Document and Entity Information	12 Months Ended
Document and Entity Information	Dec. 31, 2018 shares
Document And Entity Information [Abstract]
Trading Symbol	ORTX
Entity Registrant Name	ORCHARD THERAPEUTICS PLC
Entity Central Index Key	0001748907
Entity Current Reporting Status	Yes
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Well-known Seasoned Issuer	No
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Document Type	20-F
Document Period End Date	Dec. 31, 2018
Amendment Flag	false
Entity Common Stock, Shares Outstanding	85,865,557
Entity Shell Company	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	false

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash	$ 335,844	$ 89,856
Trade and other receivables	2,153	1,247
Prepaid expenses and other current assets	6,935	2,247
Research and development tax credit receivable	10,585	871
Total current assets	355,517	94,221
Non-current assets:
Property and equipment, net	5,476	2,713
Restricted cash	3,837	0
Other long-term assets	1,212	360
Total non-current assets	10,525	3,073
Total assets	366,042	97,294
Current liabilities:
Accounts payable	18,125	3,891
Accrued expenses and other current liabilities	29,780	6,864
Total current liabilities	47,905	10,755
Other long-term liabilities	6,799	134
Total liabilities	54,704	10,889
Commitments and contingencies (Note 12)
Shareholders’ equity:
Ordinary shares, £0.10 par value, authority to allot up to a maximum nominal value of £13,023,851.50 and £675,413 of shares at December 31, 2018 and 2017, respectively;85,865,557 and 8,927,121 shares issued and outstanding at December 31, 2018 and 2017, respectively.	10,924	1,145
Additional paid-in capital	587,490	6,808
Accumulated other comprehensive income	3,163	4,127
Accumulated deficit	(290,239)	(59,744)
Total shareholders’ equity	311,338	86,405
Total liabilities, convertible preferred shares and shareholders’ equity	$ 366,042	97,294
Convertible Preferred Shares
Shareholders’ equity:
Convertible preferred shares, £0.00001 par value; 33,771,174 shares authorized as of December 31, 2017; 33,277,678 shares issued and outstanding as of December 31, 2017; aggregate liquidation preference of $139,954 as of December 31, 2017.		$ 134,069

Consolidated Balance Sheets (Parenthetical)	Dec. 31, 2018 GBP (£) £ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2017 GBP (£) £ / shares shares
Ordinary Shares, Par Value \| £ / shares	£ 0.10		£ 0.10
Ordinary Shares, Authorized \| £	£ 13,023,851.50		£ 675,413
Ordinary Shares, Issued	85,865,557	8,927,121	8,927,121
Ordinary Shares, Outstanding	85,865,557	8,927,121	8,927,121
Convertible Preferred Shares
Convertible Preferred Shares, Par Value \| (per share)		$ 0.00001	£ 0.00001
Convertible Preferred Shares, Authorized		33,771,174	33,771,174
Convertible Preferred Shares, Issued		33,277,678	33,277,678
Convertible Preferred Shares, Outstanding	0	33,277,678	33,277,678
Convertible Preferred Shares, Liquidation Preference Value \| $		$ 139,954

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Product sales, net	$ 2,076
Type of Revenue [Extensible List]	us-gaap:ProductMember	us-gaap:ProductMember	us-gaap:ProductMember
Costs and operating expenses
Cost of product sales	$ 422
Type of Cost, Good or Service [Extensible List]	us-gaap:ProductMember	us-gaap:ProductMember	us-gaap:ProductMember
Research and development	$ 205,319	$ 32,527	$ 16,206
Selling, general and administrative	31,366	5,985	2,997
Total costs and operating expenses	237,107	38,512	19,203
Loss from operations	(235,031)	(38,512)	(19,203)
Other income (expense):
Interest income	1,116		3
Change in fair value of tranche obligations			289
Other income (expense)	4,390	(1,179)	(154)
Total other income (expense), net	5,506	(1,179)	138
Net loss before income tax	(229,525)	(39,691)	(19,065)
Income tax expense	(970)	(53)	(20)
Net loss attributable to ordinary shareholders	(230,495)	(39,744)	(19,085)
Other comprehensive (loss) income
Foreign currency translation adjustment	(964)	4,398	(271)
Total comprehensive loss	$ (231,459)	$ (35,346)	$ (19,356)
Net loss per share attributable to ordinary shareholders, basic and diluted	$ (10.22)	$ (4.48)	$ (2.69)
Weighted average number of ordinary shares outstanding, basic and diluted	22,559,389	8,872,768	7,100,528

Consolidated Statement of Convertible Preferred Shares and Shareholders' Equity (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stockholders Equity, Beginning Balance	$ 86,405	$ (16,524)	$ (572)
Temporary equity, Beginning balance, Shares	33,277,678
Reclassification of convertible preferred shares Value		83,951
Issuance of convertible preferred shares, net of issuance costs, Value	$ 242,744	50,118
Share-based compensation expense	6,766	$ 1,019	$ 204
Exercise of share options, Value	$ 28
Exercise of share options, Shares	14,547	0	0
Ordinary shares committed to be issued as part of license agreements		$ 1,534	$ 465
Ordinary shares issued as part of license agreements, Value	$ 1,385	1,653	2,735
Issuance of ordinary shares in initial public offering net of issuance, Value	205,469
Foreign currency translation adjustment	(964)	4,398	(271)
Net loss	(230,495)	(39,744)	(19,085)
Stockholders Equity, Ending Balance	311,338	$ 86,405	(16,524)
Temporary equity, Ending balance, Shares		33,277,678
Convertible Preferred Shares
Stockholders Equity, Beginning Balance	$ 134,069
Temporary equity, Beginning balance, Shares		11,204,199
Temporary equity, Beginning balance, Value		$ 16,970
Stockholders Equity, Beginning Balance, Shares	33,277,678
Temporary equity, Issuance of convertible preferred shares, net of issuance costs, Value		$ 66,981	$ 16,970
Temporary equity, Issuance of convertible preferred shares, net of issuance costs, Shares		14,693,207	11,204,199
Reclassification of convertible preferred shares Value		$ 83,951
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
Issuance of convertible preferred shares, net of issuance costs, Value	$ 242,744	$ 50,118
Issuance of convertible preferred shares, net of issuance costs, Shares	26,891,222	7,380,272
Effect of corporate reorganization, including conversion of preferred shares to ordinary shares	$ (376,813)
Effect of corporate reorganization, including conversion of preferred shares to ordinary shares	(60,168,900)
Stockholders Equity, Ending Balance		$ 134,069
Temporary equity, Ending balance, Shares			11,204,199
Temporary equity balance, Values			$ 16,970
Stockholders Equity, Ending Balance, Shares		33,277,678
Ordinary Shares
Stockholders Equity, Beginning Balance	$ 1,145	$ 957	$ 343
Stockholders Equity, Beginning Balance, Shares	8,927,121	7,446,931	2,697,151
Conversion of ordinary shares to deferred shares			(80,030)
Exercise of share options, Value	$ 2
Exercise of share options, Shares	14,545
Ordinary shares issued as part of license agreements, Value	$ 83	$ 188	$ 614
Ordinary shares issued as part of license agreements, Shares	651,419	1,480,190	4,829,810
Effect of corporate reorganization, including conversion of preferred shares to ordinary shares	$ 7,647
Effect of corporate reorganization, including conversion of preferred shares to ordinary shares	60,168,900
Issuance of ordinary shares in initial public offering net of issuance, Value	$ 2,047
Issuance of ordinary shares in initial public offering net of issuance, Shares	16,103,572
Stockholders Equity, Ending Balance	$ 10,924	$ 1,145	$ 957
Stockholders Equity, Ending Balance, Shares	85,865,557	8,927,121	7,446,931
Additional Paid-in Capital
Stockholders Equity, Beginning Balance	$ 6,808	$ 2,790
Share-based compensation expense	6,766	1,019	$ 204
Exercise of share options, Value	26
Ordinary shares committed to be issued as part of license agreements		1,534	465
Ordinary shares issued as part of license agreements, Value	1,302	1,465	2,121
Effect of corporate reorganization, including conversion of preferred shares to ordinary shares	369,166
Issuance of ordinary shares in initial public offering net of issuance, Value	203,422
Stockholders Equity, Ending Balance	587,490	6,808	2,790
Accumulated Other Comprehensive Income (Loss)
Stockholders Equity, Beginning Balance	4,127	(271)
Foreign currency translation adjustment	(964)	4,398	(271)
Stockholders Equity, Ending Balance	3,163	4,127	(271)
Accumulated Deficit
Stockholders Equity, Beginning Balance	(59,744)	(20,000)	(915)
Net loss	(230,495)	(39,744)	(19,085)
Stockholders Equity, Ending Balance	$ (290,239)	$ (59,744)	$ (20,000)

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities
Net loss	$ (230,495)	$ (39,744)	$ (19,085)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	1,199	302	6
Share-based compensation	6,766	1,019	204
Amortization of provision on loss contract	(6,300)
Non-cash consideration for licenses and milestones	94,776	3,126	3,089
Change in fair value of tranche obligation liability			(289)
Changes in components of operating assets and liabilities:
Trade and other receivables	(927)	(1,168)
Research and development tax credit receivable, prepaids and other assets	(15,946)	(2,737)	(639)
Accounts payable	14,848	1,930	666
Accrued expenses and other current liabilities	31,663	4,672	1,460
Other long-term liabilities	6,880	113	22
Net cash used in operating activities	(97,536)	(32,487)	(14,566)
Cash flows from investing activities
Purchases of property and equipment	(4,032)	(1,559)	(190)
Net cash used in investing activities	(4,032)	(1,559)	(190)
Cash flows from financing activities
Issuance of convertible preferred shares, net of issuance costs	149,367	115,696	18,034
Issuance of ADRs in initial public offering, net of issuance costs	205,469
Proceeds from share options	28
Net cash provided by financing activities	354,864	115,696	18,034
Effect of exchange rate changes on cash	(3,471)	4,709	(751)
Net increase in cash and restricted cash	249,825	86,359	2,527
Cash and restricted cash —beginning of year	89,856	3,497	970
Cash and restricted cash —end of year	339,681	89,856	3,497
Supplemental disclosure of non-cash investing and financing activities
Conversion of promissory note to convertible preferred shares			946
Issuance of tranche obligations with convertible preferred shares			2,459
Settlement of tranche obligations		1,402	$ 451
Property and equipment included in accrued expenses and accounts payable at period end		$ 1,247
Convertible preferred shares issued for licenses	$ 93,391

Nature of Business and Basis of Presentation

12 Months Ended

Dec. 31, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Nature of Business and Basis of Presentation

1. Nature of Business and Basis of Presentation

Orchard Therapeutics plc (the “Company”), a commercial-stage fully-integrated biopharmaceutical company dedicated to transforming the lives of patients with serious and life- threatening rare diseases through autologous ex vivo gene therapies. The Company’s gene therapy approach seeks to transform a patient’s own, or autologous, hematopoietic stem cells (HSCs) into a gene-modified drug product to treat the patient’s disease through a single administration. The Company has acquired and developed a portfolio of autologous ex vivo gene therapies focused on three franchises in which it accumulates expertise, including primary immune deficiencies, inherited metabolic disorders and hemoglobinopathies. The Company’s programs include Strimvelis, the first autologous ex vivo gene therapy approved by the EMA for ADA-SCID, three clinical programs in advanced registrational studies in metachromatic leukodystrophy (“MLD”), Wiskott–Aldrich syndrome (“WAS”) and adenosine deaminase severe combined immunodeficiency (“ADA-SCID”), other clinical programs in X-linked chronic granulomatous disease (“X-CGD”) and transfusion-dependent beta-thalassemia (“TDBT”), as well as an extensive preclinical pipeline.

