective for the Company beginning January 1, 2018. The Company does not currently engage in sale transactions with its wholly owned subsidiaries. Adoption of this standard did not have a material impact on the Company’s consolidated financial statements.

In January 2016, the FASB issued ASU 2016-01, Financial Instruments – Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities (“ASU 2016-01”), which amended the guidance on the recognition and measurement of financial assets and financial liabilities. The new guidance requires that equity investments (except those accounted for under the equity method of accounting, or those that result in consolidation of the investee) are measured at fair value with changes in fair value recognized in net income. The guidance also requires the use of an exit price when measuring the fair value of financial instruments for disclosure purposes, eliminates the requirement to disclose the methods and significant assumptions used to estimate the fair value that is required to be disclosed for financial instruments measured at amortized cost and requires separate presentation of financial assets and financial liabilities by measurement category and form of financial asset. The guidance became effective for the fiscal year beginning January 1, 2018, including interim periods within that fiscal year. Adoption of ASU 2016-01 did not have a material impact on the Company’s consolidated financial statements as the Company does not hold any equity securities.

In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASU 2014-09”), which supersedes existing revenue recognition guidance under U.S. GAAP. The standard’s core principle is that a company will recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the Company expects to be entitled in exchange for those goods or services. The standard defines a five-step process to achieve this principle and will require companies to use more judgment and make more estimates than under the current guidance. The Company expects that these judgments and estimates will include identifying performance obligations in the customer contract, estimating the amount of variable consideration to include in the transaction price and allocating the transaction price to each separate performance obligation. ASU 2014-09 also requires additional disclosure about the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts. In August 2015, the FASB issued ASU 2015-14, Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date, which delays the effective date of ASU 2014-09 such that the standard is effective for public entities for annual period beginning after December 15, 2017, including interim periods within those fiscal years. For all other entities, the guidance is effective beginning after December 15, 2018, and interim periods within annual periods beginning after December 15, 2019. Early adoption of the standard is permitted for annual periods beginning after December 15, 2016, including interim periods within those fiscal years. The Company adopted these revenue standards on January 1, 2017. Prior to 2018, the Company had no sources of revenue. In 2018, the Company had its first sales of Strimvelis and have applied this guidance to our revenue recognition, and as such there was no impact from the adoption of ASC 606 in prior periods.

Recently issued accounting pronouncements not yet adopted

In June 2016, the FASB issued new accounting guidance which requires entities to record expected credit losses for certain financial instruments, including trade receivables, as an allowance that reflects the entity's current estimate of credit losses expected to be incurred. For available-for-sale debt securities in unrealized loss positions, the new standard requires allowances to be recorded instead of reducing the amortized cost of the investment. The new standard became effective for the Company on January 1, 2020. We currently do not expect this guidance to have a significant impact on our consolidated financial statements and related disclosures.

The Company has considered other recent accounting pronouncements and concluded that they are either not applicable to the business, or that the effect is not expected to be material to the financial statements as a result of future adoption.

Fair Value Measurements and Marketable Securities

12 Months Ended

Dec. 31, 2019

Fair Value Disclosures [Abstract]

Fair Value Measurements and Marketable Securities

3. Fair Value Measurements and Marketable Securities

The following tables present information about the Company’s financial assets that have been measured at fair value as of December 31, 2019 and indicate the fair value of the hierarchy of the valuation inputs utilized to determine such fair value. In general, fair values determined by Level 1 inputs utilize quoted prices (unadjusted) in active markets for identical assets or liabilities. Fair value determined by Level 2 inputs utilize observable inputs other than Level 1 prices, such as quoted prices for similar assets or liabilities, quoted market prices in markets that are not active or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the related assets or liabilities. Fair values determined by Level 3 inputs are unobservable data points for the asset or liability, and include situations where there is little, if any, market activity for the asset or liability. During the year ended December 31, 2019, there were no transfers between Level 1 and Level 2 financial assets.

The following table summarizes the Company’s cash equivalents and marketable securities as of December 31, 2019 (in thousands)

	Fair Value Measurements as of December 31, 2019 Using:
	Level 1		Level 2		Level 3		Total
Cash equivalents
Money market funds	$	202	$	—	$	—	$	202
U.S. government securities		—		3,159		—		3,159
Commercial paper		—		9,792		—		9,792
Total cash equivalents	$	202	$	12,951	$	—	$	13,153
Marketable securities
Corporate bonds	$	—	$	259,900	$	—		259,900
Commercial paper		—		46,037		—		46,037
Total marketable securities	$	—	$	305,937	$	—	$	305,937
Total	$	202	$	318,888	$	—	$	319,090

The Company had no cash equivalents and marketable securities at December 31, 2018.

The carrying amount reflected in the consolidated balance sheets for research and development tax incentive receivable, trade receivables, other receivables, accounts payable, and accrued expenses approximate fair value due to their short-term maturities. The carrying value of the Company’s outstanding notes payable approximates fair value (a Level 2 fair value measurement), reflecting interest rates currently available to the Company.

Marketable Securities

The following table summarizes the Company’s level 2 cash equivalents and marketable securities as of December 31, 2019, in thousands:

	Fair Value Measurements as of December 31, 2019 Using:
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
U.S. government securities	$	3,159	$	—	$	—			3,159
Corporate bonds		259,669		285		(54	)		259,900
Commercial paper		55,794		42		(7	)		55,829
Total		318,622		327		(61	)		318,888

The following table summarizes the Company’s available-for-sale debt securities by contractual maturity, as of December 31, 2019, in thousands:

Maturities in one year or less	$	250,490
Maturities between one and three years		68,398
Balance at December 31, 2019	$	318,888

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Revenue Recognition

12 Months Ended

Dec. 31, 2019

Revenue From Contract With Customer [Abstract]

Revenue Recognition

4. Revenue Recognition

The Company currently has one commercial-stage therapy, Strimvelis, for the treatment of ADA-SCID. Strimvelis is currently administered exclusively at the San Raffaele Hospital in Milan, Italy. San Raffaele Hospital will purchase and pay for Strimvelis and submit a claim to the payer. The Company’s contracted sales with San Raffaele Hospital contain a single performance obligation and the Company recognizes revenue from product sales when the Company has satisfied its performance obligation by transferring control of Strimvelis to San Raffaele Hospital. Control of the product generally transfers upon the completion of the scheduled Strimvelis treatment. The Company’s product sales represent total gross product sales of Strimvelis. The Company evaluated the variable consideration under Accounting Standards Codification (ASC) 606, Revenue from Contracts with Customers, and there is currently no variable consideration included in the transaction price for Strimvelis. The Company excludes from measurement of the transaction price all taxes assessed by a governmental authority that are both imposed concurrent with the specific revenue-producing transaction and collected by the Company from a customer. The Company does include such taxes in trade receivables on the consolidated balance sheet.

Costs to manufacture the product and those associated with administering the therapy are included in cost of product sales. As the product is sold in direct relation to a scheduled treatment, the Company estimates that there is minimal risk of product return, including the risk of product expiration.

Payment terms and conditions generally require payment for Strimvelis sales within 60 days of treatment. Strimvelis is currently distributed exclusively at the San Rafaelle Hospital, and there is currently no variable consideration included in the transaction price of Strimvelis.

Property and Equipment

12 Months Ended

Dec. 31, 2019

Property Plant And Equipment [Abstract]

Property and Equipment

5. Property and Equipment

Property and equipment consist of the following (in thousands):

	December 31,
	2019			2018
Property and equipment:
Lab equipment	$	6,377		$	4,689
Leasehold improvements		1,839			1,487
Furniture and fixtures		508			403
Office and IT equipment		184			152
Construction-in-progress		1,848			241
Property and equipment	$	10,756		$	6,972
Less: accumulated depreciation		(3,160	)		(1,496	)
Property and equipment, net	$	7,596		$	5,476

Depreciation expense for the years ended December 31, 2019, 2018 and 2017 was $1.7 million, $1.2 million and $0.3 million, respectively.

Accrued Expenses and Other Liabilities

12 Months Ended

Dec. 31, 2019

Accrued Liabilities And Other Liabilities [Abstract]

Accrued Expenses and Other Liabilities

6. Accrued Expenses and Other Liabilities

Accrued expenses and other current liabilities consisted of the following (in thousands):

	December 31,
	2019		2018
Accrued external research and development expenses	$	16,215	$	12,738
Accrued payroll and related expenses		12,381		7,372
Accrued professional fees		1,321		1,186
Accrued other		5,069		2,762
Strimvelis liability - current portion		2,994		4,170
Due to UCLA		—		1,552
Total accrued expenses and other current liabilities	$	37,980	$	29,780

Leases

12 Months Ended

Dec. 31, 2019

Leases [Abstract]

Leases

7. Leases

Operating leases

In October 2016, the Company entered into a lease agreement for laboratory space in Foster City, California, United States. The lease had a term of 5 years and originally terminated in October 2021. In June 2019, the Company assigned the lease to a third-party and were relieved of future payment obligations under the lease. Costs associated with the termination of the lease were not material.

In November 2017 and January 2019, the Company entered into lease agreements for office and laboratory space in Menlo Park, California, United States. The leases terminate in December 2020. The combined annual rental payments under both leases with the same landlord approximate $1.3 million. The Company was provided with one month of free rent in connection with the first lease. The lease agreement includes annual rent escalation provisions.

In January 2018 and December 2018, the Company entered into lease agreements for office space in London, United Kingdom, both of which terminate in January 2023. The combined annual rental payments under the lease agreements approximate $0.9 million.

In March 2018, the Company entered into a lease agreement for office space in Boston, Massachusetts, United States, which terminates in September 2022. The annual rental payments approximate $0.3 million. The lease agreement includes annual rent escalation provisions.

In July 2019, the Company entered into a lease agreement for office space in Boston, Massachusetts, United States, which commences for accounting purposes in January 2020. The lease terminates in September 2026. The annual rental payments approximate $0.8 million. The lease agreement includes annual rent escalation provisions.

Fremont operating lease agreement

In December 2018, the Company leased manufacturing, laboratory, and office space in Fremont, California (the “Fremont facility”) which terminates in May 2030. The annual rental payments approximate $2.6 million. The lease includes annual rent escalation provisions. The Company was provided with 8 months of free rent. Subject to the terms of the lease agreement, the Company executed a $3.0 million letter of credit upon signing the lease, which may be reduced by 25% subject to reduction requirements specified therein. This amount is classified as restricted cash on the consolidated balance sheet.

Upon the adoption of ASU 2016-02, we recorded an operating lease right-of-use asset of $14.7 million and a corresponding lease liability of $14.6 million related to the Fremont facility. The lease provides for approximately $5.0 million in tenant improvement allowances to be reimbursed to the Company by the landlord. These tenant improvement allowances have been included in the calculation of the operating lease liability and is currently expected to be received in 2021. The Company will continue to assess the expected receipt of the tenant improvement allowances.

The Company intends to perform non-normal tenant improvements to the property to customize the facility to suit the Company’s unique manufacturing needs. The Company is responsible for paying directly the costs associated with the construction project. As of December 31, 2019, the Company has not broken ground or incurred significant costs associated with the construction.

Upon the start of construction, the Company is required to deposit $10.0 million in an escrow account. Subject to the terms of the lease and reduction provisions, this amount may be decreased to nil over time. As of December 31, 2019, the Company has no funds deposited in the escrow account.

Embedded operating lease arrangement

The Company is party to a manufacturing agreement for research and development and future commercial production with a contract manufacturing organization. The agreement with the contract manufacturing organization was novated to the Company as part of the GSK Agreement (see Note 11). The agreement contains an operating lease for the use of dedicated manufacturing suites. Prior to the novation of the contract, GSK paid the contract manufacturing organization upfront fees of €6.8 million for license costs and manufacturing suite GMP readiness upon the achievement of certain contractual milestones. After novation of the contract to the Company as part of the GSK Agreement, the Company gained the ability to control the underlying manufacturing suites.

The Company makes bi-annual pre-payments to the contract manufacturing organization for manufacturing costs. The fair value of the lease component of the agreement was determined utilizing information from the agreement about the value of the suites, and a portion of the pre-payment amount is allocated to the lease component. With the adoption of ASU 2016-02, the Company recorded a right-of-use asset and corresponding lease liability for the operating lease in the amounts of $2.0 million and $2.0 million, respectively. As of December 31, 2019, the agreement with the contract manufacturing organization extends until July 1, 2022.

Summary of all lease costs recognized under ASC 842

Our facility leases described above generally contain customary provisions allowing the landlords to terminate the leases if we fail to remedy a breach of any of our obligations under any such lease within specified time periods, or upon our bankruptcy or insolvency. The leases do not include any restrictions or covenants that had to be accounted for under the lease guidance. The following table contains a summary of the lease costs recognized under ASC 842 and other information pertaining to the Company’s operating leases as of December 31, 2019 (in thousands):

Fixed lease cost	$	5,589
Variable lease cost		1,436
Total lease cost	$	7,025

Other information
Operating cash flows used for operating leases		5,738
Weighted-average remaining lease term (years)		8.2
Weighted-average discount rate		9.3	%

During the year ended December 31, 2019 there were no material right of use assets obtained in exchange for material new lease obligations. Variable lease payments are the amounts owed by the Company to a lessor that are not fixed, such as reimbursement for common area maintenance and utilities costs, and are not included in the calculation of the Company’s operating lease right of use assets or operating lease liabilities and are expensed when incurred.