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 the 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, our 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.

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. 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 our ordinary shares.

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 restrictions.

The Company is subject to risks and uncertainties common to early-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 biotechnological 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, it is uncertain when, if ever, the Company will realize significant revenue from product sales.

Through December 31, 2018, the Company funded its operations primarily with proceeds from the sale of convertible preferred shares and ADSs in the IPO. The Company has incurred recurring losses since its inception, including net losses of $230.5 million, $39.7 million, and $19.1 for the years ended December 31, 2018, 2017, and 2016, respectively. As of December 31, 2018, the Company had an accumulated deficit of $290.2 million. The Company expects to continue to generate operating losses for the foreseeable future. The 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. The Company expects that its cash on hand as of December 31, 2018 of $335.8 million, will be sufficient to fund its operations and capital expenditure requirements through at least the next twelve months.

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, Orchard Therapeutics (Europe) Limited, Orchard Therapeutics North America, and Orchard Therapeutics (Netherlands) B.V., after elimination of all intercompany accounts and transactions.

Research and development tax credit receivable as of December 31, 2017 previously included in prepaid and other current assets has been presented as a separate line item on the consolidated balance sheet to conform to current period presentation.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2018

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 Stimvelis loss provision, the fair values of ordinary and convertible preferred shares, the fair value of tranche obligations, share-based compensation 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 finance 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 $4.4 million and foreign currency transaction loss of $1.2 million and $0.2 million for the years ended December 31, 2018, 2017 and 2016, 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 to the foreign exchange rates ruling at the dates of the transactions and the balance sheet of these subsidiaries are translated at foreign exchange rates prevailing at the balance sheet date. Exchange differences arising from this translation of foreign operations are reported as an item of other comprehensive loss.

Cash and cash equivalents

The Company considers all highly liquid investments purchased with original maturities of 90 days or less at acquisition to be cash equivalents. In 2018 and 2017, the Company did not have any cash equivalents.

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 has entered into a lease transaction (Note 12) that requires a letter of credit of $3.0 million at December 31, 2018. The Company is also contractually required to maintain a cash collateral account associated with corporate credit card accounts in the amount of $0.9 million at December 31, 2018. The Company had no restricted cash at December 31, 2017. 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.

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, 2018, and 2017, 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 other receivable, accounts payable, accrued expenses and other current liabilities approximate their fair values due to the short-term nature of these assets and liabilities.

Tranche obligations

In 2016, Series A convertible preferred shares (the “Series A convertible preferred shares”) were issued in three tranches. The Company was obligated to issue second and third tranches of Series A convertible preferred shares once certain business milestones were met; these tranches were recognized as tranche obligations, which are subject to revaluation at each balance sheet date. Changes in fair value were recorded as a component of other income (expense) until the settlement of the tranche obligation. The tranche obligations settled in 2017, and no such obligations existed in 2018.

The fair values of the tranche obligations are based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. The tranche obligations are valued as a forward contract, and the values are determined using a probability-weighted present value calculation. In determining the fair values of the tranche obligations, estimates and assumptions impacting fair value included the fair value of the Company’s convertible preferred shares, risk-free interest rates, the probability and estimated timing of the tranche closings, expected dividend yield and expected volatility of the price of the underlying convertible preferred shares. The Company determines the per share fair value of the underlying convertible preferred shares using the option pricing model (“OPM”), which considers the preferred share price paid by investors, the time to liquidity and volatility. In the OPM, the timing of the liquidity event determines the assumed life in the Black-Scholes calculation. The Company estimates a time to liquidity taking into account the future tranche funding. If the future tranche is not expected to be funded, a liquidity event will be assumed to have occurred. If the tranche is expected to be funded, a longer-term liquidity event is assumed to have occurred. Volatility is estimated based on the daily trading histories of comparable public companies. The risk-free interest rate is determined by reference to the United States Treasury yield curve. The Company estimated a 0% dividend yield based on the expected dividend yield and the fact that it has never paid or declared a dividend.

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 fixed assets of $1.7 million and $3.8 million located in the United Kingdom and United States, respectively, as of December 31, 2018, and $0.5 million and $2.2 million located in the United Kingdom and United States, respectively, as of December 31, 2017.

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, and external costs of outside vendors engaged to conduct clinical development activities and clinical trials, as well as to 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 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-related contracts. These agreements are cancelable, and related costs are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies or clinical trials, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates 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 at the fair market value of the Company’s ADS at the date of grant as approved by the board.

Prior to the IPO, given the absence of an active market for the Company’s ordinary shares, the board of directors, the members of which the Company believes have extensive business, finance, and venture capital experience, was required to estimate the fair value of the Company’s ordinary share at the time of each grant of a share-based award. The board of directors determined the estimated fair value of the Company’s equity instruments based on a number of objective and subjective factors, including external market conditions affecting the biotechnology industry sector. The Company and the board of directors utilized various valuation methodologies in accordance with the framework of the American Institute of Certified Public Accountants’ Technical Practice Aid, Valuation of Privately-Held Company Equity Securities Issued as Compensation, to estimate the fair value of its ordinary shares. Each valuation methodology includes estimates and assumptions that require the Company’s judgment. These estimates and assumptions include a number of objective and subjective factors in determining the value of the Company’s ordinary shares at each grant date, including the following factors: (1) prices paid for the Company’s convertible preferred shares, which the Company had sold to outside investors in arm’s-length transactions, and the rights, preferences, and privileges of the Company’s convertible preferred shares and ordinary shares; (2) valuations performed by an independent valuation specialist; (3) the Company’s stage of development; (4) the fact that the grants of share-based awards involved illiquid securities in a private company; and (5) the likelihood of achieving a liquidity event for the ordinary shares underlying the share-based awards, such as an IPO or sale of the Company, given prevailing market conditions.

Ordinary share valuations were prepared using the OPM to estimate the Company’s enterprise value. The OPM treats ordinary and convertible preferred shares as call options on the total equity value of a company, with exercise prices based on the value thresholds at which the allocation among the various holders of a company’s securities changes. Under this method, the ordinary shares have value only if the funds available for distribution to shareholders exceeded the value of the convertible preferred shares liquidation preferences at the time of the liquidity event, such as a strategic sale or a merger. A discount for lack of marketability of the ordinary shares is then applied to arrive at an indication of value for the ordinary shares. The hybrid method is a probability weighted expected return method, PWERM, where the equity value in one or more scenarios is calculated using an OPM. The PWERM is a scenario-based methodology that estimates the fair value of ordinary shares based upon an analysis of future values for the company, assuming various outcomes. The ordinary shares’ value is based on the probability-weighted present value of expected future investment returns considering each of the possible outcomes available as well as the rights of each share class. The future value of the ordinary shares under each outcome is discounted back to the valuation date at an appropriate risk-adjusted discount rate and probability weighted to arrive at an indication of value for the ordinary shares.

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, 2018, 2017 and 2016, other comprehensive loss included a loss of $1.0 million, a gain of $4.4 million, and a loss of $0.3 million and, respectively, related to foreign currency translation adjustments.

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 9). 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 $18.4 million. 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. As of December 31, 2018, the total Strimvelis loss provision liability was $10.3 million. During the year-ended December 31, 2018 the Company amortized $6.3 million as a credit to research and development expense. The effects of foreign currency translation for the year-ended December 31, 2018 reduced the liability by $1.7 million.

Research and development income tax credit

As a company that carries out extensive research and development activities, the Company seeks to benefit from one of two U.K. research and development tax relief programs, the Small and Medium-sized Enterprises R&D Tax Credit Program (“SME Program”) and the Research and Development Expenditure program (“RDEC Program”). 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. Such credits are accounted for as reductions in research and development expense in the period in which the expenditures were incurred.

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 RDEC Program for the years ended December 31, 2018, 2017 and 2016. The Company has qualified under the more favorable SME regime for the year ended December 31, 2018.

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. As such the Company has recorded a United Kingdom research and development tax credit as an offset to research and development expense in the consolidated statements of operations and comprehensive loss of $10.2 million, $0.7 million, and $0.2 million for the years ended December 31, 2018, 2017, and 2016, respectively. As of December 31, 2018, and 2017, the Company’s tax incentive receivable from the United Kingdom government was $10.6 million and $0.9 million, respectively. The effects of foreign currency translation for the year-ended December 31, 2018 reduced the receivable by $0.5 million. These amounts have not yet been paid to the Company by HMRC.

Income taxes

The Company is subject to United Kingdom corporate taxation. Due to the nature of its business, the Company has generated losses since inception and has therefore not paid United Kingdom corporation tax. The Company’s income tax credit recognized represents the sum of the research and development tax credits recoverable in the United Kingdom and income tax payable in the United States.

Unsurrendered United Kingdom losses may be carried forward indefinitely to be offset against future taxable profits, subject to numerous utilization criteria and restrictions. The amount that can be offset each year is limited to £5.0 million plus an incremental 50% of United Kingdom taxable profits.

Value Added Tax (“VAT”), is broadly charged on all taxable supplies of goods and services by VAT-registered businesses, and is generally applicable to our operations in the United Kingdom and European Union. Under current rates, an amount of 20% of the value, as determined for VAT purposes, of the goods or services supplied is added to all sales invoices and is payable to HMRC. Similarly, VAT paid on purchase invoices associated with our U.K. subsidiary is generally reclaimable from HMRC.

The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in the Company’s tax returns. Deferred tax assets and liabilities are determined on the basis of the differences between the consolidated financial statements and tax basis of assets and liabilities using substantively enacted tax rates in effect for the year in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered in the future and, to the extent the Company believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies.

The Company is 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 or United States tax law and requires judgement and estimates.

The Company accounts for uncertainty in income taxes by recognizing in the consolidated financial statements 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.

Net product sales

During the year, the Company made its first sales of Strimvelis, which is currently distributed exclusively at the San Raffaele Hospital in Milan, Italy. Strimvelis sales are currently under a buy-and-bill model where the treatment center purchases and pays for the product and submits a claim to the payer. 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’s net product sales represent total gross product sales of Strimvelis. All sales are recognized when control is transferred, which occurs upon the completion of the scheduled Strimvelis treatment. Transduction costs 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, 2018, 2017, and 2016.

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 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 January 2017, the FASB issued ASU No. 2017-01, Business Combinations (Topic 805) Clarifying the Definition of a Business (“ASU 2017-01”). ASU 2017-01 clarifies the definition of a business by adding guidance to assist entities in evaluating whether transactions should be accounted for as acquisitions of assets or businesses. The definition of a business affects many areas of accounting including acquisitions, disposals, goodwill and consolidation. The ASU is effective for public entities for fiscal years beginning after December 15, 2017. For all other entities, the guidance is effective for annual periods beginning after December 15, 2018, and interim periods within annual periods beginning after December 15, 2019. Early application is permitted for transactions for which the acquisition date occurs before the effective date when the transaction has not been reported in financial statements that have been issued or made available for issuance. As such, the Company adopted this standard effective as of January 1, 2016 and applied the guidance to our analysis of arrangements entered into during the years ended during the year ended December 31, 2016 and subsequent reporting periods.

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 March 2016, the FASB issued ASU No. 2016-09, Compensation—Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting (“ASU 2016-09”). ASU 2016-09 addresses several aspects of the accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, an option to recognize gross share compensation expense with actual forfeitures recognized as they occur, and classification on the statement of cash flows. Certain of these changes are required to be applied retrospectively, while other changes are required to be applied prospectively. ASU 2016-09 is effective for public entities for annual periods beginning after December 15, 2016, and interim periods within those annual periods. For all other entities, the guidance is effective for annual periods beginning after December 15, 2017, and interim periods within annual periods beginning after December 15, 2018. Early adoption is permitted for any entity in any interim or annual period and an entity that elects early adoption must adopt all of the amendments in the same period. The Company early adopted ASU 2016-09 effective as of January 1, 2016. The adoption of ASU 2016-09 did not have a material impact on the Company’s financial position, results of operations or cash flows.

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 November 2015, the FASB issued ASU No. 2015-17, Income Taxes (Topic 740): Balance Sheet Classification of Deferred Taxes (“ASU 2015-17”), which requires deferred tax liabilities and assets to be classified as non-current in the consolidated balance sheet. ASU 2015-17 is effective for public entities for annual periods beginning after December 15, 2016, and interim periods within those annual periods. For all other entities, the guidance is effective for annual periods beginning after December 15, 2017, and interim periods within annual periods beginning after December 15, 2018. Early adoption is permitted and the Company elected to early adopt the standard on January 1, 2016. The adoption of ASU 2015-17 had no material impact on the Company’s financial position, results of operations or cash flows as the company has recorded a full valuation allowance on deferred tax assets for the period ended December 31, 2016 and subsequent reporting periods.

In November 2014, the FASB issued ASU No. 2014-16, Determining Whether the Host Contract in a Hybrid Financial Instrument Issued in the Form of a Share Is More Akin to Debt or to Equity (“ASU 2014-16”). The guidance requires an entity to determine the nature of the host contract by considering all stated and implied substantive terms and features of the hybrid financial instrument, weighing each term and feature on the basis of the relevant facts and circumstances (commonly referred to as the whole-instrument approach). The Company adopted the standard modified retrospectively to all periods presented on the required effective date of January 1, 2016, and its adoption had no impact on the Company’s financial position, results of operations or cash flows.

In August 2014, the FASB issued ASU No. 2014-15, Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern (“ASU 2014-15”). The amendments in this update explicitly require a company’s management to assess an entity’s ability to continue as a going concern and to provide related footnote disclosures in certain circumstances. The new standard is effective for all entities for annual periods ending after December 15, 2016 and for annual and interim periods thereafter. Early adoption is permitted. The Company adopted ASU 2014-15 as of the required effective date of December 31, 2016. This guidance relates to footnote disclosure only, and its adoption had no impact on the Company’s financial position, results of operations or cash flows.

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 July 2017, the FASB issued ASU No. 2017-11, Earnings Per Share (Topic 260), Distinguishing Liabilities from Equity (Topic 480), Derivatives and Hedging (Topic 815) (Part I) Accounting for Certain Financial Instruments with Down Round Features, (Part II) Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception (“ASU 2017-11”). Part I applies to entities that issue financial instruments such as warrants, convertible debt or convertible preferred stock that contain down-round features. Part II replaces the indefinite deferral for certain mandatorily redeemable noncontrolling interests and mandatorily redeemable financial instruments of nonpublic entities contained within ASC Topic 480 with a scope exception and does not impact the accounting for these mandatorily redeemable instruments. ASU 2017-11 is required to be adopted for public entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018. For all other entities, the guidance is effective for fiscal years beginning after December 15, 2019, and interim periods within fiscal years beginning after December 15, 2020. Early adoption is permitted for all entities. The Company does not expect ASU 2017-11 to have a material impact on the Company’s financial position.

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). The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease, respectively. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases today. In January 2018, the FASB issued ASU 2018-01, Leases (Topic 842), (“ASU 2018-01”), which adds two practical expedients to the new lease guidance. Topic 842 is effective for the Company in its annual periods beginning after December 15, 2019. The Company is currently evaluating the impact that the adoption of ASU 2016-02 will have on its consolidated financial statements.

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 of Financial Assets and Liabilities

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Fair value of Financial Assets and Liabilities

3. Fair value of Financial Assets and Liabilities

The Company had no financial assets measured at fair value on a recurring basis at December 31, 2018 or 2017.

The following table presents information about the Company’s financial liabilities that have been measured at fair value on a recurring basis as of December 31, 2016 (there were no financial liabilities measured at fair value on a recurring basis as of December 31, 2018 or 2017):

	Fair Value Measurements as of December 31, 2016 Using:
	Level 1		Level 2		Level 3		Total
	(in thousands)
Liabilities:
Tranche obligations	$	—	$	—	$	1,402	$	1,402
	$	—	$	—	$	1,402	$	1,402

The tranche obligations in the table above represents the Company’s obligation to issue for sale Series A convertible preferred shares once certain business milestones were met. The fair value of the tranche obligations was based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. The tranche obligations are valued as a forward contract as described in Note 2. The Company assessed these assumptions and estimates on a quarterly basis as additional information impacting the assumptions was obtained. The Company recognized changes in fair value of these tranche obligations as a component of other income (expense) in its consolidated statement of operations and comprehensive loss.

Estimates and assumptions impacting the fair value measurement included the fair value of the Company’s convertible preferred shares, risk-free interest rate, the probability and estimated timing of each tranche closing, expected dividend yield and expected volatility of the price of the underlying convertible preferred shares (Note 2). Significant changes to the fair value of the underlying shares would have resulted in a significant change in the fair value measurements.

The tranche obligations were settled when the respective second and third tranches of Series A convertible preferred shares were issued in July 2016 and January 2017.

The following assumptions were used in valuing the tranche obligations:

		Year Ended December 31, 2016
Risk-free interest rate		0.00 - 0.53%
Expected dividend yield		0.00%
Expected term (in years)		0.00 - 0.92
Expected volatility		75.5 - 89.9%
Fair value of convertible preferred shares		$1.00 - $1.58

The following table provides a summary of the changes in fair value of the tranche obligation liability measured at fair value on a recurring basis using significant unobservable inputs during the years ended December 31, 2016 and 2017 (in thousands):

	Tranche Obligations
	(in thousands)
Balance at December 31, 2015	$	—
Issuance of tranche obligations to purchase convertible preferred shares		2,459
Change in fair value of second tranche obligation		(424	)
Settlement of second tranche obligation upon issuance of convertible preferred shares		(451	)
Change in fair value of third tranche obligation		135
Effect of exchange rate changes on tranche obligation		(317	)
Balance at December 31, 2016		1,402
Settlement of third tranche obligation upon issuance of convertible preferred shares		(1,402	)
Balance at December 31, 2017	$	—

- 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, 2018

Revenue From Contract With Customer [Abstract]

Revenue Recognition

4. Revenue Recognition

The Company adopted the new accounting guidance under ASC606 regarding recognition of revenue from customers as of January 1, 2018. Prior to 2018, the Company had no revenue, and the adoption of this guidance resulted in no cumulative adjustment to the Company’s consolidated financial statements.

The Company currently has one commercial-stage therapy, Strimvelis, for the treatment of ADA-SCID. During the year, the Company made its first sales of Strimvelis, which is currently distributed exclusively at the San Raffaele Hospital in Milan, Italy. Strimvelis sales are currently under a buy-and-bill model where the treatment center purchases and pays the Company for the product and submits a claim to the payer.

The Company’s net product sales represent total gross product sales of Strimvelis, less any allowances based on contractual terms or the arrangement with the treatment center. All sales are recognized when control is transferred, which follows the Company’s verification of a scheduled Strimvelis treatment. Transduction costs 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. 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.

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, 2018

Property Plant And Equipment [Abstract]

Property and Equipment

5. Property and Equipment

Property and equipment consist of the following:

	December 31,
	2018			2017
	(in thousands)
Property and equipment:
Lab equipment	$	4,930		$	2,708
Leasehold improvements		1,487			244
Furniture and fixtures		403			59
Office and IT equipment		152			12
Property and equipment	$	6,972			3,023
Less: accumulated depreciation		(1,496	)		(310	)
Property and equipment, net	$	5,476		$	2,713

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

Accrued Expenses and Other Liabilities

12 Months Ended

Dec. 31, 2018

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:

	December 31,
	2018		2017
	(in thousands)
Accrued external research and development expenses	$	12,738	$	1,834
Accrued payroll and related expenses		7,372		2,090
Accrued professional fees		1,186		394
Accrued other		2,762		279
Strimvelis liability - current portion		4,170		—
Deferred UCLA reimbursement		—		2,267
Due to UCLA		1,552		—
Total accrued expenses and other current liabilities	$	29,780	$	6,864

Shareholders' Equity and Convertible Preferred Shares

12 Months Ended

Dec. 31, 2018

Shareholders Equity And Convertible Preferred Shares [Abstract]

Shareholders’ Equity and Convertible Preferred Shares

7. Shareholders’ Equity and Convertible Preferred Shares

Ordinary shares

As of December 31, 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, 2018, the Company has not declared any dividends.

As of December 31, 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.

As of December 31, 2017, the voting, dividend and liquidation rights of the holders of the Company’s ordinary shares are subject to and qualified by the rights, powers and preferences of the holders of the Convertible Preferred Shares. Each ordinary share entitles the holder to one vote, together with the holders of Convertible Preferred Shares, on all matters submitted to the shareholders for a vote. The holders of Convertible Preferred Shares are entitled to elect a total of three directors of the Company. The holders of ordinary shares are entitled to elect the remaining directors of the Company by vote of a majority of such shares. Ordinary shareholders are entitled to receive dividends, as may be declared by the board of directors, if any, subject to Liquidation Preferences. Through December 31, 2017, no cash dividends have been declared or paid.

As of December 31, 2017, the Company had authority to allot ordinary shares up to a maximum nominal value of £675,413, with a nominal value of £0.00001 per share. The authority has taken into consideration the conversion of outstanding Convertible Preferred Shares of 33,277,678 as of December 31, 2017; 500,596 ordinary shares the Company committed to issue as part of its license and research agreements as of December 31, 2017; 4,153,196 for the exercise of outstanding share options, as of December 31, 2017; and 2,354,595 shares remaining available for future issuance under the 2016 Share Option Plan as of December 31, 2017.

Initial Public Offering and Corporate Reorganization

On November 2, 2018, the Company closed its 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, and commissions and offering expenses paid by the Company of $4.2 million.