As of December 31, 2019, future minimum commitments under ASC 842 under the Company’s property leases were as follows (in thousands):

Due in:
2020	$	6,498
2021		5,383
2022		5,039
2023		3,614
2024		3,715
Thereafter		18,871
Total future minimum lease payments		43,120
Less: Leases not yet commenced	$	(5,073	)
Less: imputed interest		(16,835	)
Total operating lease payments	$	21,212

As of December 31, 2018, future minimum commitments under ASC 840 under the company’s leases were as follows (in thousands):

Due in:
2019	$	3,303
2020		4,910
2021		4,135
2022		3,921
2023		2,844
Thereafter		20,386
Total future minimum lease payments	$	39,499

Notes Payable

12 Months Ended

Dec. 31, 2019

Debt Disclosure [Abstract]

Notes Payable

8. Notes Payable

In May 2019, the Company entered into a senior term facilities agreement (the “Credit Facility”) with MidCap Financial (Ireland) Limited (“MidCap Financial”), as agent, and additional lenders from time to time (together with MidCap Financial, the “Lenders”), to borrow up to $75.0 million in term loans. To date, the Company has borrowed $25.0 million under an initial term loan. The remaining $50.0 million under the Credit Facility may be drawn down in the form of a second and third term loan, the second term loan being a $25.0 million term loan available no earlier than September 30, 2019 and no later than December 31, 2020 upon submission of certain regulatory filings and evidence of the Company having $100.0 million in cash and cash equivalent investments; and the third term loan being a $25.0 million term loan available no earlier than July 1, 2020 and no later than September 30, 2021 upon certain regulatory approvals and evidence of the Company having $125.0 million in cash and cash equivalent investments. As of December 31, 2019, the Company had met the criteria to draw down the second term loan of $25.0 million.

The term loans under the Credit Facility will terminate on the fifth anniversary of the Closing Date (the “Loan Maturity Date”). Each term loan under the Credit Facility bears interest at an annual rate equal to 6% plus LIBOR. The Company is required to make interest-only payments on the term loan for all payment dates prior to 24 months following the date of the Credit Facility, unless the third tranche is drawn, in which case the Company is required to make interest-only payments for all payment dates prior to 36 months following the date of the Credit Facility. The term loans under the Credit Facility will begin amortizing on either the 24-month or the 36-month anniversary of the Credit Facility (as applicable), with equal monthly payments of principal plus interest to be made by the Company to the Lenders in consecutive monthly installments until the Loan Maturity Date. In addition, a final payment of 4.5% is due on the Loan Maturity Date. The Company accrues the final payment amount of $1.1 million associated with the first term loan, to outstanding debt by charges to interest expense using the effective-interest method from the date of issuance through the maturity date.

The Credit Facility includes affirmative and negative covenants. The affirmative covenants include, among others, covenants requiring the Company to maintain their legal existence and governmental approvals, deliver certain financial reports, maintain insurance coverage, maintain property, pay taxes, satisfy certain requirements regarding accounts and comply with laws and regulations. The negative covenants include, among others, restrictions on the Company transferring collateral, incurring additional indebtedness, engaging in mergers or acquisitions, paying dividends or making other distributions, making investments, creating liens, amending material agreements and organizational documents, selling assets, changing the nature of the business and undergoing a change in control, in some cases subject to certain exceptions. The Company is also subject to an ongoing minimum cash financial covenant in which the Company must maintain unrestricted cash in an amount not less than $20.0 million following the utilization of the second term loan and not less than $35.0 million following the utilization of the third term loan.

As of December 31, 2019, notes payable consist of the following (in thousands):

	December 31,
	2019		2018
Notes payable, net of issuance costs	$	24,541	$	—
Accretion related to final payment		158		—
Notes payable, long term	$	24,699	$	—

As of December 31, 2019, the estimated future principal payments due are as follows (in thousands):

	Aggregate Minimum Payments
2020	$	—
2021		4,861
2022		8,333
2023		8,334
2024		4,597
Thereafter		—
Total		26,125
Less unamortized portion of final payment		(967	)
Less unamortized debt issuance costs		(459	)
Notes payable, long term	$	24,699

During the year ended December 31, 2019, the Company recognized $1.5 million of interest expense related to the initial Term Loan. The effective annual interest rate as of December 31, 2019 on the outstanding debt under the Term Loan was approximately 10.5%.

Shareholders' Equity and Convertible Preferred Shares

12 Months Ended

Dec. 31, 2019

Shareholders Equity And Convertible Preferred Shares [Abstract]

Shareholders’ Equity and Convertible Preferred Shares

9. Shareholders’ Equity and Convertible Preferred Shares

Ordinary shares

As of December 31, 2019, and 2018, each holder of ordinary shares is entitled to one vote per ordinary share and to receive dividends when and if such dividends are recommended by the board of directors and declared by the shareholders. As of December 31, 2019, and 2018, the Company has not declared any dividends.

As of December 31, 2019, and 2018, the Company had authority to allot ordinary shares up to a maximum nominal value of £13,023,851.50 with a nominal value of £0.10 per share.

Initial Public Offering, Follow-on Public Offering and Corporate Reorganization

In November 2018, the Company completed its initial public offering (“IPO”) of ADSs. In the IPO, the Company sold an aggregate of 16,103,572 ADSs representing the same number of ordinary shares at a public offering price of $14.00 per ADS, including a partial exercise by the underwriters of their option to purchase additional ADSs. Net proceeds were $205.5 million, after deducting underwriting discounts of $15.8 million, and commissions and offering expenses paid by the Company of $4.2 million.

In June 2019, the Company completed its follow-on public offering of ADSs. The Company sold an aggregate of 9,725,268 ADSs representing the same number of ordinary shares at a public offering price of $14.25 per ADS, including a partial exercise by the underwriters of their option to purchase additional ADSs. Net proceeds were $129.7 million, after deducting underwriting discounts of $8.3 million, and commissions and offering expenses paid by the Company of $0.6 million.

Immediately prior to the completion of the IPO, all outstanding convertible preferred shares of the Company were converted into their respective class of preferred shares on a one-for-0.8003 basis. All ordinary shares were consolidated on a one-for-0.8003 basis. Following completion of these steps, and immediately prior to the completion of the IPO, each share outstanding was re-designated as an ordinary share on a one-for-one basis. Accordingly, all share and per share amounts for all periods presented in the accompanying condensed consolidated financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect this reverse split. In addition, all share options for all periods presented have been adjusted retroactively to reflect this reverse split.

Additionally, as part of the corporate reorganization associated with the Company’s IPO, each ordinary share with a nominal value of £0.00001 was redenominated as an ordinary share with a nominal value of £0.10. Accordingly, equity accounts for all periods presented in the condensed consolidated financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect the effects of the redenomination of our ordinary shares.

Other ordinary share issuances

In November 2016, as amended in September 2018, the Company entered into a license and development agreement with Oxford BioMedica U.K. Limited (“Oxford BioMedica”). As consideration for the rights and licenses granted to the Company under the license and development agreement, the Company issued 588,220 ordinary shares to Oxford BioMedica in December 2016. The Company also agreed to grant additional ordinary shares upon achievement of specified milestones. In November 2017, the first milestone was achieved, and the Company was obligated to issue an additional 150,826 shares. The shares issued in 2016 and 2017 were recorded based on their fair values at the time the agreement was executed of $0.5 million and $0.1 million, respectively. In August 2018, the terms of the arrangement were modified to extend milestone achievement dates under the agreement, and a second milestone was met, and the company issued an additional 150,826 shares. The shares issued in 2018 were recorded based on their fair value at the time the agreement was modified of $1.4 million. The amounts were recorded to research and development expense in the years ended December 31, 2018, 2017, and 2016, respectively.

In February 2016, and amended in July 2017, the Company entered into a license agreement (the “UCLB/UCLA License Agreement”) with UCL Business PLC (“UCLB”), which is the commercialization company of University College London, and The Regents of the University of California (“UCLA”), pursuant to which the Company issued nil, 1,224,094, and 3,441,290 ordinary shares in 2018, 2017 and 2016, respectively, to UCLB. The shares were recorded at their fair values as of the time the agreement was executed or modified, which was an aggregate of $3.8 million. Amounts totaling $1.7 million and $2.1 million were recorded to research and development expense for the years ended December 31, 2017 and 2016, respectively.

In 2016 and 2017, the Company entered into several license agreements with various academic and health care institutions to in-license certain intellectual property rights and know-how relevant to its programs. Pursuant to these agreements, the Company issued 256,096 and 800,380 ordinary shares in 2017 and 2016, respectively. The share commitments were recorded to research and development expense based on their fair values as of the time the respective agreement was executed or modified. The amounts were $1.4 million and $0.5 million in 2017 and 2016, respectively.

As of December 31, 2019, and 2018, the Company had outstanding 96,923,729 and 85,865,557 ordinary shares, respectively.

Share-based Compensation

12 Months Ended

Dec. 31, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Share-based Compensation

10. Share-based Compensation

The Company maintains three equity compensation plans; the Orchard Therapeutics Limited Employee Share Option Plan with Non-Employee Sub-Plan and U.S. Sub-Plan (the “2016 Plan”), the Orchard Therapeutics plc 2018 Share Option and Incentive Plan (the “2018 Plan”), and the 2018 Employee Share Purchase Plan (the “ESPP”). The board of directors has determined not to make any further awards under the 2016 plan following the Company’s IPO. On March 22, 2019, pursuant to the evergreen provisions in the 2018 Plan and the ESPP, the Company increased (i) the number of ordinary shares available for issuance under the 2018 Plan by 4,293,278, and (ii) the number of ordinary shares available for issuance under the ESPP by 858,656. As of December 31, 2019, 4,687,029 shares remained available for grant under the 2018 Plan, and 1,580,887 shares remained available for grant under the ESPP.

Prior to the Company’s IPO, the Company granted options to United States employees and non-employees at exercise prices deemed by the board of directors to be equal to the fair value of the ordinary share at the time of grant, and granted options to United Kingdom and European Union employees and non-employees at an exercise price equal to the par value of the ordinary shares of £0.00001. After the IPO, options are now granted at exercise prices equal to the fair value of the Company’s ordinary shares on the grant date for all employees. The vesting period is determined by the board of directors, which is generally four years. An option’s maximum term is ten years.

Options and option valuation

The assumptions used in the Black-Scholes option pricing model to determine the fair value of the share options granted to employees, non-employees, and directors during the year ended December 31, 2019, 2018, and 2017 were as follows:

	Year Ended December 31,
	2019	2018	2017
Risk-free interest rate	1.39 - 2.62%	2.66 - 3.03%	1.52 - 2.30%
Expected term (in years)	5.50 - 6.08	5.00 - 6.08		6.08
Expected volatility	70.1 - 72.1%	64.3 - 68.6%	77.8% - 80.0%
Expected dividend rate	0.00%	0.00%	0.00%

The Company uses the simplified method as prescribed by the Securities and Exchange Commission Staff Accounting Bulletin No. 107, Share-Based Payment, to calculate the expected term as it does not have sufficient historical exercise data to provide a reasonable basis upon which to estimate the expected term for options granted to employees and utilizes the contractual term for options granted to non-employees. The expected term is applied to the stock option grant group as a whole, as the Company does not expect substantially different exercise or post-vesting termination behavior among its employee population. The expected volatility is based on the historical volatility of a representative group of companies with similar characteristics to the Company, including those in the early stages of product development with a similar and therapeutic focus. For these analyses, the Company selects companies with comparable characteristics to its own including enterprise value, risk profiles, position within the industry, and with historical share price information sufficient to meet the expected term of the options. The risk-free interest rate is based on a U.S. treasury instrument whose term is consistent with the expected term of the stock options. The Company accounts for forfeitures as they occur.

Options

The following table summarizes option activity under the plans for the year ended December 31, 2019 (in thousands, except share and per share amounts):

	Shares			Weighted Average Exercise Price per Share		Weighted Average Remaining Contractual Life (Years)		Aggregate Intrinsic Value
Outstanding at December 31, 2018		10,203,432		$	3.04		8.97	$	129,551
Granted		4,681,282			13.57
Exercised		(1,209,335	)		1.65
Forfeited		(1,459,239	)		7.36
Outstanding and expected to vest at December 31, 2019		12,216,140		$	6.61		8.31	$	91,133
Exercisable, as of December 31, 2019		4,528,094		$	3.59		7.72	$	46,131

The weighted average exercise price of options granted to United Kingdom employees in 2018 was the nominal value of the underlying shares. The weighted average exercise price of options granted to United States employees in 2018 was $5.74 per share.

The aggregate intrinsic value of options is calculated as the difference between the exercise price of the options and the fair value of the Company’s ordinary shares for those options that had exercise prices lower than the fair value of the Company’s ordinary shares. During 2019, the total intrinsic value of share options exercised was $17.2 million. During 2018 and 2017 the total intrinsic value of share options exercised was not material.

The weighted average grant date fair value of the options granted during the years ended December 31, 2019, 2018, and 2017 was $8.67 per share, $5.23 per share, and $2.70 per share, respectively. All share options have 10-year terms.

Restricted Share Units

The Company has issued performance-based restricted share units (“RSUs”) to certain executives and members of its senior management, with vesting linked to the achievement of three specific regulatory and research and development milestones and one market condition based upon the volume weighted-average price (“VWAP”) of the Company’s ADSs for a certain period. Upon achievement of any of the aforementioned milestones, one third of the RSU’s will vest, and the award will become fully vested upon achievement of three of the four performance conditions.

The maximum aggregate total fair value of the outstanding performance based RSUs is $10.1 million as of December 31, 2019. The fair value associated with the shares that could vest based on the market-based condition is being recognized as expense over an average derived service period of 1.3 years. The fair value associated with the performance-based conditions will be recognized when achievement of the milestones becomes probable, if at all. The Company determined that, as of December 31, 2019, none of the regulatory and research and development milestones were deemed probable.

The following table summarizes award activity for the year-end December 31, 2019:

	Shares			Weighted Average Fair Value per Share
Unvested at December 31, 2018		219,922		$	15.48
Granted		428,000			12.29
Vested		—
Forfeited		(91,500	)		12.13
Unvested at December 31, 2019		556,422		$	13.58

The amount of compensation cost recognized for the years ended December 31, 2019, 2018 and 2017 for the market condition associated with the performance based RSUs was $1.2 million, $0.1 million and nil, respectively.

Share-based compensation

Share-based compensation expense related to share options, restricted share unit awards, and the employee stock purchase plan was classified in the consolidated statements of operations and comprehensive loss as follows:

	Year Ended December 31,
	2019		2018		2017
Research and development	$	7,425	$	2,740	$	615
Selling, general and administrative		11,999		4,026		404
Total	$	19,424	$	6,766	$	1,019

The Company had 7,688,046 unvested options outstanding as of December 31, 2019. As of December 31, 2019, total unrecognized compensation cost related to unvested stock option grants was approximately $48.0 million. This amount is expected to be recognized over a weighted average period of approximately 2.6 years. As of December 31, 2019, the total unrecognized compensation cost related to performance based RSUs is a maximum of $8.8 million, dependent upon achievement of milestones.