Immediately prior to the completion of the IPO, all outstanding Convertible Preferred Shares of Orchard Therapeutics plc were converted into their respective class of preferred shares of Orchard Therapeutics plc 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 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 our 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 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 Orchard 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 as of 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 agreements under the plan, and the 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 800,380 and 256,096 ordinary shares in 2016 and 2017, 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, 2018, and 2017, the Company had outstanding 85,865,557 and 8,927,121 ordinary shares, respectively.

Convertible preferred shares

As of December 31, 2018, there were no Convertible Preferred Shares outstanding due to our corporate reorganization and IPO. As of December 31, 2017, the Articles, as further amended and restated (the “Amended Articles”), authorized a total of 33,771,174 convertible preferred shares with a par value of £0.00001 per share, of which 16,806,299 shares have been designated as Series A convertible preferred shares and 16,964,875 shares have been designated as Series B convertible preferred shares (the “Series B convertible preferred shares”).

Until September 2017, the Series A and Series B convertible preferred shares (collectively, the “Convertible Preferred Shares”) were classified in temporary equity as the Convertible Preferred Shares were contingently redeemable. A contingent redemption feature, which is at the option of the Company, could have been exercised by a holder of the Convertible Preferred Shares while that holder controlled a majority of the Company’s board of directors. The Convertible Preferred Shares did not become redeemable as the contingency had not been met or determined to be probable.

In September 2017, the Company’s board of directors was expanded so that the holder of the Convertible Preferred Shares no longer controlled the Company’s board of directors through a majority of seats. Based on this change, the redemption feature from September 2017 onward is exercisable only in an event that is within the control of the Company. At that date, the Convertible Preferred Shares were reclassified to permanent equity within shareholders’ equity on the Company’s consolidated balance sheets. In August 2018, the Company issued Series C convertible preferred shares, which were classified as permanent equity within shareholders’ equity on the Company’s consolidated balance sheets.

Preferred share financings

In February 2016, the Company issued 5,335,333 Series A convertible preferred shares at a price of £1.25 per share (the “Series A Original Issue Price”) of which 4,811,937 Series A convertible preferred shares were issued for net proceeds of $8.5 million and 523,396 Series A convertible preferred shares were issued in settlement of the Notes.

In May 2016, the Company issued and sold 266,767 Series A convertible preferred shares at a price of £1.25 per share for net proceeds of $0.4 million.

In July 2016, the Company issued and sold 5,335,333 Series A convertible preferred shares at a price of £1.25 per share for net proceeds of $8.7 million.

In August 2016, the Company issued and sold 266,766 Series A convertible preferred shares at a price of £1.25 per share for net proceeds of $0.4 million.

In January 2017, the Company issued and sold 5,335,333 Series A convertible preferred shares at a price of £1.25 per share for net proceeds of $8.2 million.

In February 2017, the Company issued and sold 266,766 Series A convertible preferred shares at a price of £1.25 per share for net proceeds of $0.4 million.

In March 2017, the Company issued and sold 5,805,376 Series B convertible preferred shares at a price of £5.022 per share (the “Series B Original Issue Price”) for net proceeds of $36.0 million.

In August 2017, the Company issued and sold 3,285,731 Series B convertible preferred shares at a price of £5.022 per share for net proceeds of $21.0 million.

In October 2017, the Company issued and sold 4,655,985 Series B convertible preferred shares at a price of £5.022 per share for net proceeds of $30.8 million.

In December 2017, the Company issued and sold 2,724,288 Series B convertible preferred shares at a price of £5.022 per share for net proceeds of $18.3 million.

In December 2017, the Company received proceeds of $1.0 million for 150,706 Series B convertible preferred shares, which were subsequently issued in January 2018.

In August 2018, the Company issued and sold 13,942,474 Series C convertible preferred shares at a price of $10.76 per share for net proceeds of $147.1 million.

As of December 31, 2018, there were no Convertible Preferred Shares outstanding due to our corporate reorganization and IPO. As of December 31, 2017, Convertible Preferred Shares consisted of the following:

	December 31, 2017 (in thousands, except share amounts)
	Shares Authorized		Shares Issued and Outstanding		Carrying Value		Liquidation Preference(a)		Ordinary shares Issuable Upon Conversion
Series A convertible preferred shares		16,806,298		16,806,298	$	26,994	$	28,337		16,806,298
Series B convertible preferred shares		16,964,876		16,471,380		107,075		111,617		16,471,380
		33,771,174		33,277,678	$	134,069	$	139,954		33,277,678

(a)	Amounts were translated into United States dollars using the spot rate as of December 31, 2017.

There were no Convertible Preferred Shares outstanding as of December 31, 2018. The holders of the Convertible Preferred Shares have the following rights and preferences as of December 31, 2017:

Voting

Each Series A and Series B preferred share shall confer one right to vote at all general meetings and to receive and vote on proposed written resolutions of the Company.

Conversion

Each Series A and Series B preferred share was convertible, at the option of the holder, at any time and from time to time, and without the payment of additional consideration, into an ordinary share as is determined by dividing the applicable Series A or Series B Original Issue Price by the respective Series A or Series B Conversion Price.

The Series A Conversion Prices were equal to each applicable Series A Original Issue Price as noted above. The Series B Conversion Prices were equal to each applicable Series B Original Issue Price as noted above. As of December 31, 2017, each Preferred Share was convertible into one ordinary share.

As set forth in the Amended Articles, the Series A and B Conversion Prices were adjusted when there is a deemed issuance of additional convertible preferred shares issued at a price lower than Series A and Series B Original Issue Prices or issuance of an instrument with rights that could dilute the interest of Series A and B holders. In addition, each Preferred Share would be automatically converted into an ordinary share at the applicable conversion ratio then in effect for each series of Convertible Preferred Shares upon the earlier of (i) the closing of a firm commitment underwritten public offering of its ordinary shares with gross proceeds to the Company of at least $50.0 million and at a price per share of not less than £6.0262, subject to appropriate adjustment in the event of any share split, share dividend, combination or other similar recapitalization, or (ii) a date specified vote or written consent of the holders of a majority of Convertible Preferred Shares, voting together as a single class on an as-if-converted to ordinary shares basis.

Dividends

The holders of the Series A convertible preferred shares, Series B convertible preferred shares, and ordinary shares were entitled to receive non-cumulative dividends, if and when declared by the Company’s board of directors, subject to shareholder consent. The Series A convertible preferred shares, Series B convertible preferred shares and ordinary shares ranked equally in all respects (on an as converted basis) for the purpose of any dividend that is declared or paid. On a distribution of assets on a liquidation, share sale, asset sale or IPO, the holders of Series A convertible preferred shares, and Series B convertible preferred shares were entitled to receive any declared but unpaid dividend, in the order of the priority set out in Liquidation Preference above, on each outstanding Series A convertible preferred share and Series B convertible preferred share. No dividends were declared or paid during the year ended December 31, 2017 and 2018.

Liquidation preference

In the event of a distribution of assets on liquidation or a return of capital (other than a conversion, redemption or purchase of shares), the surplus remaining after settling the Company’s assets and liabilities will be distributed to the individuals holding ordinary shares, Series A and Series B convertible preferred shares on a pro rata basis (as if the ordinary shares and the Convertible Preferred Shares constituted one class) as described in the Amended Articles, except if the per share amount for Series A and Series B convertible preferred shares results in a price per share less than its original issue price. If the price per share is less than the original issue price for preferred shareholders, the shareholders will be paid an amount equal to the subscription price and the remainder of the assets will be distributed on a pro rata basis to the remaining ordinary shareholders.

Redemption

The Amended Articles do not provide redemption rights to the holders of Convertible Preferred Shares.

Deferred shares

Deferred shares are a unit of equity in the Company. All deferred shares can be repurchased at any time by the Company at a purchase price of £0.00001 per share. Deferred shares have no rights attached to them, are not convertible to any other class of shares and are not redeemable. The entire class of deferred shares is entitled to a total of £1.25 from the distribution of assets on a liquidation or return of capital event.

In 2016, the Company converted 80,030 ordinary shares of an investor to deferred shares. In March 2017, the Company repurchased 80,030 deferred shares at £0.00001 per share and simultaneously cancelled them.

There were no deferred shares outstanding as of December 31, 2017 and 2018.

Share-based Compensation

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Share-based Compensation

8. Share-based Compensation

In September 2016, the Company adopted the Orchard Therapeutics Limited Employee Share Option Plan with Non-Employee Sub-Plan and U.S. Sub-Plan (the “2016 Plan”). The 2016 Plan provided for the Company to grant incentive and non-qualified options to officers, directors, consultants, and advisors to purchase the Company’s ordinary shares prior to the IPO. The board of directors has determined not to make any further awards under the 2016 plan following the Company’s IPO.

In October 2018, as part of the Company’s reorganization and IPO, the Company adopted the Orchard Therapeutics plc 2018 Share Option and Incentive Plan (the “2018 Plan”). The 2018 Plan provides for grants in the form of incentive and non-qualified options, share appreciation rights, restricted shares, and restricted share units. The Company issues new ordinary shares upon exercise of share options. The Company has initially reserved 4,254,741 ordinary shares, or the initial limit, for the issuance of awards under the 2018 Plan. As of December 31, 2018, 3,953,726 shares remained available for future grant under the plan. The number of ordinary shares reserved for issuance will automatically increase each January 1, beginning January 1, 2019, by 5% of the outstanding number of ordinary shares on the immediately preceding December 31, or such lesser number of shares as determined by the board of directors.

In October 2018, the Company adopted the 2018 Employee Share Purchase Plan (the “ESPP”) under which eligible employees may contribute up to 15% of their base compensation toward bi-annual purchases of the Company’s ordinary shares. The ESPP initially reserved and authorized the issuance of up to a total of 850,948 ordinary shares to participating employees. The number of ordinary shares reserved for issuance will automatically increase by the least of (i) 1% of the outstanding number of ordinary shares on the immediately preceding December 31; (ii) 1,500,000 shares or (iii) such number of shares as determined by the ESPP administrator. The purchase price for each ordinary share is the lesser of 85% of the market price on the first business day or last business day of the offering period. Share-based compensation expense related to this plan was $0.1 million for the year ended December 31, 2018.

Prior to the Company’s IPO, the Company typically 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 grant options to United Kingdom employees at an exercise price equal to the par value of the ordinary shares of £0.00001. After the IPO, options are typically granted at exercise prices equal to the fair value of the Company’s ordinary shares on the grant date. The vesting period is determined by the board of directors, which is generally four years. An option’s maximum term is ten years.

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, 2018, 2017, and 2016 were as follows:

	Year Ended December 31,
	2018	2017	2016
Risk-free interest rate% - %	2.66% - 3.03%	1.52% - 2.30%	1.52% - 2.40%
Expected term (in years)	5.00 - 6.08	6.08	6.08 - 9.75
Expected volatility% - %	64.27 - 68.58%	77.80% - 80.00%	77.80% -79.70%
Expected dividend rate%	0.00%	0.00%	0.00%

Expected Term: The expected term for employees represents the period that the options granted are expected to be outstanding and is determined using the simplified method (based on the mid-point between the vesting date and the end of the contractual term). The expected term is applied to the share option grant group as a whole, as the Company does not expect substantially different exercise or post-vesting termination behavior among its employee population. Prior to the adoption of ASU 2018-07, expected term for non-employee grants was the contractual term of the options. After the adoption of ASU 2018-07, the expected term of share options granted to non-employees is determined in the same manner as share options granted to employees.

Expected Volatility: The Company used an average historical stock price volatility of comparable public companies within the biotechnology and pharmaceutical industry that were deemed to be representative of future share price trends as the Company does not have significant trading history for its ordinary shares.

Risk-Free Interest Rate: The Company based the risk-free interest rate over the expected term of the options on the constant maturity rate of United States Treasury securities with similar maturities as of the date of the grant.

Expected Dividend Rate: The Company has not paid and does not anticipate paying any dividends in the near future. Therefore, the expected dividend yield was zero.

Fair value of underlying ordinary shares: Prior to the IPO, the Company determined the fair value of the underlying ordinary shares based on input from management and approved by the board of directors, as described in Note 2. Subsequent to the IPO, the Company determined the fair value of the underlying ordinary shares based on the close price of our ordinary shares on the grant date.

Options

The following table summarizes option activity under the plans for the year ended December 31, 2018:

	Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Life		Aggregate Intrinsic Value
	(in thousands, except share and per share amounts)
Options outstanding at December 31, 2017		4,153,196		$	1.20		9.28	$	10,483
Granted		6,303,465			4.23
Exercised		(14,547	)		1.96
Cancelled		(238,682	)		2.55
Options outstanding at December 31, 2018		10,203,432			3.04		8.97		129,551
Vested as of December 31, 2018		2,022,399			1.11		8.20		29,568

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.

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 2018, the total intrinsic value of share options exercised was not material. There were no share option exercises in 2017 or 2016.

The weighted average grant date fair value of the options granted during the years ended December 31, 2018, 2017, and 2016 was $5.23 per share, $2.70 per share and $0.92 per share, respectively.

Restricted Share Units

In November 2018, the Company issued performance-based restricted share units (“RSUs”) to our Chief Executive Officer covering a maximum of 219,922 ordinary shares. The performance-based RSUs will vest, if at all, based upon the Company achieving three specific regulatory and research and development milestones, or 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 performance-based RSUs is $4.5 million. The fair value associated with the shares that could vest based on the market-based condition is being recognized as expense over the 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, 2018, none of the regulatory and research development milestones were deemed probable.

The following table summarizes RSU award activity for the year ended December 31, 2018:

	Shares		Weighted Average Fair Value
Unvested at December 31, 2017		—	$	—
Granted		219,922		15.48
Vested		—		—
Forfeited		—		—
Unvested at December 31, 2018		219,922		15.48

The amount of compensation cost recognized for the years ended December 31, 2018 and 2017 for the market condition associated with the performance-based RSUs was $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,
	2018		2017		2016
	(in thousands)
Research and development	$	2,740	$	615	$	181
Selling, general and administrative		4,026		404		23
Total	$	6,766	$	1,019	$	204

The Company had 8,181,033 unvested options outstanding as of December 31, 2018. As of December 31, 2018, total unrecognized compensation cost related to unvested stock option grants was approximately $33.3 million. This amount is expected to be recognized over a weighted average period of approximately 2.96 years. As of December 31, 2018, the total unrecognized compensation cost related to performance-based RSUs is a maximum of $4.5 million, depending upon achievement of the milestones.

License and Research Arrangements

12 Months Ended

Dec. 31, 2018

License And Research Arrangements [Abstract]

License and Research Arrangements

9. License and Research Arrangements

GSK asset purchase and license agreement

In April 2018, the Company entered into 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”). This complements and enhances the Company’s current portfolio.

The portfolio of programs and options acquired consists of:

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

•	One earlier stage clinical gene therapy program for TDBT;

•	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 such option rights have lapsed as of the date of this Annual Report.

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 as of date of acquisition), which includes an upfront payment of £10.0 million ($14.2 million at the acquisition date) and 12,455,252 Series B-2 convertible preferred shares of the Company issued to GSK at £65.8 million ($93.4 million at the acquisition date), 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 Series B-2 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 PRV from the United States Food and Drug Administration for each of the programs for MLD, WAS and TDBT, the first of which GSK retained beneficial ownership. 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 TDBT. 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, 2018, 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 our 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 (Note 2). During the period ended December 31, 2018, the Company amortized $6.3 million as a credit to research and development expenses associated with the loss provision. The effects of foreign currency translation for the year-ended December 31, 2018 reduced the liability by $1.7 million. The balance of the liability as of December 31, 2018 was $10.3 million. The consideration transferred in the asset acquisition was measured at cost, including transaction costs, assets and equity interests transferred by the acquirer, and liabilities incurred by the acquirer as noted below:

	Consideration
	(in thousands)
Upfront cash paid for GSK Agreement	$	14,186
Series B-2 convertible preferred shares issued to GSK		93,391
Transaction costs		780
Liabilities:
Strimvelis liability		18,351
Inventory purchase liability		6,893
Total consideration transferred:	$	133,601

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 percentage from the high single-digits to low double-digit for the TDBT product, upon marketing approval, calculated as percentages of aggregate annual net sales of the TDBT 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, both effective April 11, 2018. The TSA outlines several activities that the Company has 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/Ospedale San Raffaele and an ongoing manufacturing agreement.

Telethon-OSR research and development collaboration and license agreement

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, TDBT, 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 and options granted, the Company will be required to make payments to Telethon-OSR upon achievement of certain product development milestones and pay Telethon-OSR a fee in connection with the exercise of an option for each collaboration program. 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 and aggregate of approximately €31.0 million in milestone payments upon achievement of certain product development milestones and exercises of options under the Telethon-OSR agreements.

UCLB/UCLA License Agreement

In February 2016, and amended in July 2017, the Company entered into 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.

The Company recorded $0.2 million, $1.8 million, and $4.6 million of research and development costs in respect of UCLB, which comprise the upfront payments, issuance of ordinary shares and payments for clinical data, for the years ended December 31, 2018, 2017, and 2016, respectively.

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 is obligated to make payments to the parties of up to an aggregate of $38.9 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.

In connection with the UCLB/UCLA License Agreement, in February 2016 the Company sold an aggregate of 800,298 Series A convertible preferred shares at a price of £1.25 per share (Note 14).

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 for 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 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. If future milestones are met, the Company may become obligated to issue more ordinary shares.

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.

The Company may also pay low single-digit percentage royalties on 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 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 we may be obligated to reimburse UCLA for up to $5.5 million for research activities upon achievement of certain milestones. 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 royalties based on a low single digit royalty percentage 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, payable within 10 days of election. No such election has been made as of the date of this Annual Report. 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 year ended December 31, 2018 and 2017, the Company recorded $3.0 million and $5.0 million as a reduction of research and development expenses related to the UCLA Research Agreement. As of December 31, 2018, the Company recorded $1.6 million in accrued expenses for amounts which it is obligated to reimburse to UCLA under the July 2018 grant. As of December 31, 2017, the Company recorded $2.3 million within accrued expense and other liabilities on the Company’s consolidated balance sheet related to the advance of reimbursements for research activities.

Other license and research agreements

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. As part of the consideration related to these license agreements, the total share commitment was 1,030,786 and 375,380 ordinary shares, respectively. The Company made cash payments of nil, $2.7 million and $0.4 million 2018, 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 nil, $1.4 million and $0.5 million in 2018, 2017 and 2016, respectively. In addition, the Company also committed to make certain clinical and regulatory milestone payments in the aggregate of $31.8 million as well as single-digit percentage royalties on net sales of products and services associated with the in-licensed technology.

Income Taxes

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Income Taxes

10. Income Taxes

The components of loss from operations before income taxes for the years ended December 31, 2018, 2017, and 2016 are as follows:

	December 31,
	2018			2017			2016
	(in thousands)
U.K.		(230,543	)		(39,422	)		(19,105	)
Non-U.K.		1,018			(269	)		40
Loss before taxes	$	(229,525	)	$	(39,691	)	$	(19,065	)

The provision for income taxes for the years ended December 31, 2018, 2017, and 2016 was computed at the United Kingdom statutory income tax rate. The income tax provision for the years then ended comprised:

	December 31,
	2018			2017		2016
	(in thousands)
Current provision expense
Federal—United States	$	607		$	—	$	—
State—United States		444			16		17
United Kingdom		—			—		—
Total current provision expense		1,051			16		17
Deferred provision expense
Federal—United States		(31	)		—		—
State—United States		(50	)		37		3
United Kingdom		—			—		—
Total deferred provision expense		(81	)		37		3
Total provision for income taxes	$	970		$	53	$	20

A reconciliation of income tax expense computed at the United Kingdom statutory income tax rate to income taxes as reflected in the consolidated financial statements is as follows:

	December 31,
	2018			2017			2016
	(in thousands)
Income taxes at United Kingdom statutory rate	$	(43,526	)	$	(7,640	)	$	(3,831	)
State income taxes		370			41			14
Permanent differences		293			115			75
Tax credits		—			(286	)		(99	)
Foreign rate differential		20			(40	)		6
Change in valuation allowance		43,562			7,827			3,855
Impact of United States tax reform		159			36			—
Other		92			—			—
Total provision expense for income taxes	$	970		$	53		$	20

Significant components of the Company’s deferred tax assets and liabilities as of December 31, 2018 and 2017 consist of the following:

	December 31,
	2018			2017
	(in thousands)
Deferred tax assets
Net operating loss carryforwards	$	29,436		$	9,483
Research and development credits		—			356
Share-based compensation		1,297			147
Amortization		19,451			2,156
Accruals		184			28
Other		1,946			—
Total deferred tax assets	$	52,314		$	12,170
Valuation allowance		(51,281	)		(11,882	)
Net deferred tax assets	$	1,033		$	288
Deferred tax liabilities
Depreciation	$	(991	)	$	(328	)
Other non-current liabilities (net deferred tax assets and liabilities)	$	42		$	(40	)

As of December 31, 2018, the Company had approximately $155.2 million of United Kingdom net operating loss carryforwards.

As of December 31, 2017, the Company has approximately $48.4 million of United Kingdom net operating loss carryforwards with an indefinite life (but may be subject to certain utilization restrictions).

The Company has evaluated the positive and negative evidence bearing upon its ability to realize its deferred tax assets, which primarily comprise net operating loss carryforwards and research and development credits. Management has considered the Company’s history of cumulative net losses in the United Kingdom, estimated future taxable income and prudent and feasible tax planning strategies and has concluded that it is more likely than not that the Company will not realize the benefits of its United Kingdom deferred tax assets. Accordingly, a full valuation allowance has been established against these net deferred tax assets as of December 31, 2018 and 2017, respectively. The Company reevaluates the positive and negative evidence at each reporting period.

Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2018, 2017, and 2016 related primarily to the increase in net operating loss carryforwards and were as follows:

	December 31,
	2018			2017			2016
	(in thousands)
Valuation allowance as of beginning of year	$	(11,882	)	$	(3,503	)		—
Decreases recorded as benefit to income tax provision		604			—			—
Increases recorded to income tax provision		(44,166	)		(7,827	)		(3,855	)
Effect of foreign currency translation		4,163			(552	)		352
Valuation allowance as of end of year	$	(51,281	)	$	(11,882	)	$	(3,503	)

The Company files tax returns as prescribed by the tax laws of the jurisdictions in which it operates. In the normal course of business, the Company is subject to examination by federal and state jurisdictions, where applicable. There are currently no pending tax examinations. The Company's tax years are still open under statute from December 31, 2015, to the present. The resolution of tax matters is not expected to have a material effect on the Company's consolidated financial statements.

Net Loss Per Share

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

Net Loss Per Share

11. Net Loss Per Share

The following table sets forth the computation of basic and diluted net loss per share:

	Year Ended December 31
	2018			2017			2016
	(In thousands, except per share and share amounts)
Net loss	$	(230,495	)	$	(39,744	)	$	(19,085	)
Net loss attributable to ordinary shareholders	$	(230,495	)	$	(39,744	)	$	(19,085	)
Weighted average ordinary shares outstanding, basic and diluted		22,559,389			8,872,768			7,100,528
Net loss per share attributable to ordinary shareholders, basic and diluted	$	(10.22	)	$	(4.48	)	$	(2.69	)

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,
	2018		2017		2016
Convertible preferred shares		—		33,277,678		11,204,199
Share options		9,179,247		3,612,288		1,809,442
Unvested performance-based restricted share units		219,922		—		—
		9,399,169		36,889,966		13,013,641

Commitments and Contingencies

12 Months Ended

Dec. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

12. Commitments and Contingencies

Operating lease agreements

In October 2016, the Company entered into a lease agreement for laboratory space in Foster City, California, United States. The lease has a term of 5 years and expires in October 2021. The annual rental expense approximates $0.2 million. The Company was provided with one month of free rent.

In November 2017, the Company entered into a lease agreement for laboratory space in Menlo Park, California, United States. The lease expires in November 2020. The annual rental expense approximates $0.8 million. The Company was provided with one month of free rent.

In January 2018, the Company entered into a lease agreement for additional office space in London, United Kingdom. The lease has a term of five years and terminates in January 2023. The annual rental expense approximates $0.8 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 expense approximates $0.3 million.

In December 2018, the Company leased additional office space in London, United Kingdom. The lease commenced on December 7, 2018 and terminates on January 7, 2023. The annual rental expense approximates $0.1 million.

Fremont lease agreement

In December 2018, the Company leased manufacturing and office space in Fremont, California, which terminates in May 2030. The annual rent expense approximates $2.4 million. 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.

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 and as such the Company will be deemed for accounting purposes only to be the owner of the construction project, even though it is not the legal owner. As of December 31, 2018, no construction has begun related to the facility. The lease provides for approximately $5.0 million in tenant improvement allowances to be reimbursed to the Company by the landlord, which will be amortized into rental expense over the term of the lease.

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, 2018, no construction has begun, and the Company has no funds deposited in the escrow account.

Future minimum lease payments

The following table summarizes the future minimum lease payments due under all operating leases as of December 31, 2018:

Due in:	(in thousands)
2019	$	3,303
2020		4,910
2021		4,135
2022		3,921
2023		2,844
Thereafter		20,386
Total	$	39,499

The Company recognizes rent expense on a straight-line basis over the respective lease period and has recorded deferred rent for rent expense incurred but not yet paid.

The Company recorded rent expense of $2.4 million, $0.7 million and $0.2 million for the years ended December 31, 2018, 2017 and 2016, respectively.

Other funding commitments

The Company has entered into several license agreements (Note 9). In connection with these agreements the Company is required to make milestone payments and annual license maintenance payments not met at December 31, 2018 and 2017 or royalties on future sales of specified products. The Company determined that no milestone payments that have not already been accrued were probable as of December 31, 2018.

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, 2018, and December 31, 2017, there was $0.8 million and $nil included within prepayments relates to prepaid instalments against these minimum commitments. The Company is committed to make further payments totaling $10.1 million between January 2019 and March 2021.

Legal proceedings

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

Related-Party Transactions

12 Months Ended

Dec. 31, 2018

Related Party Transactions

14. Related-party Transactions

UCLB

Subsequent to our Series C preferred share financing in August 2018, UCLB is no longer a principal shareholder, and is no longer considered an affiliated entity of the Company as of December 31, 2018.

UCL Technology Fund LP (“UCLTF”) is affiliated with UCLB. On February 6, 2016, UCLB through its associate UCLTF, entered into a Subscription and Shareholders’ Agreement with the Company to purchase an aggregate of 800,298 Series A shares (Note 9). At the same time, UCLB also entered into the UCLB/UCLA License Agreement (Note 9), through which the Company was granted licenses to certain intellectual property rights controlled by UCLB and UCLA to develop and commercialize gene therapy products in certain fields and territories. In 2016, the Company also agreed to sponsor a short-term research program with UCLB with total program costs of $0.5 million. In 2018, 2017, and 2016 the Company incurred $0.2 million, $0.2 million, and $0.4 million of consulting fees, with an affiliate of UCLB, respectively.

GSK

In April 2018, the Company entered into 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-OSR (See Note 9). As consideration for the license the Company paid an upfront fee of $14.1 million, incurred an inventory purchase liability of $6.9 million, and issued 12,455,252 Series B 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 contract, 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.

The Company and GSK have also separately executed a Transition Services Agreement (“TSA”) as well as an Inventory Sale Agreement, both effective April 11, 2018. The TSA outlined several activities that the Company 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 is expired in December 2018. 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, 2018, the company had $6.0 million in accrued expenses and accounts payable associated with the GSK TSA.

Convertible preferred shares

In February 2016, entities affiliated with F-Prime Capital purchased 16,006,000 Series A convertible preferred shares.

In December 2017, entities affiliated with F-Prime Capital and Scottish Investment Trust purchased 2,400,900 and 3,201,200 Series B convertible preferred shares.

In December 2017, the Company sold to its Chief Executive Officer, Chief Medical Officer and Senior Vice President of Business Development and Alliance Management 39,825, 9,955 and 3,982 Series B convertible preferred shares.

In August 2018, entities affiliated with Deerfield Management Company and Scottish Mortgage Investment Trust purchased 4,647,500 and 697,125 Series C convertible preferred shares at a price of

In August 2018, the Company sold to its Chief Executive Officer, Chief Financial Officer, and various members of its board of directors 24,979, 9,294, and 90,158 Series C convertible preferred shares.

All convertible preferred shares were converted to ordinary shares as part of the Company’s IPO.

Initial public offering

In November 2018, entities affiliated with Deerfield Management Company, RA Capital Management LLC, Temasek Holdings (Private) Limited, and Scottish Mortgage Investment Trust purchased 3,376,100, 2,057,432, 1,000,000 and 925,000 ADSs, respectively, in the IPO. Subsequent to the IPO and as of December 31, 2018, each of these entities holds more than 5% of the Company’s share capital.

In November 2018, our Chief Executive Officer, Chief Financial Officer, and members of the board of directors purchased 18,500, 5,000, and 13,000 ADRs, respectively, in the IPO.

Subsequent Events

12 Months Ended

Dec. 31, 2018

Subsequent Events [Abstract]

Subsequent Events

15. Subsequent Events

Grants of share options and performance-based restricted share units under the 2018 Plan

On January 2, 2019, the Company granted options to employees for the purchase of an aggregate of 117,280 ordinary shares, at a weighted average exercise price of $14.98 per share. The aggregate grant-date fair value of these options was $1.1 million, which will be recognized as share-based compensation expense over the vesting period of four years.

On January 16, 2019, the Company granted options to senior management and employees for the purchase of an aggregate of 2,470,423 ordinary shares, at a weighted average exercise price of $12.54 per share. The aggregate grant-date fair value of these options was $19.8 million, which will be recognized as share-based compensation expense over the vesting period of approximately four years. The Company also granted performance-based RSUs to certain of its executives covering a maximum of 219,500 ordinary shares. These performance-based RSUs will vest, if at all, based upon attainment of certain regulatory and market-based milestones, but must vest by December 31, 2021 or else be forfeited. The maximum aggregate total fair value of these RSUs that could be recognized over this period is $3.3 million.

On February 1, 2019, the Company granted options to employees for the purchase of an aggregate of 95,800 ordinary shares, at a weighted averaged exercise price of $12.86 per share. The aggregate grant-date fair value of these options was $0.8 million, which will be recognized as share-based compensation expense over the vesting period of four years.

On March 1, 2019, the Company granted options to employees for the purchase of an aggregate of 24,700 ordinary shares, at a weighted averaged exercise price of $16.89 per share. The aggregate grant-date fair value of these options was $0.3 million, which will be recognized as share-based compensation expense over the vesting period of four years.

On March 13, 2019, the Company granted performance-based RSUs to certain members of its senior management covering 108,000 ordinary shares. These performance-based RSUs will vest, if at all, based upon attainment of certain regulatory and market-based milestones, but must vest by December 31, 2021 or else be forfeited. The maximum aggregate total fair value of these RSUs that could be recognized over this period is estimated to be $1.9 million.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Use of estimates

Concentration of credit risk

Foreign currency translation

Cash and cash equivalents

Restricted cash

Restricted Cash

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.

•

•	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.

Tranche obligations

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 at the fair market value of the Company’s ADS at the date of grant as approved by the board.

Valuation of RSUs

Comprehensive loss

Strimvelis loss provision

Research and development income tax credit

Income taxes

Net product sales

Net income (loss) per share

The Company reported a net loss attributable to ordinary shareholders for the years ended December 31, 2018, 2017, and 2016.