License and Research Agreements

12 Months Ended

Dec. 31, 2019

License And Research Arrangements [Abstract]

License and Research Arrangements

11. License and Research Arrangements

GSK asset purchase and license agreement

In April 2018, the Company completed an asset purchase and license agreement (the “GSK Agreement”) with subsidiaries of GSK to acquire a portfolio of autologous ex vivo gene therapy assets and licenses, for rare diseases and option rights on three additional programs in preclinical development from Telethon Foundation and San Raffaele Hospital (“Telethon-OSR”). The portfolio of programs and options acquired consisted of:

•

Two late-stage clinical gene therapy programs in ongoing registrational trials for MLD and WAS;

•

One earlier stage clinical gene therapy program for TDT;

•

Strimvelis, the first autologous ex vivo gene therapy for ADA-SCID which was approved for marketing by the European Medicines Agency in 2016; and

•

Option rights exercisable upon completion of clinical proof of concept studies for three additional earlier-stage development programs, which option rights have all subsequently lapsed.

The Company accounted for the GSK Agreement as an asset acquisition, since the asset purchase and licensing arrangement did not meet the definition of a business pursuant to ASC 805, Business Combinations. Total consideration of £94.2 million ($133.6 million at the acquisition date), which includes an upfront payment of £10.0 million ($14.2 million at the acquisition date) and 12,455,252 convertible preferred shares of the Company issued to GSK at an aggregate value of £65.8 million ($93.4 million at the acquisition date), a loss contract on the Strimvelis program valued at £12.9 million ($18.4 million), an inventory purchase liability valued at £4.9 million ($6.9 million) and transaction costs of £0.6 million ($0.8 million). The Company allocated £94.2 million ($133.6 million) to in-process research and development expense (based on the fair value of the underlying programs in development). The convertible preferred shares were converted to ordinary shares as part of our IPO in November 2018.

The Company is required to use commercially reasonable efforts to obtain a Priority Review Voucher (“PRV”) from the United States Food and Drug Administration for each of the programs for MLD, WAS and TDT, the first of which GSK retained beneficial ownership over. GSK also has an option to acquire, at a price pursuant to an agreed upon formula, any PRV granted to the Company thereafter for MLD, WAS and TDT. If GSK does not exercise this option to purchase any PRV, the Company may sell the PRV to a third party and must share any proceeds in excess of a specified sale price equally with GSK. For accounting purposes, as of December 31, 2019, the Company does not consider the attainment of a PRV from the United States Food and Drug Administration to be probable.

As part of the GSK Agreement the Company is required to use its best endeavors to make Strimvelis commercially available in the European Union until such time as an alternative gene therapy, such as the Company’s OTL-101 product candidate, is commercially available for patients in Italy, and at all times at the San Raffaele Hospital in Milan, provided that a minimum number of patients continue to be treated at this site. Strimvelis is not currently expected to generate sufficient cash flows to overcome the costs of maintaining the product and certain regulatory commitments; therefore, the Company recorded a liability associated with the loss contract of £12.9 million ($18.4 million at the acquisition date) associated with the loss expected due to this obligation. This liability is being amortized over the remaining period of expected sales of Strimvelis as a credit to research and development expenses (see Note 2).

The Company will pay GSK non-refundable royalties and milestone payments in relation to the gene therapy programs acquired and OTL-101. The Company will pay a flat mid-single digit percentage royalty on the combined annual net sales of ADA-SCID products, which includes Strimvelis and the Company-developed product candidate, OTL-101. The Company will also pay tiered royalty rates at a percentage beginning in the mid-teens up to twenty percent for the MLD and WAS products, upon marketing approval, calculated as percentages of aggregate cumulative net sales of the MLD and WAS products, respectively. The Company will pay a tiered royalty at a percentage from the high single-digits to low double-digit for the TDT product, upon marketing approval, calculated as percentages of aggregate annual net sales of the TDT product. These royalties owed to GSK are in addition to any royalties owed to other third parties under various license agreements for the GSK programs. In aggregate, the Company may pay up to £90.0 million in milestone payments upon achievement of certain sales milestones applicable to GSK. The Company’s royalty obligations with respect to MLD and WAS may be deferred for a certain period in the interest of prioritizing available capital to develop each product. The Company’s royalty obligations are subject to reduction on a product-by-product basis in the event of market control by biosimilars and will expire in April 2048. Other than Strimvelis, these royalty and milestone payments were not determined to be probable and estimable at the date of the acquisition and are not included as part of consideration.

The Company and GSK also separately executed a Transition Services Agreement (“TSA”) as well as an Inventory Sale Agreement, in April 2018. The TSA outlined several activities that the Company had requested GSK to assist with during the transition period, including but not limited to utilizing GSK to sell, market and distribute Strimvelis, and assist with regulatory, clinical and non-clinical activities for the other non-commercialized products which were ongoing at the date of the GSK Agreement. The TSA expired in December 2018.

In connection with the Company’s entering into the GSK Agreement, GSK assigned rights and obligations to certain contracts, which include among others, the original license agreement with Telethon-OSR and an ongoing manufacturing agreement.

Telethon-OSR research and development collaboration and license agreements

In connection with the Company’s entering into the GSK Agreement, the Company also acquired and assumed agreements with Telethon Foundation and San Raffaele Hospital, together referred to as Telethon-OSR, for the research, development and commercialization of autologous ex vivo gene therapies for ADA-SCID, WAS, MLD, TDT, as well as options over three additional earlier-stage development programs. The Company’s options under the agreement with Telethon-OSR with respect to the earlier-stage programs have lapsed.

As consideration for the licenses, the Company will be required to make payments to Telethon-OSR upon achievement of certain product development milestones. Additionally, the Company will be required to pay to Telethon-OSR a tiered mid-single to low-double digit royalty percentage on annual sales of licensed products covered by patent rights on a country-by-country basis, as well as a low double-digit percentage of sublicense income received from any certain third-party sublicenses of the collaboration programs. These royalties are in addition to those payable to GSK under the GSK Agreement. The Company may pay up to an aggregate of approximately €31.0 million ($34.8 million at December 31, 2019) in milestone payments upon achievement of certain product development milestones for the program.

In May 2019, the Company entered into a license agreement with Telethon-OSR, under which Telethon-OSR granted to the Company an exclusive worldwide license for the research, development, manufacture and commercialization of Telethon-OSR’s ex vivo autologous HSC lentiviral based gene therapy for the treatment of mucopolysaccharidosis type I (“MPS-I”), including the Hurler variant. Under the terms of the agreement, Telethon-OSR received €15.0 million in upfront and milestone payments from the Company upon entering into the agreement, resulting in $17.2 million in in-process research and development expense. The Company is also required to make milestone payments contingent upon certain development, regulatory and commercial milestones are achieved. Additionally, the Company will be required to pay Telethon a tiered mid-single to low-double digit royalty percentage on annual net sales of licensed products.

UCLB/UCLA License Agreement

In February 2016, and amended in July 2017, the Company completed the UCLB/UCLA license agreement, under which the Company has been granted exclusive and non-exclusive, sublicensable licenses under certain intellectual property rights controlled by UCLB and UCLA to develop and commercialize gene therapy products in certain fields and territories.

In exchange for these rights, in 2016, the Company made upfront cash payments consisting of $0.8 million for the license to the joint UCLB/UCLA technology and $1.1 million for the license to the UCLB technology and manufacturing technology. The Company also issued an aggregate of 4,665,384 ordinary shares to UCLB, of which 1,224,094, and 3,441,290 ordinary shares were issued in 2017 and 2016, respectively. The Company recorded research and development expense based on the fair value of the ordinary shares as of the time the agreement was executed or modified. The Company was also obligated to make an additional cash payment for clinical data. In 2017, the Company paid $0.8 million in relation to clinical data acquired. The Company recorded the payments to research and development expense.

Under the UCLB/UCLA License Agreement, the Company is also obligated to pay an annual administration fee of $0.1 million on the first, second and third anniversary of the agreement date. Additionally, the Company may become obligated to make payments to the parties of up to an aggregate of £19.9 million upon the achievement of specified regulatory milestones as well as royalties ranging from low to mid-single-digit percentage on net sales of the applicable gene therapy product.

The Company recorded $0.1 million of research and development costs in respect of the UCLB/UCLA license agreement, which comprise the upfront payments, issuance of ordinary shares and payments for clinical data, for each of the years ended December 31, 2019, 2018 and 2017, respectively.

Unless terminated earlier by either party, the UCLB/UCLA license agreement will expire on the 25^th anniversary of the agreement.

Oxford BioMedica license, development and supply agreement

In November 2016, and amended in September 2018, the Company entered into an arrangement with Oxford BioMedica whereby Oxford BioMedica granted an exclusive intellectual property license to the Company for the purposes of research, development, and commercialization of collaboration products, and will provide process development services, and manufacture clinical and commercial GMP-grade lentiviral vectors to the Company (“Oxford BioMedica Agreement”). As part of the consideration to rights and licenses granted under the Oxford BioMedica Agreement, the Company issued 588,220 ordinary shares to Oxford BioMedica. The Company is also obligated to make certain development milestone payments in the form of issuance of additional ordinary shares if the milestones are achieved. In November 2017, the first milestone was achieved, and the Company was committed to issue another 150,826 ordinary shares, and issued these shares in 2018. In September 2018, the second and third milestones were achieved, and the Company issued 150,826 ordinary shares. No milestones were met during the year ended December 31, 2019.

The Company recorded $0.5 million to research and development expense upon execution of the Oxford BioMedica Agreement in 2016 and $0.1 million upon achievement of the first development milestone in 2017. The Company recorded $1.4 million upon achievement of the second and third development milestones in 2018. The expense recognized in 2016 and 2017 was determined based on the ordinary shares’ fair value as of the time the agreement was executed. The expense recognized in 2018 was determined based on the ordinary shares’ fair value as of the time the agreement was modified in September 2018. There was no expense recorded associated with the license agreement in year ended December 31, 2019 as no milestones were met or became probable during the period.

The Company may also pay low single-digit percentage royalties on annual net sales of collaborated product generated under the Oxford BioMedica Agreement.

UCLA/CIRM research agreement

In January 2017, the Company and UCLA executed a subcontract agreement (“UCLA Research Agreement”), whereby the Company would provide UCLA certain research and development services related to autologous ex vivo lentiviral gene therapy in ADA-SCID as part of UCLA’s existing ADA-SCID research program that is being funded by the California Institute for Regenerative Medicine (“CIRM”). The original amount of total reimbursement the Company could have received under the UCLA Research Agreement was $10.4 million. Through June 30, 2018, the Company received and recognized $7.3 million from this agreement. In July 2018, a transfer of the sponsorship took place and the Company became the awardee under the program funded by CIRM, and the Company received an award that superseded the previous award noted above. The total reimbursement the Company may receive under the new award is $8.5 million, of which the Company is obligated to reimburse UCLA for up to $5.5 million for research activities, subject to achievement of certain milestones outlined in the CIRM grant. Reimbursement may be received from CIRM during the period from January 2017 to December 2021. Under the terms of the CIRM grants, the Company is obligated to pay a low single-digit percentage royalty on net sales of CIRM-funded product candidates or CIRM-funded technology. The Company has the option to decline any and all amounts awarded by CIRM. As an alternative to revenue sharing, the Company has the option to elect to convert the award to a loan, in which case the full loan amount becomes payable within 10 days of election. No such election has been made as of the date. The reimbursements are recognized as a reduction in research and development expense for research activities that have taken place. In the event the reimbursement is received in advance of research activities, it is recognized within other liabilities. The Company accrues the sales-based royalties associated with CIRM-funded products when payment becomes probable. To date, no royalties have been accrued.

For the years ended December 31, 2019, 2018 and 2017, the Company recorded nil, $3.0 million, and $5.0 million, respectively, as a reduction of research and development expenses related to the UCLA Research Agreements. As of December 31, 2019, and 2018, the Company recorded nil and $1.6 million in accrued expenses for amounts which it is obligated to reimburse to UCLA under the July 2018 grant, respectively.

Income Taxes

12 Months Ended

Dec. 31, 2019

Income Tax Disclosure [Abstract]

Income Taxes

12. Income Taxes

The components of income (loss) from operations before income taxes for the years ended December 31, 2019, 2018, and 2017 are as follows (in thousands):

	December 31,
	2019			2018			2017
U.K.	$	(173,118	)	$	(230,543	)	$	(39,422	)
Non-U.K.		7,456			1,018			(269	)
Loss before taxes	$	(165,662	)	$	(229,525	)	$	(39,691	)

The (benefit from) provision for income taxes for the years ended December 31, 2019, 2018, and 2017 are as follows (in thousands):

	December 31,
	2019			2018			2017
Current (benefit) provision
Federal—United States	$	888		$	607		$	—
State—United States		(275	)		444			16
United Kingdom		—			—			—
Other		89			—			—
Total current (benefit) provision		702			1,051			16
Deferred (benefit) provision
Federal—United States		(2,820	)		(31	)		—
State—United States		(122	)		(50	)		37
United Kingdom		—			—			—
Total deferred (benefit) provision		(2,942	)		(81	)		37
Total (benefit) provision for income taxes	$	(2,240	)	$	970		$	53

The following table presents a reconciliation of income tax (benefit) expense computed at the U.K. statutory income tax rate to the effective income tax rate as reflected in the consolidated financial statements (in thousands):

	December 31,
	2019			2018			2017
Income taxes at United Kingdom statutory rate	$	(31,475	)	$	(43,526	)	$	(7,640	)
Change in valuation allowance		16,507			43,562			7,827
Reduction in research expense for credits granted		9,787			—			—
Change in tax rates		8,109			—			—
Tax credits		(3,372	)		—			(286	)
U.S. Deduction for foreign derived intangible income		(2,058	)
Permanent differences		344			293			115
U.S. state income taxes		(238	)		370			41
Foreign rate differential		156			20			(40	)
Impact of United States tax reform		—			159			36
Other		—			92			—
Total (benefit) provision for income taxes	$	(2,240	)	$	970		$	53

The Company’s income tax (benefit) expense for the year ended December 31, 2019 compared to the year ended December 31, 2018 increased primarily related to benefits associated with its U.S. federal research and development tax credits and U.S. foreign derived intangibles income deduction (“FDII”).

The Company accounts for income taxes in accordance with ASC Topic 740. Deferred income tax assets and liabilities are determined based upon temporary differences between the financial reporting and tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. The following table presents the principal components of the Company’s deferred tax assets and liabilities as of December 31, 2019 and 2018 (in thousands):

	December 31,
	2019			2018
Deferred tax assets
Net operating loss carryforwards	$	45,358		$	29,436
Amortization		21,741			19,451
Research and development credits		1,244			—
Share-based compensation		3,604			1,297
Accruals		1,286			184
Lease Liability		4,406			—
Other		1			1,946
Total deferred tax assets		77,640			52,314
Valuation allowance		(70,153	)		(51,281	)
Net deferred tax assets		7,487			1,033
Deferred tax liabilities
Operating lease right of use assets		(3,975	)		—
Depreciation		(528	)		(991	)
Other non-current liabilities (net deferred tax assets and liabilities)	$	2,984		$	42

As of December 31, 2019 and 2018, the Company had cumulative U.K. net operating loss carryforwards of approximately $266.8 million and $155.2 million, respectively. Unsurrendered U.K. losses may be carried forward indefinitely, subject to numerous utilization criteria and restrictions and are fully offset by a valuation allowance.

For the year ended December 31, 2019, the Company had cumulative U.S. federal general business and U.S. state research and development tax credit carryforwards of approximately $0.1 million and $1.4 million, respectively, available to reduce future U.S. tax liabilities. The U.S. federal general business tax credits will expire at various dates through 2039. The U.S. state tax credit carryforwards can be carried forward indefinitely and are fully offset by a valuation allowance.

The U.S. tax credit carryforwards may become subject to an annual limitation in the event of certain cumulative changes in the ownership interest of significant stockholders over a three-year period in excess of 50 percent, as defined under Sections 382 and 383 of the Internal Revenue Code, respectively, as well as similar state provisions. This could limit the amount of tax credits that can be utilized annually to offset future U.S. tax liabilities. The amount of the annual limitation is determined based on the value of the Company immediately prior to the ownership change. Subsequent ownership changes may further affect the limitation in future years. The Company has completed several financings since its inception that it believes may have resulted in a change in control as defined by Sections 382 and 383 of the Internal Revenue Code.

In measuring the Company’s deferred tax assets, the Company considers all available evidence, both positive and negative, to determine whether, based on the weight of that evidence, a valuation allowance is needed for all or some portion of the deferred tax assets. Significant judgment is required in considering the relative impact of the negative and positive evidence, and weight given to each category of evidence is commensurate with the extent to which it can be objectively verified. The more negative evidence that exists, the more positive evidence is necessary, and the more difficult it is to support a conclusion that a valuation allowance is not needed. Additionally, the Company utilizes the "more likely than not" criteria established in FASB ASC Topic 740 to determine whether the future tax benefit from the deferred tax assets should be recognized. As a result, the Company has established valuation allowances on the deferred tax assets in jurisdictions that have incurred net operating losses and in which it is more likely than not that such losses will not be utilized in the foreseeable future.

As of each reporting date, we consider new evidence, both positive and negative, that could impact our view with regard to future realization of our deferred tax assets. In the fourth quarter of 2019, Management has considered the Company’s history of cumulative net losses in the U.K., along with estimated future taxable income and has concluded that it is more likely than not that the Company will not realize the benefits of its U.K. deferred tax assets and U.S. state research and development tax credits. Accordingly, the Company has maintained a full valuation allowance against these net deferred tax assets as of December 31, 2019 and 2018, respectively.

Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2019, 2018, and 2017 related primarily to the increase in U.K. net operating loss carryforwards and U.K. amortization of intangible assets were as follows (in thousands):

	December 31,
	2019			2018			2017
Valuation allowance as of beginning of year	$	(51,281	)	$	(11,882	)	$	(3,503	)
Decreases recorded as benefit to income tax provision		—			604			—
Increases recorded to income tax provision		(16,507	)		(44,166	)		(7,827	)
Effect of foreign currency translation		(2,365	)		4,163			(552	)
Valuation allowance as of end of year	$	(70,153	)	$	(51,281	)	$	(11,882	)

The Company applies the authoritative guidance on accounting for and disclosure of uncertainty in tax positions, which requires the Company to determine whether a tax position of the Company is more likely than not to be sustained upon examination, including resolution of any related appeals of litigation processes, based on the technical merits of the position. For tax positions meeting the more likely than not threshold, the tax amount recognized in the financial statements is reduced by the largest benefit that has a greater than fifty percent likelihood of being realized upon the ultimate settlement with the relevant taxing authority. There were no material uncertain tax positions as of December 31, 2019 and 2018.

The Company will recognize interest and penalties related to uncertain tax positions in income tax expense when in a taxable income position. As of December 31, 2019 and 2018, the Company had no accrued interest or penalties related to uncertain tax positions and no amounts have been recognized in the Company’s statement of operations.

The Company and its subsidiaries file income tax returns in the U.K., the U.S., and various foreign jurisdictions. Generally, the tax years 2016 through 2018 remain open to examination by the major taxing jurisdictions to which the Company is subject. To the extent the Company has tax attribute carryforwards, the tax years in which the attribute was generated may still be adjusted upon examination by the federal, state, or foreign tax authorities, if such tax attributes are utilized in a future period.

Net Loss Per Share

12 Months Ended

Dec. 31, 2019

Earnings Per Share [Abstract]

Net Loss Per Share

13. Net Loss Per Share

The following table sets forth the computation of basic and diluted net loss per share (in thousands, except share and per share amounts):

	Year Ended December 31
	2019			2018			2017
Net loss attributable to ordinary shareholders	$	(163,422	)	$	(230,495	)	$	(39,744	)
Weighted average ordinary shares outstanding, basic and diluted		93,240,355			22,559,389			8,872,768
Net loss per share attributable to ordinary shareholders, basic and diluted	$	(1.75	)	$	(10.22	)	$	(4.48	)

Since the Company was in a loss position for all periods presented, basic net loss per share is the same as diluted net loss per share for all periods as the inclusion of all shares convertible into ordinary shares outstanding would have been anti-dilutive.

The following securities, presented based on amounts outstanding at each period end, are considered to be ordinary share equivalents, but were not included in the computation of diluted net loss per ordinary share because to do so would have been anti-dilutive:

	December 31,
	2019		2018		2017
Convertible preferred shares		—		—		33,277,678
Share options		10,056,864		9,179,247		3,612,288
Unvested shares from share plan and consulting agreement		751,496		219,922		—
		10,808,360		9,399,169		36,889,966

Commitments and Contingencies

12 Months Ended

Dec. 31, 2019

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

14. Commitments and Contingencies

Commitment with contract manufacturing organization

The Company has entered into agreements with contract manufacturing organizations relating to the provision of manufacturing services and purchase of clinical material to be used in clinical trials that include minimum purchase commitments. As of December 31, 2019, and December 31, 2018, there was nil and $0.8 million included within prepayments relates to prepaid instalments against these minimum commitments. As of December 31, 2019, the Company is committed to make further payments totaling $9.1 million between January 2020 and July 2022.

Other funding commitments

The Company has entered into several license agreements (Note 11). In connection with these agreements the Company is required to make milestone payments and annual license maintenance payments not met at December 31, 2019 and 2018 or royalties on future sales of specified products. The Company determined that no milestone payments were probable other than those already accrued as of December 31, 2019.

Consulting Agreement

In December 2019, the Company entered into a consulting agreement with non-employee advisor whereby the Company is obligated to make cash payments of $0.1 million per year and to issue up to 91,034 ordinary shares, which vest annually over a four year period, and 92,035 ordinary shares upon attainment of certain clinical development and regulatory milestones. As of December 31, 2019, the Company recorded immaterial research and development expense associated with the share-based awards with service conditions. No expense was recorded associated with the performance-based conditions.

Legal proceedings

The Company is not a party to any litigation and does not have contingency reserves established for any litigation liabilities.

Benefit Plans

12 Months Ended

Dec. 31, 2019

Compensation And Retirement Disclosure [Abstract]

Benefit Plans

15. Benefit Plans

The Company makes contributions to private defined contribution pension plans on behalf of its employees. The Company matches its employee contributions up to five percent of each employee’s annual salary based on the jurisdiction the employees are located. The Company paid $1.3 million, $0.6 million, and $0.2 million in matching contributions for the years ended December 31, 2019, 2018 and 2017, respectively.

Related-Party Transactions

12 Months Ended

Dec. 31, 2019

Related Party Transactions

16. Related-party Transactions

GSK

In April 2018, the Company completed the GSK Agreement with subsidiaries of GSK (See Note 11). As consideration under the agreement the Company paid an upfront fee of $14.2 million, purchased inventory of $6.9 million, paid $0.8 million in transaction costs, and issued 12,455,252 convertible preferred shares valued at $93.4 million. Additionally, as part of the GSK Agreement, the Company obtained, and is responsible for maintaining the commercial availability of Strimvelis. The Company recorded a loss provision of $18.4 million associated with the agreement, as the costs to maintain Strimvelis are expected to significantly exceed revenues. The issuance of the convertible preferred shares made GSK a principal shareholder in the Company.

In 2019, the Company made $7.2 million in payments to settle accounts payable due to GSK associated with the TSA and royalties associated with sales of Strimvelis incurred during 2018. Additionally, during 2019, the Company made a $3.6 million payment associated with the inventory purchase liability incurred upon entering into the agreement, and $0.1 million in royalties associated with Strimvelis sales during the year. In 2018 the Company paid $14.0 million in pass-through research and development and royalty costs with GSK associated with the TSA. As of December 31, 2019, and 2018, the Company had inventory purchase liability in accrued research and development expenses of $3.3 million and $6.2 million, respectively.

Selected Quarterly Financial Information (unaudited)

12 Months Ended

Dec. 31, 2019

Quarterly Financial Information Disclosure [Abstract]

Selected Quarterly Financial Information (unaudited)

17. Selected Quarterly Financial Information (unaudited)

The following table summarizes the unaudited quarterly financial data for the last two fiscal years (in thousands):

	2019
	First Quarter			Second Quarter (1)			Third Quarter			Fourth Quarter			Full year
Total revenues	$	-		$	-		$	1,918		$	595		$	2,513
Total costs and operating expenses		28,283			54,152			43,330			49,621			175,386
Loss from operations		(28,283	)		(54,152	)		(41,412	)		(49,026	)		(172,873	)
Net loss attributable to ordinary shareholders		(30,739	)		(50,530	)		(36,737	)		(45,416	)		(163,422	)
Weighted average ordinary shares outstanding - basic and diluted		87,010,596			89,712,916			97,817,847			98,243,915			93,240,355
Earnings per share	$	(0.35	)	$	(0.56	)	$	(0.38	)	$	(0.46	)	$	(1.75	)
	2018
	First Quarter			Second Quarter (2)			Third Quarter			Fourth Quarter			Full year
Total revenues	$	-		$	-		$	1,387		$	689		$	2,076
Total costs and operating expenses		13,698			158,412			35,477			29,520			237,107
Loss from operations		(13,698	)		(158,412	)		(34,090	)		(28,831	)		(235,031	)
Net loss attributable to ordinary shareholders		(15,311	)		(156,233	)		(33,888	)		(25,063	)		(230,495	)
Weighted average ordinary shares outstanding - basic and diluted		9,983,754			10,115,335			10,294,498			59,435,325			22,559,389
Earnings per share	$	(1.53	)	$	(15.45	)	$	(3.29	)	$	(0.42	)	$	(10.22	)

(1)

The increase in total costs and operating expenses in the second quarter of 2019 is due to the $17.2 million in-process research and development charge resulting from the licensing of a clinical stage development program for the treatment of MPS-I from Telethon-OSR (see Note 11).

(2)

The significant increase in total costs and operating expenses in the second quarter of 2018 is due to the $133.6 million in-process research and development charge resulting from the GSK Agreement (see Note 11).

As disclosed in Note 2, the Company adopted ASC 842 on December 31, 2019, with an effective date of January 1, 2019. In previously issued interim financial statements within the nine months ended September 30, 2019, the Company applied the guidance in ASC 840 for its leases. In connection with the adoption of ASC 842, there are no changes required to the amounts presented in the previously issued 2019 interim condensed consolidated statements of operations and comprehensive loss.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2019

Accounting Policies [Abstract]

Use of estimates

Concentration of credit risk

Foreign currency translation

Cash and cash equivalents

The Company considers all highly liquid investments purchased with original maturities of 90 days or less at the date of acquisition to be cash equivalents.

Marketable securities

Restricted cash

Restricted Cash

	December 31,
	2019		2018
Cash and cash equivalents	$	19,053	$	335,844
Restricted cash		4,264		3,837
Total cash, cash equivalents and restricted cash shown in the statement of cash flows	$	23,317	$	339,681

Property and equipment

Property and equipment are recorded at cost and depreciated or amortized using the straight-line method over the following estimated useful lives.

Property and equipment:		Estimated useful life
Lab equipment		5-10 years
Leasehold improvements		Shorter of lease term or estimated useful life
Furniture and fixtures		4 years
Office and computer equipment		3-5 years

Impairment of long-lived assets

Fair value measurements

•

Level 1—Quoted prices in active markets for identical assets or liabilities.

•

Segment information

Research and development costs

Research contract costs and accruals

Share-based compensation

Valuation of Stock Options

The fair value of each option is estimated on the date of grant using the Black-Scholes option pricing model. Assumptions used in the option pricing model include the following:

Expected term. The expected term of the Company’s share options has been determined utilizing the “simplified method” for awards that qualify as “plain-vanilla” options.

Fair value of ordinary shares. Options granted subsequent to the Company’s IPO are issued with an exercise price equal to the fair market value of the Company’s ADS at the date of grant.