Recently adopted accounting pronouncements

Recently issued accounting pronouncements not yet adopted

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

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

Fair Value of Financial Assets and Liabilities (Tables)

12 Months Ended

Dec. 31, 2018

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

Schedule of Financial Liabilities Measured at Fair Value on Recurring Basis

	Fair Value Measurements as of December 31, 2016 Using:
	Level 1		Level 2		Level 3		Total
	(in thousands)
Liabilities:
Tranche obligations	$	—	$	—	$	1,402	$	1,402
	$	—	$	—	$	1,402	$	1,402

Schedule of Assumptions Used in Valuing of Tranche Obligations

The following assumptions were used in valuing the tranche obligations:

		Year Ended December 31, 2016
Risk-free interest rate		0.00 - 0.53%
Expected dividend yield		0.00%
Expected term (in years)		0.00 - 0.92
Expected volatility		75.5 - 89.9%
Fair value of convertible preferred shares		$1.00 - $1.58

Schedule of Changes in Fair Value of Tranche Obligation Liability Measured at Fair Value on Recurring Basis using Significant Unobservable Inputs

	Tranche Obligations
	(in thousands)
Balance at December 31, 2015	$	—
Issuance of tranche obligations to purchase convertible preferred shares		2,459
Change in fair value of second tranche obligation		(424	)
Settlement of second tranche obligation upon issuance of convertible preferred shares		(451	)
Change in fair value of third tranche obligation		135
Effect of exchange rate changes on tranche obligation		(317	)
Balance at December 31, 2016		1,402
Settlement of third tranche obligation upon issuance of convertible preferred shares		(1,402	)
Balance at December 31, 2017	$	—

Property and Equipment (Tables)

12 Months Ended

Dec. 31, 2018

Property Plant And Equipment [Abstract]

Schedule of Property and Equipment

Property and equipment consist of the following:

	December 31,
	2018			2017
	(in thousands)
Property and equipment:
Lab equipment	$	4,930		$	2,708
Leasehold improvements		1,487			244
Furniture and fixtures		403			59
Office and IT equipment		152			12
Property and equipment	$	6,972			3,023
Less: accumulated depreciation		(1,496	)		(310	)
Property and equipment, net	$	5,476		$	2,713

Accrued Expenses and Other Liabilities (Tables)

12 Months Ended

Dec. 31, 2018

Accrued Liabilities And Other Liabilities [Abstract]

Schedule of Accrued Expenses and Other Liabilities

Accrued expenses and other current liabilities consisted of the following:

	December 31,
	2018		2017
	(in thousands)
Accrued external research and development expenses	$	12,738	$	1,834
Accrued payroll and related expenses		7,372		2,090
Accrued professional fees		1,186		394
Accrued other		2,762		279
Strimvelis liability - current portion		4,170		—
Deferred UCLA reimbursement		—		2,267
Due to UCLA		1,552		—
Total accrued expenses and other current liabilities	$	29,780	$	6,864

Shareholders’ Equity and Convertible Preferred Shares (Tables)

12 Months Ended

Dec. 31, 2018

Shareholders Equity And Convertible Preferred Shares [Abstract]

Schedule of Convertible Preferred Shares

As of December 31, 2017, Convertible Preferred Shares consisted of the following:

	December 31, 2017 (in thousands, except share amounts)
	Shares Authorized		Shares Issued and Outstanding		Carrying Value		Liquidation Preference(a)		Ordinary shares Issuable Upon Conversion
Series A convertible preferred shares		16,806,298		16,806,298	$	26,994	$	28,337		16,806,298
Series B convertible preferred shares		16,964,876		16,471,380		107,075		111,617		16,471,380
		33,771,174		33,277,678	$	134,069	$	139,954		33,277,678

(a)	Amounts were translated into United States dollars using the spot rate as of December 31, 2017.

Share-based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Assumptions Used in Black-Scholes Option Pricing Model

	Year Ended December 31,
	2018	2017	2016
Risk-free interest rate% - %	2.66% - 3.03%	1.52% - 2.30%	1.52% - 2.40%
Expected term (in years)	5.00 - 6.08	6.08	6.08 - 9.75
Expected volatility% - %	64.27 - 68.58%	77.80% - 80.00%	77.80% -79.70%
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, 2018:

	Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Life		Aggregate Intrinsic Value
	(in thousands, except share and per share amounts)
Options outstanding at December 31, 2017		4,153,196		$	1.20		9.28	$	10,483
Granted		6,303,465			4.23
Exercised		(14,547	)		1.96
Cancelled		(238,682	)		2.55
Options outstanding at December 31, 2018		10,203,432			3.04		8.97		129,551
Vested as of December 31, 2018		2,022,399			1.11		8.20		29,568

Summary of RSU Award Activity

The following table summarizes RSU award activity for the year ended December 31, 2018:

	Shares		Weighted Average Fair Value
Unvested at December 31, 2017		—	$	—
Granted		219,922		15.48
Vested		—		—
Forfeited		—		—
Unvested at December 31, 2018		219,922		15.48

Share-based Compensation Expense

	Year Ended December 31,
	2018		2017		2016
	(in thousands)
Research and development	$	2,740	$	615	$	181
Selling, general and administrative		4,026		404		23
Total	$	6,766	$	1,019	$	204

License and Research Arrangements (Tables)

12 Months Ended

Dec. 31, 2018

License And Research Arrangements [Abstract]

Summary of Consideration in Asset Acquisition

The consideration transferred in the asset acquisition was measured at cost, including transaction costs, assets and equity interests transferred by the acquirer, and liabilities incurred by the acquirer as noted below:

	Consideration
	(in thousands)
Upfront cash paid for GSK Agreement	$	14,186
Series B-2 convertible preferred shares issued to GSK		93,391
Transaction costs		780
Liabilities:
Strimvelis liability		18,351
Inventory purchase liability		6,893
Total consideration transferred:	$	133,601

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Schedule of Income before Income Tax

The components of loss from operations before income taxes for the years ended December 31, 2018, 2017, and 2016 are as follows:

	December 31,
	2018			2017			2016
	(in thousands)
U.K.		(230,543	)		(39,422	)		(19,105	)
Non-U.K.		1,018			(269	)		40
Loss before taxes	$	(229,525	)	$	(39,691	)	$	(19,065	)

Schedule of Provision for Income Taxes

	December 31,
	2018			2017		2016
	(in thousands)
Current provision expense
Federal—United States	$	607		$	—	$	—
State—United States		444			16		17
United Kingdom		—			—		—
Total current provision expense		1,051			16		17
Deferred provision expense
Federal—United States		(31	)		—		—
State—United States		(50	)		37		3
United Kingdom		—			—		—
Total deferred provision expense		(81	)		37		3
Total provision for income taxes	$	970		$	53	$	20

Schedule of Reconciliation of Income Tax Expense

A reconciliation of income tax expense computed at the United Kingdom statutory income tax rate to income taxes as reflected in the consolidated financial statements is as follows:

	December 31,
	2018			2017			2016
	(in thousands)
Income taxes at United Kingdom statutory rate	$	(43,526	)	$	(7,640	)	$	(3,831	)
State income taxes		370			41			14
Permanent differences		293			115			75
Tax credits		—			(286	)		(99	)
Foreign rate differential		20			(40	)		6
Change in valuation allowance		43,562			7,827			3,855
Impact of United States tax reform		159			36			—
Other		92			—			—
Total provision expense for income taxes	$	970		$	53		$	20

Schedule of Deferred Tax Assets and Liabilities

Significant components of the Company’s deferred tax assets and liabilities as of December 31, 2018 and 2017 consist of the following:

	December 31,
	2018			2017
	(in thousands)
Deferred tax assets
Net operating loss carryforwards	$	29,436		$	9,483
Research and development credits		—			356
Share-based compensation		1,297			147
Amortization		19,451			2,156
Accruals		184			28
Other		1,946			—
Total deferred tax assets	$	52,314		$	12,170
Valuation allowance		(51,281	)		(11,882	)
Net deferred tax assets	$	1,033		$	288
Deferred tax liabilities
Depreciation	$	(991	)	$	(328	)
Other non-current liabilities (net deferred tax assets and liabilities)	$	42		$	(40	)

Schedule of Changes in Valuation Allowance for Deferred Tax Assets Related Primarily to Increase in Net Operating Loss Carryforwards

	December 31,
	2018			2017			2016
	(in thousands)
Valuation allowance as of beginning of year	$	(11,882	)	$	(3,503	)		—
Decreases recorded as benefit to income tax provision		604			—			—
Increases recorded to income tax provision		(44,166	)		(7,827	)		(3,855	)
Effect of foreign currency translation		4,163			(552	)		352
Valuation allowance as of end of year	$	(51,281	)	$	(11,882	)	$	(3,503	)

Net Loss Per Share (Tables)

12 Months Ended

Dec. 31, 2018

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:

	Year Ended December 31
	2018			2017			2016
	(In thousands, except per share and share amounts)
Net loss	$	(230,495	)	$	(39,744	)	$	(19,085	)
Net loss attributable to ordinary shareholders	$	(230,495	)	$	(39,744	)	$	(19,085	)
Weighted average ordinary shares outstanding, basic and diluted		22,559,389			8,872,768			7,100,528
Net loss per share attributable to ordinary shareholders, basic and diluted	$	(10.22	)	$	(4.48	)	$	(2.69	)

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

	December 31,
	2018		2017		2016
Convertible preferred shares		—		33,277,678		11,204,199
Share options		9,179,247		3,612,288		1,809,442
Unvested performance-based restricted share units		219,922		—		—
		9,399,169		36,889,966		13,013,641

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Schedule of Future Minimum Lease Payments Due Under Operating Leases

Future minimum lease payments

The following table summarizes the future minimum lease payments due under all operating leases as of December 31, 2018:

Due in:	(in thousands)
2019	$	3,303
2020		4,910
2021		4,135
2022		3,921
2023		2,844
Thereafter		20,386
Total	$	39,499

Nature of Business and Basis of Presentation - Additional Information (Details) $ / shares in Units, $ in Thousands		12 Months Ended
	Nov. 02, 2018 USD ($) shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Nov. 02, 2018 £ / shares	Nov. 02, 2018 $ / shares
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Reverse stock split		one-for-0.8003
Payments of stock issuance costs		$ 4,200
Net Loss		230,500	$ 39,700	$ 19,100
Accumulated deficit		290,239	59,744
Cash		$ 335,844	$ 89,856
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
Nominal value per share \| £ / shares					£ 0.00001
Redenomination of Nominal value per share \| £ / shares					£ 0.10
American Depositary Shares \| Underwriter Discounts and Commissions
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Payments of stock issuance costs	$ 15,800

Summary of Significant Accounting Policies - Additional Information (Detail) £ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) Segment	Dec. 31, 2018 GBP (£) Segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) Tranche
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.	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)	$ 4,400,000		$ (1,200,000)	$ (200,000)
Proceeds from lines of credit	3,000,000
Restricted cash, related to lease transaction's letter of credit	3,837,000		0
Cash collateral associated with corporate credit card accounts	$ 900,000		0
Expected dividend yield	0.00%	0.00%
Fixed assets	$ 5,476,000		2,713,000
Other comprehensive income (loss) related to foreign currency translation adjustments	(964,000)		4,398,000	(271,000)
Tax credit			286,000	99,000
Tax incentive receivable	10,585,000		871,000
Increase (decrease) in tax receivable	$ (500,000)
Value added tax percentage	20.00%	20.00%
GSK Asset Purchase and License Agreement
Summary Of Significant Accounting Policies [Line Items]
Liability associated with the loss contract	$ 18,400,000
Remaining liability	10,300,000
Amortization as credit to research and development expense	6,300,000
Decrease in liability in effect of foreign exchange translation	$ 1,700,000
United Kingdom
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments \| Segment	3	3
Fixed assets	$ 1,700,000		500,000
Tax incentive receivable	$ 10,600,000		900,000
Amount that will be offset against future taxable profits \| £		£ 5.0
Incremental percentage of taxable profits	50.00%	50.00%
United Kingdom \| Research and Development Expense
Summary Of Significant Accounting Policies [Line Items]
Tax credit	$ 10,200,000		700,000	$ 200,000
European Union
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments \| Segment	3	3
United States
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments \| Segment	3	3
Fixed assets	$ 3,800,000		$ 2,200,000
Series A Convertible Preferred Shares
Summary Of Significant Accounting Policies [Line Items]
Number of Tranches \| Tranche				3