Valuation of RSUs

Comprehensive loss

	Currency Translation Adjustment			Unrealized Gain (Loss) on Marketable Securities		Accumulated Other Comprehensive Income (Loss)
Balance at December 31, 2016	$	(271	)	$	—	$	(271	)
Other comprehensive (loss) income, net of tax		4,398			—		4,398
Balance at December 31, 2017	$	4,127		$	—	$	4,127
Other comprehensive (loss) income, net of tax		(964	)		—		(964	)
Balance at December 31, 2018	$	3,163		$	—	$	3,163
Other comprehensive (loss) income, net of tax		(1,387	)		266		(1,121	)
Balance at December 31, 2019	$	1,776		$	266	$	2,042

Leases

Strimvelis loss provision

	Twelve Months Ended
	December 31,
	2019			2018
Balance at beginning of period	$	10,339		$	-
Provisions		—			18,351
Amortization of loss provision		(3,855	)		(6,366	)
Foreign currency translation		306			(1,646	)
Balance at end of period	$	6,790		$	10,339

United Kingdom Research and development income tax credits

Income taxes

Product sales

Net income (loss) per share

The Company reported a net loss attributable to ordinary shareholders for the years ended December 31, 2019, 2018, and 2017.

Recently adopted accounting pronouncements

	January 1, 2019 Prior to ASC 842 Adoption		ASC 842 Adjustment			January 1, 2019 as Adjusted
Consolidated balance sheet data (in thousands):
Operating lease right-of-use assets	$	—	$	22,348		$	22,348
Prepaid rent (1)		373		(373	)		—
Deferred rent (2)		181		(181	)		—
Deferred rent, non-current (3)		357		(357	)		—
Operating lease liabilities		—		3,746			3,746
Operating lease liabilities, net of current portion		—		18,748			18,748

(1)

Included in Prepaid expenses and other current assets in 2018 consolidated balance sheet

(2)

Included in Accrued expenses and other current liabilities in 2018 consolidated balance sheet

(3)

Included in Other long-term liabilities in 2018 consolidated balance sheet

Recently issued accounting pronouncements not yet adopted

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2019

Accounting Policies [Abstract]

Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash

The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported in the consolidated balance sheet that sum to the total of the amounts reported in the consolidated statement of cash flows (in thousands):

	December 31,
	2019		2018
Cash and cash equivalents	$	19,053	$	335,844
Restricted cash		4,264		3,837
Total cash, cash equivalents and restricted cash shown in the statement of cash flows	$	23,317	$	339,681

Schedule of Estimated Useful Lives of Property and Equipment

Property and equipment are recorded at cost and depreciated or amortized using the straight-line method over the following estimated useful lives.

Property and equipment:		Estimated useful life
Lab equipment		5-10 years
Leasehold improvements		Shorter of lease term or estimated useful life
Furniture and fixtures		4 years
Office and computer equipment		3-5 years

Components of Accumulated Other Comprehensive Loss

For the years ended December 31, 2019, 2018 and 2017, the components of accumulated other comprehensive loss are detailed as follows (in thousands):

	Currency Translation Adjustment			Unrealized Gain (Loss) on Marketable Securities		Accumulated Other Comprehensive Income (Loss)
Balance at December 31, 2016	$	(271	)	$	—	$	(271	)
Other comprehensive (loss) income, net of tax		4,398			—		4,398
Balance at December 31, 2017	$	4,127		$	—	$	4,127
Other comprehensive (loss) income, net of tax		(964	)		—		(964	)
Balance at December 31, 2018	$	3,163		$	—	$	3,163
Other comprehensive (loss) income, net of tax		(1,387	)		266		(1,121	)
Balance at December 31, 2019	$	1,776		$	266	$	2,042

Schedule of Changes in Loss Provision

The following table below outlines the changes to the Strimvelis loss provision for the periods ended December 31, 2019 and 2018 (in thousands):

	Twelve Months Ended
	December 31,
	2019			2018
Balance at beginning of period	$	10,339		$	-
Provisions		—			18,351
Amortization of loss provision		(3,855	)		(6,366	)
Foreign currency translation		306			(1,646	)
Balance at end of period	$	6,790		$	10,339

Summary of Financial Impact on Company's Condensed Consolidated Balance Sheet upon Recording of Adoption Entries for ASU 2016-02

The following table summarizes the financial impact on the Company’s consolidated balance sheet upon recording of the adoption entries for ASU 2016-02 on January 1, 2019 (in thousands):

	January 1, 2019 Prior to ASC 842 Adoption		ASC 842 Adjustment			January 1, 2019 as Adjusted
Consolidated balance sheet data (in thousands):
Operating lease right-of-use assets	$	—	$	22,348		$	22,348
Prepaid rent (1)		373		(373	)		—
Deferred rent (2)		181		(181	)		—
Deferred rent, non-current (3)		357		(357	)		—
Operating lease liabilities		—		3,746			3,746
Operating lease liabilities, net of current portion		—		18,748			18,748

(1)

Included in Prepaid expenses and other current assets in 2018 consolidated balance sheet

(2)

Included in Accrued expenses and other current liabilities in 2018 consolidated balance sheet

(3)

Included in Other long-term liabilities in 2018 consolidated balance sheet

Fair Value Measurements and Marketable Securities (Tables)

12 Months Ended

Dec. 31, 2019

Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract]

Schedule of Cash Equivalents and Marketable Securities

The following table summarizes the Company’s cash equivalents and marketable securities as of December 31, 2019 (in thousands)

	Fair Value Measurements as of December 31, 2019 Using:
	Level 1		Level 2		Level 3		Total
Cash equivalents
Money market funds	$	202	$	—	$	—	$	202
U.S. government securities		—		3,159		—		3,159
Commercial paper		—		9,792		—		9,792
Total cash equivalents	$	202	$	12,951	$	—	$	13,153
Marketable securities
Corporate bonds	$	—	$	259,900	$	—		259,900
Commercial paper		—		46,037		—		46,037
Total marketable securities	$	—	$	305,937	$	—	$	305,937
Total	$	202	$	318,888	$	—	$	319,090

Schedule of Cash Equivalents and Marketable Securities

The following table summarizes the Company’s level 2 cash equivalents and marketable securities as of December 31, 2019, in thousands:

	Fair Value Measurements as of December 31, 2019 Using:
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
U.S. government securities	$	3,159	$	—	$	—			3,159
Corporate bonds		259,669		285		(54	)		259,900
Commercial paper		55,794		42		(7	)		55,829
Total		318,622		327		(61	)		318,888

Schedule of Available-for-Sale Debt Securities by contractual Maturity

The following table summarizes the Company’s available-for-sale debt securities by contractual maturity, as of December 31, 2019, in thousands:

Maturities in one year or less	$	250,490
Maturities between one and three years		68,398
Balance at December 31, 2019	$	318,888

Property and Equipment (Tables)

12 Months Ended

Dec. 31, 2019

Property Plant And Equipment [Abstract]

Schedule of Property and Equipment

Property and equipment consist of the following (in thousands):

	December 31,
	2019			2018
Property and equipment:
Lab equipment	$	6,377		$	4,689
Leasehold improvements		1,839			1,487
Furniture and fixtures		508			403
Office and IT equipment		184			152
Construction-in-progress		1,848			241
Property and equipment	$	10,756		$	6,972
Less: accumulated depreciation		(3,160	)		(1,496	)
Property and equipment, net	$	7,596		$	5,476

Accrued Expenses and Other Liabilities (Tables)

12 Months Ended

Dec. 31, 2019

Accrued Liabilities And Other Liabilities [Abstract]

Schedule of Accrued Expenses and Other Liabilities

Accrued expenses and other current liabilities consisted of the following (in thousands):

	December 31,
	2019		2018
Accrued external research and development expenses	$	16,215	$	12,738
Accrued payroll and related expenses		12,381		7,372
Accrued professional fees		1,321		1,186
Accrued other		5,069		2,762
Strimvelis liability - current portion		2,994		4,170
Due to UCLA		—		1,552
Total accrued expenses and other current liabilities	$	37,980	$	29,780

Leases (Tables)

12 Months Ended

Dec. 31, 2019

Leases [Abstract]

Summary of Lease Cost Under Operating Leases

The following table contains a summary of the lease costs recognized under ASC 842 and other information pertaining to the Company’s operating leases as of December 31, 2019 (in thousands):

Fixed lease cost	$	5,589
Variable lease cost		1,436
Total lease cost	$	7,025

Other information
Operating cash flows used for operating leases		5,738
Weighted-average remaining lease term (years)		8.2
Weighted-average discount rate		9.3	%

Schedule of Future Minimum Commitments Under Operating Leases

As of December 31, 2019, future minimum commitments under ASC 842 under the Company’s property leases were as follows (in thousands):

Due in:
2020	$	6,498
2021		5,383
2022		5,039
2023		3,614
2024		3,715
Thereafter		18,871
Total future minimum lease payments		43,120
Less: Leases not yet commenced	$	(5,073	)
Less: imputed interest		(16,835	)
Total operating lease payments	$	21,212

Schedule of Future Minimum Commitments Under Leases

As of December 31, 2018, future minimum commitments under ASC 840 under the company’s leases were as follows (in thousands):

Due in:
2019	$	3,303
2020		4,910
2021		4,135
2022		3,921
2023		2,844
Thereafter		20,386
Total future minimum lease payments	$	39,499

Notes Payable (Tables)

12 Months Ended

Dec. 31, 2019

Debt Disclosure [Abstract]

Summary of Notes Payable

As of December 31, 2019, notes payable consist of the following (in thousands):

	December 31,
	2019		2018
Notes payable, net of issuance costs	$	24,541	$	—
Accretion related to final payment		158		—
Notes payable, long term	$	24,699	$	—

Summary of Estimated Future Principal Payments Due

As of December 31, 2019, the estimated future principal payments due are as follows (in thousands):

	Aggregate Minimum Payments
2020	$	—
2021		4,861
2022		8,333
2023		8,334
2024		4,597
Thereafter		—
Total		26,125
Less unamortized portion of final payment		(967	)
Less unamortized debt issuance costs		(459	)
Notes payable, long term	$	24,699

Share-based Compensation (Tables)

12 Months Ended

Dec. 31, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Assumptions Used in Black-Scholes Option Pricing Model

	Year Ended December 31,
	2019	2018	2017
Risk-free interest rate	1.39 - 2.62%	2.66 - 3.03%	1.52 - 2.30%
Expected term (in years)	5.50 - 6.08	5.00 - 6.08		6.08
Expected volatility	70.1 - 72.1%	64.3 - 68.6%	77.8% - 80.0%
Expected dividend rate	0.00%	0.00%	0.00%

Summary of Option Activity

The following table summarizes option activity under the plans for the year ended December 31, 2019 (in thousands, except share and per share amounts):

	Shares			Weighted Average Exercise Price per Share		Weighted Average Remaining Contractual Life (Years)		Aggregate Intrinsic Value
Outstanding at December 31, 2018		10,203,432		$	3.04		8.97	$	129,551
Granted		4,681,282			13.57
Exercised		(1,209,335	)		1.65
Forfeited		(1,459,239	)		7.36
Outstanding and expected to vest at December 31, 2019		12,216,140		$	6.61		8.31	$	91,133
Exercisable, as of December 31, 2019		4,528,094		$	3.59		7.72	$	46,131

Summary of Award Activity

The following table summarizes award activity for the year-end December 31, 2019:

	Shares			Weighted Average Fair Value per Share
Unvested at December 31, 2018		219,922		$	15.48
Granted		428,000			12.29
Vested		—
Forfeited		(91,500	)		12.13
Unvested at December 31, 2019		556,422		$	13.58

Share-based Compensation Expense

	Year Ended December 31,
	2019		2018		2017
Research and development	$	7,425	$	2,740	$	615
Selling, general and administrative		11,999		4,026		404
Total	$	19,424	$	6,766	$	1,019

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2019

Income Tax Disclosure [Abstract]

Schedule of Income (Loss) before Income Tax

The components of income (loss) from operations before income taxes for the years ended December 31, 2019, 2018, and 2017 are as follows (in thousands):

	December 31,
	2019			2018			2017
U.K.	$	(173,118	)	$	(230,543	)	$	(39,422	)
Non-U.K.		7,456			1,018			(269	)
Loss before taxes	$	(165,662	)	$	(229,525	)	$	(39,691	)

Schedule of (Benefit from) Provision for Income Taxes

The (benefit from) provision for income taxes for the years ended December 31, 2019, 2018, and 2017 are as follows (in thousands):

	December 31,
	2019			2018			2017
Current (benefit) provision
Federal—United States	$	888		$	607		$	—
State—United States		(275	)		444			16
United Kingdom		—			—			—
Other		89			—			—
Total current (benefit) provision		702			1,051			16
Deferred (benefit) provision
Federal—United States		(2,820	)		(31	)		—
State—United States		(122	)		(50	)		37
United Kingdom		—			—			—
Total deferred (benefit) provision		(2,942	)		(81	)		37
Total (benefit) provision for income taxes	$	(2,240	)	$	970		$	53

Schedule of Reconciliation of Income Tax (Benefit) Expense

	December 31,
	2019			2018			2017
Income taxes at United Kingdom statutory rate	$	(31,475	)	$	(43,526	)	$	(7,640	)
Change in valuation allowance		16,507			43,562			7,827
Reduction in research expense for credits granted		9,787			—			—
Change in tax rates		8,109			—			—
Tax credits		(3,372	)		—			(286	)
U.S. Deduction for foreign derived intangible income		(2,058	)
Permanent differences		344			293			115
U.S. state income taxes		(238	)		370			41
Foreign rate differential		156			20			(40	)
Impact of United States tax reform		—			159			36
Other		—			92			—
Total (benefit) provision for income taxes	$	(2,240	)	$	970		$	53

Schedule of Deferred Tax Assets and Liabilities

The following table presents the principal components of the Company’s deferred tax assets and liabilities as of December 31, 2019 and 2018 (in thousands):

	December 31,
	2019			2018
Deferred tax assets
Net operating loss carryforwards	$	45,358		$	29,436
Amortization		21,741			19,451
Research and development credits		1,244			—
Share-based compensation		3,604			1,297
Accruals		1,286			184
Lease Liability		4,406			—
Other		1			1,946
Total deferred tax assets		77,640			52,314
Valuation allowance		(70,153	)		(51,281	)
Net deferred tax assets		7,487			1,033
Deferred tax liabilities
Operating lease right of use assets		(3,975	)		—
Depreciation		(528	)		(991	)
Other non-current liabilities (net deferred tax assets and liabilities)	$	2,984		$	42

Schedule of Changes in Valuation Allowance for Deferred Tax Assets Related Primarily to Increase in U.K. Net Operating Loss Carryforwards and U.K. Amortization of Intangible Assets

	December 31,
	2019			2018			2017
Valuation allowance as of beginning of year	$	(51,281	)	$	(11,882	)	$	(3,503	)
Decreases recorded as benefit to income tax provision		—			604			—
Increases recorded to income tax provision		(16,507	)		(44,166	)		(7,827	)
Effect of foreign currency translation		(2,365	)		4,163			(552	)
Valuation allowance as of end of year	$	(70,153	)	$	(51,281	)	$	(11,882	)

Net Loss Per Share (Tables)

12 Months Ended

Dec. 31, 2019

Earnings Per Share [Abstract]

Computation of Basic and Diluted Net Loss Per Share

The following table sets forth the computation of basic and diluted net loss per share (in thousands, except share and per share amounts):

	Year Ended December 31
	2019			2018			2017
Net loss attributable to ordinary shareholders	$	(163,422	)	$	(230,495	)	$	(39,744	)
Weighted average ordinary shares outstanding, basic and diluted		93,240,355			22,559,389			8,872,768
Net loss per share attributable to ordinary shareholders, basic and diluted	$	(1.75	)	$	(10.22	)	$	(4.48	)

Securities Excluded in the Computation of Diluted Net Loss Per Ordinary Share

	December 31,
	2019		2018		2017
Convertible preferred shares		—		—		33,277,678
Share options		10,056,864		9,179,247		3,612,288
Unvested shares from share plan and consulting agreement		751,496		219,922		—
		10,808,360		9,399,169		36,889,966

Selected Quarterly Financial Information (unaudited) (Tables)

12 Months Ended

Dec. 31, 2019

Quarterly Financial Information Disclosure [Abstract]

Summary of Unaudited Quarterly Financial Data

The following table summarizes the unaudited quarterly financial data for the last two fiscal years (in thousands):

	2019
	First Quarter			Second Quarter (1)			Third Quarter			Fourth Quarter			Full year
Total revenues	$	-		$	-		$	1,918		$	595		$	2,513
Total costs and operating expenses		28,283			54,152			43,330			49,621			175,386
Loss from operations		(28,283	)		(54,152	)		(41,412	)		(49,026	)		(172,873	)
Net loss attributable to ordinary shareholders		(30,739	)		(50,530	)		(36,737	)		(45,416	)		(163,422	)
Weighted average ordinary shares outstanding - basic and diluted		87,010,596			89,712,916			97,817,847			98,243,915			93,240,355
Earnings per share	$	(0.35	)	$	(0.56	)	$	(0.38	)	$	(0.46	)	$	(1.75	)
	2018
	First Quarter			Second Quarter (2)			Third Quarter			Fourth Quarter			Full year
Total revenues	$	-		$	-		$	1,387		$	689		$	2,076
Total costs and operating expenses		13,698			158,412			35,477			29,520			237,107
Loss from operations		(13,698	)		(158,412	)		(34,090	)		(28,831	)		(235,031	)
Net loss attributable to ordinary shareholders		(15,311	)		(156,233	)		(33,888	)		(25,063	)		(230,495	)
Weighted average ordinary shares outstanding - basic and diluted		9,983,754			10,115,335			10,294,498			59,435,325			22,559,389
Earnings per share	$	(1.53	)	$	(15.45	)	$	(3.29	)	$	(0.42	)	$	(10.22	)

(1)

(2)

Nature of the Business and Basis of Presentation - Additional Information (Details) $ / shares in Units, $ in Thousands			1 Months Ended	3 Months Ended								12 Months Ended
	Nov. 02, 2018 USD ($) $ / shares shares	Nov. 01, 2018	Jun. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($) $ / shares	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2019 £ / shares	Dec. 31, 2018 £ / shares	Dec. 31, 2017 £ / shares
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Reverse stock split		one-for-0.8003
Ordinary Shares, Par Value \| £ / shares															£ 0.10	£ 0.10
Payments of stock issuance costs												$ 605	$ 4,200
Net loss				$ 45,416	$ 36,737	$ 50,530	$ 30,739	$ 25,063	$ 33,888	$ 156,233	$ 15,311	163,422	230,495	$ 39,744
Accumulated deficit				453,661				$ 290,239				453,661	$ 290,239
Cash, cash equivalents and marketable securities				$ 325,000								$ 325,000
American Depositary Shares \| Initial Public Offering
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Shares issued \| shares	16,103,572
Issue price per share \| $ / shares	$ 14.00
Proceeds from shares issued	$ 205,500
Payments of stock issuance costs	4,200
American Depositary Shares \| Underwriter Discounts and Commissions
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Payments of stock issuance costs	$ 15,800		$ 600
American Depositary Shares \| Follow On Public Offering
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Shares issued \| shares			9,725,268
Issue price per share \| $ / shares			$ 14.25			$ 14.25
Proceeds from shares issued			$ 129,700
Payments of stock issuance costs			$ 8,300
Corporate Reorganization
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Ordinary Shares, Par Value \| £ / shares															£ 0.10		£ 0.00001

Summary of Significant Accounting Policies - Additional Information (Details)	3 Months Ended				12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2019 USD ($) Segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Description of significant off-balance sheet risk					The Company has no significant off-balance sheet risk, such as foreign currency contracts, options contracts, or other foreign hedging arrangements.
Foreign currency transaction gain (loss)					$ 1,500,000	$ 4,400,000	$ (1,200,000)
Cash collateral associated with corporate credit cards and other leases	$ 1,300,000				1,300,000
Impairment losses					0
Property and equipment	7,596,000		$ 5,476,000		7,596,000	5,476,000
Product sales	595,000	$ 1,918,000	689,000	$ 1,387,000	2,513,000	2,076,000
Operating lease right-of-use-assets	19,415,000				19,415,000
Strimvelis loss provision current	3,000,000		4,200,000		3,000,000	4,200,000
Strimvelis loss provision non current	3,800,000		6,100,000		3,800,000	6,100,000
Tax credit					3,372,000		286,000
Tax incentive receivable, current	14,934,000		10,585,000		14,934,000	10,585,000
Tax incentive receivable, long term	13,710,000				13,710,000
Increase (decrease) in tax receivable					700,000	(500,000)
Tax incentive received amount					$ 200,000
GSK Asset Purchase and License Agreement
Summary Of Significant Accounting Policies [Line Items]
Liability associated with the loss contract			18,400,000			18,400,000
United Kingdom
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments \| Segment					3
Property and equipment	2,600,000		1,700,000		$ 2,600,000	1,700,000
Tax incentive receivable	28,600,000				28,600,000
Tax incentive receivable, current	14,900,000		10,600,000		14,900,000	10,600,000
Tax incentive receivable, long term	13,700,000				$ 13,700,000
Incremental percentage of taxable profits					50.00%
United Kingdom \| Research and Development Expense
Summary Of Significant Accounting Policies [Line Items]
Tax credit					$ 17,600,000	10,200,000	$ 700,000
Change in estimate associated to research and development tax credit from prior years					$ 4,100,000
European Union
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments \| Segment					3
Product sales					$ 2,500,000	2,100,000
United States
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments \| Segment					3
Property and equipment	5,000,000		$ 3,800,000		$ 5,000,000	3,800,000
Operating lease right-of-use-assets	15,700,000				15,700,000
United Kingdom and European Union
Summary Of Significant Accounting Policies [Line Items]
Operating lease right-of-use-assets	3,700,000				3,700,000
Fremont Lease Agreement [Member]
Summary Of Significant Accounting Policies [Line Items]
Proceeds from lines of credit					3,000,000	$ 3,000,000
Operating lease right-of-use-assets	$ 14,700,000				$ 14,700,000

Summary of Significant Accounting Policies - Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Summary Of Significant Accounting Policies [Abstract]
Cash and cash equivalents	$ 19,053	$ 335,844
Restricted cash	4,264	3,837
Total cash, cash equivalents and restricted cash shown in the statement of cash flows	$ 23,317	$ 339,681	$ 89,856	$ 3,497

Summary of Significant Accounting Policies - Schedule of Estimated Useful Lives of Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2019
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment useful life, term	Shorter of lease term or estimated useful life
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment useful life	4 years
Minimum \| Lab Equipment
Property Plant And Equipment [Line Items]
Property and equipment useful life	5 years
Minimum \| Office and Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment useful life	3 years
Maximum \| Lab Equipment
Property Plant And Equipment [Line Items]
Property and equipment useful life	10 years
Maximum \| Office and Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment useful life	5 years

Summary of Significant Accounting Policies - Components of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income Loss [Line Items]
Stockholders Equity, Beginning Balance	$ 311,338	$ 86,405	$ (16,524)
Other comprehensive (loss) income, net of tax	(1,121)	(964)	4,398
Stockholders Equity, Ending Balance	299,193	311,338	86,405
Currency Translation Adjustment
Accumulated Other Comprehensive Income Loss [Line Items]
Stockholders Equity, Beginning Balance	3,163	4,127	(271)
Other comprehensive (loss) income, net of tax	(1,387)	(964)	4,398
Stockholders Equity, Ending Balance	1,776	3,163	4,127
Unrealized Gain (Loss) on Marketable Securities
Accumulated Other Comprehensive Income Loss [Line Items]
Other comprehensive (loss) income, net of tax	266
Stockholders Equity, Ending Balance	266
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income Loss [Line Items]
Stockholders Equity, Beginning Balance	3,163	4,127	(271)
Other comprehensive (loss) income, net of tax	(1,121)	(964)	4,398
Stockholders Equity, Ending Balance	$ 2,042	$ 3,163	$ 4,127

Summary of Significant Accounting Policies - Schedule of Changes in Loss Provision (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Summary Of Significant Accounting Policies [Abstract]
Balance at beginning of period	$ 10,339
Provisions		$ 18,351
Amortization of loss provision	(3,855)	(6,366)
Foreign currency translation	306	(1,646)
Balance at end of period	$ 6,790	$ 10,339

Summary of Significant Accounting Policies - Summary of Financial Impact on Company's Condensed Consolidated Balance Sheet upon Recording of Adoption Entries for ASU 2016-02 (Details) - USD ($)
$ in Thousands

Dec. 31, 2019

Jan. 01, 2019

New Accounting Pronouncements Or Change In Accounting Principle [Line Items]

Operating lease right-of-use-assets

$ 19,415

Operating lease liabilities

5,892

Operating lease liabilities, net of current portion

$ 15,320

ASU 2016-02

New Accounting Pronouncements Or Change In Accounting Principle [Line Items]

Operating lease right-of-use-assets

$ 22,348

Operating lease liabilities

3,746

Operating lease liabilities, net of current portion

18,748

Prior to ASC 842 Adoption

New Accounting Pronouncements Or Change In Accounting Principle [Line Items]

Prepaid rent (1)

^[1]

373

Deferred rent (2)

^[2]

181

Deferred rent, non-current (3)

^[3]

357

ASC 842 Adjustment | ASU 2016-02

New Accounting Pronouncements Or Change In Accounting Principle [Line Items]

Operating lease right-of-use-assets

22,348

Prepaid rent (1)

^[1]

(373)

Deferred rent (2)

^[2]

(181)

Deferred rent, non-current (3)

^[3]

(357)

Operating lease liabilities

3,746

Operating lease liabilities, net of current portion

$ 18,748

[1]	Included in Prepaid expenses and other current assets in 2018 consolidated balance sheet
[2]	Included in Accrued expenses and other current liabilities in 2018 consolidated balance sheet
[3]	Included in Other long-term liabilities in 2018 consolidated balance sheet

Fair Value Measurements and Marketable Securities - Additional Information (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Fair value of financial assets transfers between level 1 to level 2	$ 0
Fair value of financial assets transfers between level 2 to level 1	0
Cash equivalents	$ 13,153	$ 0
Marketable securities		$ 0

Fair Value Measurements and Marketable Securities - Schedule of Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Cash equivalents
Total cash equivalents	$ 13,153	$ 0
Marketable securities
Total marketable securities	305,937
Total	319,090
Level 1
Cash equivalents
Total cash equivalents	202
Marketable securities
Total	202
Level 2
Cash equivalents
Total cash equivalents	12,951
Marketable securities
Total marketable securities	305,937
Total	318,888
Money Market Funds
Cash equivalents
Total cash equivalents	202
Money Market Funds \| Level 1
Cash equivalents
Total cash equivalents	202
U.S. Government Securities
Cash equivalents
Total cash equivalents	3,159
U.S. Government Securities \| Level 2
Cash equivalents
Total cash equivalents	3,159
Corporate Bonds
Marketable securities
Total marketable securities	259,900
Corporate Bonds \| Level 2
Marketable securities
Total marketable securities	259,900
Commercial Paper
Cash equivalents
Total cash equivalents	9,792
Marketable securities
Total marketable securities	46,037
Commercial Paper \| Level 2
Cash equivalents
Total cash equivalents	9,792
Marketable securities
Total marketable securities	$ 46,037

Fair Value Measurements and Marketable Securities - Schedule of Level 2 Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair Value		$ 0
Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 318,622
Gross Unrealized Gains	327
Gross Unrealized Losses	(61)
Fair Value	318,888
Level 2 \| U.S. Government Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	3,159
Fair Value	3,159
Level 2 \| Corporate Bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	259,669
Gross Unrealized Gains	285
Gross Unrealized Losses	(54)
Fair Value	259,900
Level 2 \| Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	55,794
Gross Unrealized Gains	42
Gross Unrealized Losses	(7)
Fair Value	$ 55,829

Fair Value Measurements and Marketable Securities - Schedule of Available-for-Sale Debt Securities by contractual Maturity (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Fair Value Disclosures [Abstract]
Maturities in one year or less	$ 250,490
Maturities between one and three years	68,398
Balance at December 31, 2019	$ 318,888

Revenue Recognition - Additional Information (Details)	12 Months Ended
Revenue Recognition - Additional Information (Details)	Dec. 31, 2019
Revenue From Contract With Customer [Abstract]
General payment terms, maximum	60 days

Property and Equipment - Schedule of Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Property and equipment	$ 10,756	$ 6,972
Less: accumulated depreciation	(3,160)	(1,496)
Property and equipment, net	7,596	5,476
Lab Equipment
Property Plant And Equipment [Line Items]
Property and equipment	6,377	4,689
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment	1,839	1,487
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment	508	403
Office and IT Equipment
Property Plant And Equipment [Line Items]
Property and equipment	184	152
Construction-In-Progress
Property Plant And Equipment [Line Items]
Property and equipment	$ 1,848	$ 241

Property and Equipment - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Depreciation expense	$ 1,675	$ 1,199	$ 302

Accrued Expenses and Other Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Accrued Liabilities And Other Liabilities [Abstract]
Accrued external research and development expenses	$ 16,215	$ 12,738
Accrued payroll and related expenses	12,381	7,372
Accrued professional fees	1,321	1,186
Accrued other	5,069	2,762
Strimvelis liability - current portion	2,994	4,170
Due to UCLA		1,552
Total accrued expenses and other current liabilities	$ 37,980	$ 29,780

Leases - Additional Information (Details) $ in Thousands, € in Millions	1 Months Ended							12 Months Ended
	Jul. 31, 2019 USD ($)	Jan. 31, 2019	Dec. 31, 2018	Mar. 31, 2018 USD ($)	Jan. 31, 2018	Nov. 30, 2017	Oct. 31, 2016	Dec. 31, 2019 USD ($)	Dec. 31, 2019 EUR (€)	Dec. 31, 2018 USD ($)
Operating Leases [Line Items]
Operating lease, term of contract							5 years
Operating leases expiration term							Oct. 31, 2021
Operating lease right-of-use-assets								$ 19,415
Operating lease, liability, noncurrent								15,320
G S K
Operating Leases [Line Items]
Upfront fee payment to contract manufacturing organization for license costs and manufacturing suite \| €									€ 6.8
Office Space Menlo Park Lease Agreement [Member]
Operating Leases [Line Items]
Operating leases expiration term						Dec. 31, 2020
Operating lease duration for free rent						1 month
Laboratory Space Menlo Park Lease Agreement [Member]
Operating Leases [Line Items]
Operating leases expiration term		Dec. 31, 2020
Office and Laboratory Space Menlo Park Lease Agreement [Member]
Operating Leases [Line Items]
Annual rental payment								1,300
Five Years Lease Term Terminates In January 2023 [Member] \| Office Space London United Kingdom Lease Agreement [Member]
Operating Leases [Line Items]
Operating leases expiration term			Jan. 31, 2023		Jan. 31, 2023
Annual rental payment								900
Boston [Member]
Operating Leases [Line Items]
Operating leases expiration term	Sep. 30, 2026			Sep. 30, 2022
Annual rental payment	$ 800			$ 300
Operating leases Commenced term	Jan. 31, 2020
Fremont Lease Agreement [Member]
Operating Leases [Line Items]
Operating leases expiration term										May 31, 2030
Annual rental payment										$ 2,600
Operating lease duration for free rent										8 months
Reduction of standby letter of credit amount percentage										25.00%
Proceeds from lines of credit								3,000		$ 3,000
Operating lease right-of-use-assets								14,700
Operating lease, liability, noncurrent								14,600
Tenant improvements allowance								5,000
Escrow deposit								10,000
Increase decrease in escrow deposit								$ 0
GSK Agreement [Member]
Operating Leases [Line Items]
Operating leases expiration term								Jul. 01, 2022	Jul. 01, 2022
Operating lease right-of-use-assets								$ 2,000
Operating lease, liability, noncurrent								$ 2,000

Leases - Summary of Lease Cost Under Operating Leases (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($)
Leases [Abstract]
Fixed lease cost	$ 5,589
Variable lease cost	1,436
Total lease cost	7,025
Operating cash flows used for operating leases	$ 5,738
Weighted-average remaining lease term (years)	8 years 2 months 12 days
Weighted-average discount rate	9.30%

Leases - Schedule of Future Minimum Commitments Under Operating Leases (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Due in:
2020	$ 6,498
2021	5,383
2022	5,039
2023	3,614
2024	3,715
Thereafter	18,871
Total future minimum lease payments	43,120
Less: Leases not yet commenced	(5,073)
Less: imputed interest	(16,835)
Total operating lease payments	$ 21,212

Leases - Schedule of Future Minimum Commitments Under Leases (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Due in:
2019	$ 3,303
2020	4,910
2021	4,135
2022	3,921
2023	2,844
Thereafter	20,386
Total future minimum lease payments	$ 39,499

Notes Payable - Additional Information (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	May 31, 2019	Dec. 31, 2019
Debt Instrument [Line Items]
Line of credit facility agreement date	May 31, 2019
Credit facility maximum borrowings	$ 75.0
Credit facility, remaining borrowings	$ 50.0
Line of credit facility interest payments term	24 months
Line of credit facility amortization term	36 months
Line of credit facility, frequency of payments		monthly payments
Line of credit facility, percentage of final payment	4.50%
Final payment amount	$ 1.1
Line of credit facility interest rate description		Each term loan under the Credit Facility bears interest at an annual rate equal to 6% plus LIBOR. The Company is required to make interest-only payments on the term loan for all payment dates prior to 24 months following the date of the Credit Facility, unless the third tranche is drawn, in which case the Company is required to make interest-only payments for all payment dates prior to 36 months following the date of the Credit Facility. The term loans under the Credit Facility will begin amortizing on either the 24-month or the 36-month anniversary of the Credit Facility (as applicable), with equal monthly payments of principal plus interest to be made by the Company to the Lenders in consecutive monthly installments until the Loan Maturity Date. In addition, a final payment of 4.5% is due on the Loan Maturity Date.
Line of credit facility covenants description		The Company is also subject to an ongoing minimum cash financial covenant in which the Company must maintain unrestricted cash in an amount not less than $20.0 million following the utilization of the second term loan and not less than $35.0 million following the utilization of the third term loan
Term Loan
Debt Instrument [Line Items]
Debt instrument, interest expense		$ 1.5
Debt instrument, effective annual interest rate		10.50%
LIBOR
Debt Instrument [Line Items]
Debt instrument, annual interest rate	6.00%
Minimum \| Second Tranche (Term Loan Available from September 30, 2019 to December 31, 2020)
Debt Instrument [Line Items]
Line of credit facility minimum cash financial covenant	$ 20.0
Minimum \| Third Tranche (Term Loan Available from July 1, 2020 to September 30, 2021)
Debt Instrument [Line Items]
Line of credit facility minimum cash financial covenant	35.0
Initial Term Loan
Debt Instrument [Line Items]
Proceeds from lines of credit		$ 25.0
Second Tranche (Term Loan Available from September 30, 2019 to December 31, 2020)
Debt Instrument [Line Items]
Credit facility maximum borrowings	25.0	$ 25.0
Line of Credit Facility, Covenant Terms		upon submission of certain regulatory filings and evidence of the Company having $100.0 million in cash and cash equivalent investments
Second Tranche (Term Loan Available from September 30, 2019 to December 31, 2020) \| Minimum
Debt Instrument [Line Items]
Cash and cash equivalent investments	100.0
Third Tranche (Term Loan Available from July 1, 2020 to September 30, 2021)
Debt Instrument [Line Items]
Credit facility maximum borrowings	25.0
Line of Credit Facility, Covenant Terms		upon certain regulatory approvals and evidence of the Company having $125.0 million in cash and cash equivalent investments
Third Tranche (Term Loan Available from July 1, 2020 to September 30, 2021) \| Minimum
Debt Instrument [Line Items]
Cash and cash equivalent investments	$ 125.0

Notes Payable - Summary of Notes Payable (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Long Term Notes And Loans [Abstract]
Notes payable, net of issuance costs	$ 24,541
Accretion related to final payment	158
Notes payable, long term	$ 24,699

Notes Payable - Summary of Estimated Future Principal Payments Due (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Long Term Debt By Maturity [Abstract]
2021	$ 4,861
2022	8,333
2023	8,334
2024	4,597
Total	26,125
Less unamortized portion of final payment	(967)
Less unamortized debt issuance costs	(459)
Notes payable, long term	$ 24,699

Shareholders' Equity and Convertible Preferred Shares - Additional Information (Details)		1 Months Ended	12 Months Ended
	Nov. 02, 2018 USD ($) $ / shares shares	Jun. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) VotingRight	Dec. 31, 2018 USD ($) VotingRight	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) shares	Dec. 31, 2019 GBP (£) £ / shares shares	Dec. 31, 2018 GBP (£) £ / shares shares	Aug. 31, 2018 shares	Dec. 31, 2017 £ / shares shares	Nov. 30, 2017 shares
Class Of Stock [Line Items]
Ordinary shares voting rights description			each holder of ordinary shares is entitled to one vote per ordinary share
Ordinary shares dividends declared			$ 0	$ 0
Number of ordinary voting rights \| VotingRight			1	1
Ordinary Shares, Par Value \| £ / shares							£ 0.10	£ 0.10
Issuance of ADRs in initial public offering, net of issuance costs			$ 130,270,000	$ 209,669,000
Payment of offering expenses			$ 605,000	4,200,000
Common Stock, Conversion Basis			one-for-0.8003
Ordinary Shares, Issued \| shares							96,923,729	85,865,557
Research and development			$ 117,363,000	205,319,000	$ 32,527,000
Ordinary Shares, Outstanding \| shares							96,923,729	85,865,557
License and Development Agreement \| Oxford BioMedica
Class Of Stock [Line Items]
Ordinary Shares, Issued \| shares						588,220
License and Development Agreement \| Oxford BioMedica \| Research and Development Expense
Class Of Stock [Line Items]
Fair value of shares issued				$ 1,400,000	100,000	$ 500,000
License and Development Agreement \| Oxford BioMedica \| Achievement of First Milestone
Class Of Stock [Line Items]
Ordinary Shares, Issued \| shares											150,826
License and Development Agreement \| Oxford BioMedica \| Achievement of Second Milestone
Class Of Stock [Line Items]
Ordinary Shares, Issued \| shares									150,826
License Agreement \| UCL Business PLC
Class Of Stock [Line Items]
Ordinary Shares, Issued \| shares						3,441,290		0		1,224,094
Fair value of shares issued					3,800,000
Research and development					1,700,000	$ 2,100,000
License Agreement with Academic and Health Care Institutions
Class Of Stock [Line Items]
Ordinary Shares, Issued \| shares						800,380				256,096
Research and development					$ 1,400,000	$ 500,000
Redesignation
Class Of Stock [Line Items]
Common Stock, Conversion Basis			one-for-one
Corporate Reorganization
Class Of Stock [Line Items]
Ordinary Shares, Par Value \| £ / shares							£ 0.10			£ 0.00001
American Depositary Shares \| Initial Public Offering
Class Of Stock [Line Items]
Issuance of ordinary shares in initial public offering net of issuance, Shares \| shares	16,103,572
Issue price per share \| $ / shares	$ 14.00
Issuance of ADRs in initial public offering, net of issuance costs	$ 205,500,000
Underwriting discounts and commissions	15,800,000
Payment of offering expenses	4,200,000
American Depositary Shares \| Follow On Public Offering
Class Of Stock [Line Items]
Issuance of ordinary shares in initial public offering net of issuance, Shares \| shares		9,725,268
Issue price per share \| $ / shares		$ 14.25
Issuance of ADRs in initial public offering, net of issuance costs		$ 129,700,000
Underwriting discounts and commissions		8,300,000
Payment of offering expenses		8,300,000
American Depositary Shares \| Underwriter Discounts and Commissions
Class Of Stock [Line Items]
Payment of offering expenses	$ 15,800,000	$ 600,000
Convertible Preferred Shares
Class Of Stock [Line Items]
Preferred stock conversion basis			one-for-0.8003
Maximum
Class Of Stock [Line Items]
Ordinary shares maximum nominal value \| £							£ 13,023,851.50	£ 13,023,851.50

Share-based Compensation - Additional Information (Details) $ / shares in Units, $ in Thousands		12 Months Ended
	Mar. 22, 2019 shares	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($) $ / shares	Dec. 31, 2019 £ / shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Exercise price of option \| £ / shares					£ 0.00001
Award vesting period		4 years
Maximum term of option		10 years
Weighted Average Exercise Price per Share, Granted \| $ / shares		$ 13.57
Total intrinsic value of share options exercised		$ 17,200	$ 0	$ 0
Weighted average grant date fair value of share options granted \| $ / shares		$ 8.67	$ 5.23	$ 2.70
Compensation cost recognized		$ 19,424	$ 6,766	$ 1,019
Unvested options outstanding \| shares		7,688,046
Total unrecognized compensation cost of stock option		$ 48,000
Compensation cost expected to be recognized weighted average period		2 years 7 months 6 days
Total unrecognized compensation cost		$ 8,800
Performance-based Restricted Share Units ("RSUs")
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting period		1 year 3 months 18 days
Aggregate fair value of RSUs		$ 10,100
Compensation cost recognized		$ 1,200	$ 100	$ 0
United States \| Employee
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted Average Exercise Price per Share, Granted \| $ / shares			$ 5.74
2018 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Increase in ordinary shares available for issuance \| shares	4,293,278
Shares available for grant \| shares		4,687,029
2018 ESPP
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Increase in ordinary shares available for issuance \| shares	858,656
Shares available for grant \| shares		1,580,887

Share-based Compensation - Assumptions Used in Black-Scholes Option Pricing Model (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate, minimum	1.39%	2.66%	1.52%
Risk-free interest rate, maximum	2.62%	3.03%	2.30%
Expected term (in years)			6 years 29 days
Expected volatility, minimum	70.10%	64.30%	77.80%
Expected volatility, maximum	72.10%	68.60%	80.00%
Expected dividend rate	0.00%	0.00%	0.00%
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	5 years 6 months	5 years
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	6 years 29 days	6 years 29 days

Share-based Compensation - Summary of Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Options, Outstanding at December 31, 2018	10,203,432
Number of Options, Granted	4,681,282
Number of Options, Exercised	(1,209,335)
Number of Options, Forfeited	(1,459,239)
Number of Options, Outstanding and expected to vest at December 31, 2019	12,216,140	10,203,432
Number of Options, Exercisable as of December 31, 2019	4,528,094
Weighted Average Exercise Price per Share, Outstanding at December 31, 2018	$ 3.04
Weighted Average Exercise Price per Share, Granted	13.57
Weighted Average Exercise Price per Share, Exercised	1.65
Weighted Average Exercise Price per Share, Forfeited	7.36
Weighted Average Exercise Price per Share, Outstanding and expected to vest at December 31,2019	6.61	$ 3.04
Weighted Average Exercise Price per Share, Exercisable as of December 31,2019	$ 3.59
Weighted Average Remaining Contractual Life, Outstanding at December 31, 2018		8 years 11 months 19 days
Weighted Average Remaining Contractual Life, Outstanding and expected to vest at December 31, 2019	8 years 3 months 21 days
Weighted Average Remaining Contractual Life, Exercisable as of December 31, 2019	7 years 8 months 19 days
Aggregate Intrinsic Value, Outstanding	$ 91,133	$ 129,551
Aggregate Intrinsic Value, Exercisable as of December 31, 2019	$ 46,131

Share-based Compensation - Summary of Award Activity (Details) - Performance-based Restricted Share Units ("RSUs")	12 Months Ended
	Dec. 31, 2019 $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares, Unvested at December 31, 2018 \| shares	219,922
Shares, Granted \| shares	428,000
Shares, Forfeited \| shares	(91,500)
Shares, Unvested at December 31, 2019 \| shares	556,422
Weighted Average Fair Value per share, Unvested at December 31, 2018 \| $ / shares	$ 15.48
Weighted Average Fair Value per Share, Granted \| $ / shares	12.29
Weighted Average Fair Value per share, Forfeited \| $ / shares	12.13
Weighted Average Fair Value per Share, Unvested at December 31, 2019 \| $ / shares	$ 13.58

Share-based Compensation - Share-based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation	$ 19,424	$ 6,766	$ 1,019
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation	7,425	2,740	615
Selling General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation	$ 11,999	$ 4,026	$ 404

License and Research Agreements - Additional Information (Details) € in Millions	1 Months Ended							12 Months Ended
	May 31, 2019 USD ($)	May 31, 2019 EUR (€)	Jul. 31, 2018 USD ($)	Jun. 30, 2018 USD ($)	Apr. 30, 2018 USD ($) shares	Apr. 30, 2018 GBP (£) shares	Jan. 31, 2017 USD ($)	Dec. 31, 2019 USD ($) Milestone shares	Dec. 31, 2019 GBP (£) Milestone	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2019 GBP (£) shares	Dec. 31, 2019 EUR (€) shares	Nov. 30, 2018 shares	Sep. 30, 2018 shares	Apr. 30, 2018 GBP (£)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Remaining liability								$ 6,790,000		$ 10,339,000
Ordinary shares issued \| shares								96,923,729		85,865,557			96,923,729	96,923,729
Research and development								$ 117,363,000		$ 205,319,000	$ 32,527,000
GSK Asset Purchase and License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total consideration					$ 133,600,000	£ 94,200,000
Upfront payment					14,200,000	10,000,000
Remaining liability					18,400,000												£ 12,900,000
Inventory purchase liability					6,900,000												4,900,000
Transaction costs					800,000	600,000
In-process research and development expense					$ 133,600,000	£ 94,200,000
Payment of tiered royalty, maximum percentage								20.00%					20.00%	20.00%
Milestone payments payable upon achievement of certain sales milestones								$ 34,800,000					£ 90,000,000	€ 31.0
Upfront payment description								In May 2019, the Company entered into a license agreement with Telethon-OSR, under which Telethon-OSR granted to the Company an exclusive worldwide license for the research, development, manufacture and commercialization of Telethon-OSR’s ex vivo autologous HSC lentiviral based gene therapy for the treatment of mucopolysaccharidosis type I (“MPS-I”), including the Hurler variant. Under the terms of the agreement, Telethon-OSR received €15.0 million in upfront and milestone payments from the Company upon entering into the agreement, resulting in $17.2 million in in-process research and development expense.	In May 2019, the Company entered into a license agreement with Telethon-OSR, under which Telethon-OSR granted to the Company an exclusive worldwide license for the research, development, manufacture and commercialization of Telethon-OSR’s ex vivo autologous HSC lentiviral based gene therapy for the treatment of mucopolysaccharidosis type I (“MPS-I”), including the Hurler variant. Under the terms of the agreement, Telethon-OSR received €15.0 million in upfront and milestone payments from the Company upon entering into the agreement, resulting in $17.2 million in in-process research and development expense.
GSK Asset Purchase and License Agreement \| Convertible Preferred Shares
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Shares issued for asset acquisition \| shares					12,455,252	12,455,252
Aggregate value of shares issued for asset acquisition					$ 93,400,000												£ 65,800,000
Telethon O S R License Agreements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront and milestone payments \| €		€ 15.0
In-process research and development expense for upfront and milestone payments	$ 17,200,000
UCLB/UCLA Technology
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payments												$ 800,000
UCLB Technology and Manufacturing Technology
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payments												$ 1,100,000
UCLB License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Ordinary shares issued \| shares										4,665,384	1,224,094	3,441,290
UCLB/UCLA License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Payments to acquire clinical data											$ 800,000
Annual administration fee								$ 100,000
Payments upon achievement of specified regulatory milestones \| £									£ 19,900,000
Research and development								$ 100,000		$ 100,000	100,000
Oxford BioMedica License, Development and Supply Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Ordinary shares issued \| shares								588,220					588,220	588,220	150,826	150,826
Research and development								$ 0				$ 500,000
Number of milestones met \| Milestone								0	0
Oxford BioMedica License, Development and Supply Agreement \| Achievement Of Second And Third Milestone
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development										1,400,000
Oxford BioMedica License, Development and Supply Agreement \| Achievement Of First Milestone
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development											100,000
UCLA/CIRM Research Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total Reimbursements							$ 10,400,000
Reimbursements received				$ 7,300,000
Total reimbursement under new award			$ 8,500,000
Loan payable term period							10 days
Accrued royalties										0
Research and Development Expense										(3,000,000)	$ (5,000,000)
Accrued Liabilities and Other Liabilities										$ 1,600,000
UCLA/CIRM Research Agreement \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Obligation to reimburse research activities upon achievement of certain milestones			$ 5,500,000

Income Taxes - Schedule of Components of Income (Loss) from Operations Before Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
U.K.	$ (173,118)	$ (230,543)	$ (39,422)
Non-U.K.	7,456	1,018	(269)
Net loss before income tax	$ (165,662)	$ (229,525)	$ (39,691)

Income Taxes - Schedule of (Benefit from) Provision for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current (benefit) provision
Federal—United States	$ 888	$ 607
State—United States	(275)	444	$ 16
Other	89
Total current (benefit) provision	702	1,051	16
Deferred (benefit) provision
Federal—United States	(2,820)	(31)
State—United States	(122)	(50)	37
Total deferred (benefit) provision	(2,942)	(81)	37
Total (benefit) provision for income taxes	$ (2,240)	$ 970	$ 53

Income Taxes - Schedule of Reconciliation of Income Tax (Benefit) Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income taxes at United Kingdom statutory rate	$ (31,475)	$ (43,526)	$ (7,640)
Change in valuation allowance	16,507	43,562	7,827
Reduction in research expense for credits granted	9,787
Change in tax rates	8,109	159	36
Tax credits	(3,372)		(286)
U.S. Deduction for foreign derived intangible income	(2,058)
Permanent differences	344	293	115
U.S. state income taxes	(238)	370	41
Foreign rate differential	156	20	(40)
Other		92
Total (benefit) provision for income taxes	$ (2,240)	$ 970	$ 53

Income Taxes - Schedule of Deferred Tax Assets and Liabilities Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets
Net operating loss carryforwards	$ 45,358	$ 29,436
Amortization	21,741	19,451
Research and development credits	1,244
Share-based compensation	3,604	1,297
Accruals	1,286	184
Lease Liability	4,406
Other	1	1,946
Total deferred tax assets	77,640	52,314
Valuation allowance	(70,153)	(51,281)	$ (11,882)	$ (3,503)
Net deferred tax assets	7,487	1,033
Deferred tax liabilities
Operating lease right of use assets	(3,975)
Depreciation	(528)	(991)
Other non-current liabilities (net deferred tax assets and liabilities)	$ 2,984	$ 42

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Operating Loss Carryforwards [Line Items]
Accrued interest or penalties related to uncertain tax positions	$ 0	$ 0
Earliest Tax Year
Operating Loss Carryforwards [Line Items]
Income tax year open to examination	2016
Latest Tax Year
Operating Loss Carryforwards [Line Items]
Income tax year open to examination	2018
U.S. Federal General Business
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	$ 100,000
Tax credit carryforward expiration year	2039
U.S. State Research and Development
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	$ 1,400,000
United Kingdom
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	$ 266,800,000	$ 155,200,000

Income Taxes - Schedule of Changes in Valuation Allowance for Deferred Tax Assets Related Primarily to Increase in U.K. Net Operating Loss Carryforwards and U.K. Amortization of Intangible Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Valuation allowance as of beginning of year	$ (51,281)	$ (11,882)	$ (3,503)
Decreases recorded as benefit to income tax provision		604
Increases recorded to income tax provision	(16,507)	(44,166)	(7,827)
Effect of foreign currency translation	(2,365)	4,163	(552)
Valuation allowance as of end of year	$ (70,153)	$ (51,281)	$ (11,882)

Net Loss Per Share - Schedule of basic and diluted net loss per share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share [Abstract]
Net loss attributable to ordinary shareholders									$ (163,422)	$ (230,495)	$ (39,744)
Weighted average ordinary shares outstanding, basic and diluted	98,243,915	97,817,847	89,712,916	87,010,596	59,435,325	10,294,498	10,115,335	9,983,754	93,240,355	22,559,389	8,872,768
Net loss per share attributable to ordinary shareholders, basic and diluted	$ (0.46)	$ (0.38)	$ (0.56)	$ (0.35)	$ (0.42)	$ (3.29)	$ (15.45)	$ (1.53)	$ (1.75)	$ (10.22)	$ (4.48)

Net Loss Per Share - Securities Excluded in the Computation of Diluted Net Loss Per Ordinary Share (Details) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common shares attributable to anti-dilutive shares	10,808,360	9,399,169	36,889,966
Share options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common shares attributable to anti-dilutive shares	10,056,864	9,179,247	3,612,288
Unvested Shares from Share Plan and Consulting Agreement
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common shares attributable to anti-dilutive shares	751,496	219,922
Convertible Preferred Shares
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common shares attributable to anti-dilutive shares			33,277,678

Commitments and Contingencies - Additional Information (Details) - USD ($)	7 Months Ended	12 Months Ended
	Jul. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Leased Assets [Line Items]
Prepayments			$ 800,000
Shares, vest		4 years
Total share-based compensation		$ 19,424,000	$ 6,766,000	$ 1,019,000
Consulting Agreement \| Advisor
Operating Leased Assets [Line Items]
Cash payments		$ 100,000
Shares, issued		91,034
Shares, vest		4 years
Total share-based compensation		$ 0
Consulting Agreement \| Advisor \| Clinical Development And Regulatory Milestone
Operating Leased Assets [Line Items]
Shares, issued		92,035
Scenario, Forecast [Member]
Operating Leased Assets [Line Items]
Further payments	$ 9,100,000

Benefit Plans - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Compensation And Retirement Disclosure [Abstract]
Contribution expenses	$ 1.3	$ 0.6	$ 0.2
Maximum annual contribution employer matches per employee, percent	5.00%

Related Party Transactions - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Apr. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Payments to settle accounts payable		$ 6,413	$ (14,848)	$ (1,930)
Payment on inventory purchase liability		3,300	6,200
Research and development expense		117,363	205,319	$ 32,527
G S K
Related Party Transaction [Line Items]
License upfront fee	$ 14,200
Inventory purchased	6,900
Transaction costs	$ 800
G S K \| Transition Services Agreement
Related Party Transaction [Line Items]
Payments to settle accounts payable		7,200
Payment on inventory purchase liability		3,600
Royalty Expense			14,000
Research and development expense			$ 14,000
G S K \| Strimvelis
Related Party Transaction [Line Items]
Royalty Expense		$ 100
G S K \| Convertible Preferred Shares
Related Party Transaction [Line Items]
Ordinary shares Issuable Upon Conversion	12,455,252
Preferred stock, Value	$ 93,400
Loss provision associated with contract	$ 18,400

Selected Quarterly Financial Information (unaudited) - Summary of Unaudited Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 595	$ 1,918			$ 689	$ 1,387			$ 2,513	$ 2,076
Total costs and operating expenses	49,621	43,330	$ 54,152	$ 28,283	29,520	35,477	$ 158,412	$ 13,698	175,386	237,107	$ 38,512
Loss from operations	(49,026)	(41,412)	(54,152)	(28,283)	(28,831)	(34,090)	(158,412)	(13,698)	(172,873)	(235,031)	(38,512)
Net loss attributable to ordinary shareholders	$ (45,416)	$ (36,737)	$ (50,530)	$ (30,739)	$ (25,063)	$ (33,888)	$ (156,233)	$ (15,311)	$ (163,422)	$ (230,495)	$ (39,744)
Weighted average ordinary shares outstanding, basic and diluted	98,243,915	97,817,847	89,712,916	87,010,596	59,435,325	10,294,498	10,115,335	9,983,754	93,240,355	22,559,389	8,872,768
Earnings per share	$ (0.46)	$ (0.38)	$ (0.56)	$ (0.35)	$ (0.42)	$ (3.29)	$ (15.45)	$ (1.53)	$ (1.75)	$ (10.22)	$ (4.48)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Selected Quarterly Financial Information (unaudited) - Summary of Unaudited Quarterly Financial Data (Parenthetical) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effect Of Fourth Quarter Events [Line Items]
Total costs and operating expenses	$ 49,621	$ 43,330	$ 54,152	$ 28,283	$ 29,520	$ 35,477	$ 158,412	$ 13,698	$ 175,386	$ 237,107	$ 38,512
License Of Clinical Stage Development Program \| In Process Research and Development
Effect Of Fourth Quarter Events [Line Items]
Total costs and operating expenses			$ 17,200
GSK Agreement [Member] \| In Process Research and Development
Effect Of Fourth Quarter Events [Line Items]
Total costs and operating expenses							$ 133,600