Summary of Significant Accounting Policies - Schedule of Estimated Useful Lives of Property and Equipment (Detail)	12 Months Ended
	Dec. 31, 2018
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property plant and equipment useful life, term	Shorter of lease term or estimated useful life
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property plant and equipment useful life	4 years
Minimum \| Lab Equipment
Property Plant And Equipment [Line Items]
Property plant and equipment useful life	5 years
Minimum \| Office and Computer Equipment
Property Plant And Equipment [Line Items]
Property plant and equipment useful life	3 years
Maximum \| Lab Equipment
Property Plant And Equipment [Line Items]
Property plant and equipment useful life	10 years
Maximum \| Office and Computer Equipment
Property Plant And Equipment [Line Items]
Property plant and equipment useful life	5 years

Fair Values of Assets and Liabilities - Schedule of Financial Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liabilities:
Liabilities, fair value	$ 0	$ 0
Fair Value, Measurements, Recurring
Liabilities:
Liabilities, fair value			$ 1,402,000
Fair Value, Measurements, Recurring \| Tranche Obligations
Liabilities:
Liabilities, fair value			1,402,000
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 3
Liabilities:
Liabilities, fair value			1,402,000
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 3 \| Tranche Obligations
Liabilities:
Liabilities, fair value			$ 1,402,000

Fair Value of Financial Assets And Liabilities - Schedule of Assumptions Used in Valuing of Tranche Obligations (Details) - Tranche Obligations	12 Months Ended
	Dec. 31, 2016
Risk Free Interest Rate \| Minimum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value assumptions of tranche obligations	0.00
Risk Free Interest Rate \| Maximum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value assumptions of tranche obligations	0.53
Expected Dividend Yield
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value assumptions of tranche obligations	0.00
Expected Term \| Minimum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value assumptions of tranche obligations term	0 years
Expected Term \| Maximum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value assumptions of tranche obligations term	11 months 1 day
Expected Volatility \| Minimum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value assumptions of tranche obligations	75.5
Expected Volatility \| Maximum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value assumptions of tranche obligations	89.9
Fair Value of Convertible Preferred Shares \| Minimum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value assumptions of tranche obligations	1.00
Fair Value of Convertible Preferred Shares \| Maximum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value assumptions of tranche obligations	1.58

Fair Value of Financial Assets and Liabilities - Schedule of Changes in Fair Value of Tranche Obligation Liability Measured at Fair Value on Recurring Basis using Significant Unobservable Inputs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Balance at beginning of period	$ 1,402
Issuance of tranche obligations to purchase convertible preferred shares		$ 2,459
Settlement obligation upon issuance of convertible preferred shares	(1,402)	(451)
Effect of exchange rate changes on tranche obligation		(317)
Balance at end of period		1,402
Second Tranche Obligation
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Change in fair value obligation		(424)
Settlement obligation upon issuance of convertible preferred shares		(451)
Third Tranche Obligation
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Change in fair value obligation		$ 135
Settlement obligation upon issuance of convertible preferred shares	$ (1,402)

Revenue Recognition - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Revenue From Contract With Customer [Abstract]
Revenue		$ 0
Cumulative adjustment	$ 0
General payment terms, maximum	60 days

Property and Equipment - Schedule of Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Property and equipment	$ 6,972	$ 3,023
Less: accumulated depreciation	(1,496)	(310)
Property and equipment, net	5,476	2,713
Lab Equipment
Property Plant And Equipment [Line Items]
Property and equipment	4,930	2,708
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment	1,487	244
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment	403	59
Office and IT Equipment
Property Plant And Equipment [Line Items]
Property and equipment	$ 152	$ 12

Accrued Expenses and Other Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accrued Liabilities And Other Liabilities [Abstract]
Accrued external research and development expenses	$ 12,738	$ 1,834
Accrued payroll and related expenses	7,372	2,090
Accrued professional fees	1,186	394
Accrued other	2,762	279
Strimvelis liability - current portion	4,170
Deferred UCLA reimbursement		2,267
Due to UCLA	1,552
Total accrued expenses and other current liabilities	$ 29,780	$ 6,864

Shareholders' Equity and Convertible Preferred Shares - Additional Information (Details)		1 Months Ended												12 Months Ended
	Nov. 02, 2018 USD ($) $ / shares shares	Aug. 31, 2018 USD ($) $ / shares shares	Jan. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($)	Oct. 31, 2017 USD ($)	Aug. 31, 2017 USD ($)	Jul. 31, 2017 USD ($)	Mar. 31, 2017 USD ($) shares	Feb. 28, 2017 USD ($)	Jan. 31, 2017 USD ($)	Aug. 31, 2016 USD ($)	Jul. 31, 2016 USD ($)	May 31, 2016 USD ($)	Dec. 31, 2018 USD ($) VotingRight Right shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) shares	Dec. 31, 2018 GBP (£) £ / shares shares	Dec. 31, 2018 $ / shares	Dec. 31, 2017 GBP (£) £ / shares shares	Dec. 31, 2017 $ / shares	Nov. 30, 2017 shares	Oct. 31, 2017 £ / shares shares	Aug. 31, 2017 £ / shares shares	Mar. 31, 2017 £ / shares shares	Mar. 31, 2017 $ / shares shares	Feb. 28, 2017 £ / shares shares	Jan. 31, 2017 £ / shares shares	Aug. 31, 2016 £ / shares shares	Jul. 31, 2016 £ / shares shares	May 31, 2016 £ / shares shares	Feb. 29, 2016 £ / shares 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
Number of ordinary voting rights \| VotingRight														1
Ordinary Shares, Par Value \| £ / shares																	£ 0.10		£ 0.10
Ordinary shares cash dividends declared or paid \| $															$ 0
Ordinary Shares, Issued																	85,865,557		8,927,121
Outstanding share options																	10,203,432		4,153,196
Issuance of ADRs in initial public offering, net of issuance costs \| $														$ 205,469,000
Payment of offering expenses \| $														$ 4,200,000
Common Stock, Conversion Basis														one-for-0.8003
Research and development \| $														$ 205,319,000	32,527,000	$ 16,206,000
Ordinary Shares, Outstanding																	85,865,557		8,927,121
Deferred shares par value per share \| $ / shares																		$ 0.00001
Deferred shares rights \| Right														0
Deferred share price per share from distribution of assets on liquidation or return of capital \| $ / shares																		$ 1.25
Deferred shares repurchased								80,030
Deferred shares repurchased price per share \| $ / shares																									$ 0.00001
Deferred shares outstanding																	0		0
Ordinary Shares
Class Of Stock [Line Items]
Issuance of ordinary shares in initial public offering net of issuance, Shares														16,103,572
Conversion of investor ordinary stock to deferred shares																80,030
2016 Share Option Plan
Class Of Stock [Line Items]
Outstanding share options																			4,153,196
Shares remaining available for future issuance																			2,354,595
License and Research Agreement
Class Of Stock [Line Items]
Ordinary Shares, Issued																			500,596
License and Development Agreement \| Oxford BioMedica
Class Of Stock [Line Items]
Ordinary Shares, Issued																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																					150,826
License and Development Agreement \| Oxford BioMedica \| Achievement of Second Milestone
Class Of Stock [Line Items]
Ordinary Shares, Issued		150,826
License Agreement \| UCL Business PLC
Class Of Stock [Line Items]
Ordinary Shares, Issued																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																800,380			256,096
Research and development \| $															1,400,000	$ 500,000
Corporate Reorganization
Class Of Stock [Line Items]
Ordinary Shares, Par Value \| (per share)	$ 0.10																		£ 0.00001
Redesignation
Class Of Stock [Line Items]
Common Stock, Conversion Basis														one-for-one
Convertible Preferred Shares
Class Of Stock [Line Items]
Convertible Preferred Shares, Outstanding																	0		33,277,678
Issuance of ADRs in initial public offering, net of issuance costs \| $														$ 50,000,000
Preferred stock conversion basis														one-for-0.8003
Convertible Preferred Shares, Authorized																			33,771,174
Convertible Preferred Shares, Par Value \| (per share)																			£ 0.00001	$ 0.00001
Convertible Preferred Shares, Issued																			33,277,678
Preferred stock dividend declared or paid \| $														$ 0	$ 0
American Depositary Shares \| Initial Public Offering
Class Of Stock [Line Items]
Issuance of ordinary shares in initial public offering net of issuance, 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
Payment of offering expenses \| $	4,200,000
Proceeds from shares issued \| $	$ 205,500,000
Series A Convertible Preferred Shares
Class Of Stock [Line Items]
Convertible Preferred Shares, Authorized																			16,806,299
Convertible Preferred Shares, Par Value \| £ / shares																										£ 1.25	£ 1.25	£ 1.25	£ 1.25	£ 1.25	£ 1.25
Convertible Preferred Shares, Issued																										266,766	5,335,333	266,766	5,335,333	266,767	5,335,333
Preferred stock shares issued for net proceeds														4,811,937
Proceeds from shares issued \| $									$ 400,000	$ 8,200,000	$ 400,000	$ 8,700,000	$ 400,000	$ 8,500,000
Preferred stock issued in settlement of debt														523,396
Series B Convertible Preferred Shares
Class Of Stock [Line Items]
Convertible Preferred Shares, Authorized																			16,964,875
Convertible Preferred Shares, Par Value \| £ / shares																			£ 5.022			£ 5.022	£ 5.022	£ 5.022
Convertible Preferred Shares, Issued			150,706																2,724,288			4,655,985	3,285,731	5,805,376	5,805,376
Proceeds from shares issued \| $			$ 1,000,000	$ 18,300,000	$ 30,800,000	$ 21,000,000		$ 36,000,000
Series C Convertible Preferred Shares
Class Of Stock [Line Items]
Convertible Preferred Shares, Par Value \| $ / shares		$ 10.76
Convertible Preferred Shares, Issued		13,942,474
Proceeds from shares issued \| $		$ 147,100,000
Series A and Series B Convertible Preferred Stock
Class Of Stock [Line Items]
Preferred stock conversion basis														each Preferred Share was convertible into one ordinary share
Preferred stock voting rights														Each Series A and Series B preferred share shall confer one right to vote at all general meetings and to receive and vote on proposed written resolutions of the Company
Maximum
Class Of Stock [Line Items]
Ordinary shares maximum nominal value \| £																	£ 13,023,851.50		£ 675,413
Maximum \| Convertible Preferred Shares
Class Of Stock [Line Items]
Issue price per share \| £ / shares																	£ 6.0262

Shareholders' Equity and Convertible Preferred Shares - Schedule of Convertible Preferred Shares (Details)

Dec. 31, 2017

USD ($)

Class Of Stock [Line Items]

33,771,174

33,277,678

33,277,678

$ 134,069

Liquidation Preference | $

$ 139,954

^[1]

Ordinary shares Issuable Upon Conversion

33,277,678

Series A Convertible Preferred Shares

Class Of Stock [Line Items]

16,806,298

16,806,298

16,806,298

$ 26,994

Liquidation Preference | $

$ 28,337

^[1]

Ordinary shares Issuable Upon Conversion

16,806,298

Series B Convertible Preferred Shares

Class Of Stock [Line Items]

16,964,876

16,471,380

16,471,380

$ 107,075

Liquidation Preference | $

$ 111,617

^[1]

Ordinary shares Issuable Upon Conversion

16,471,380

[1]	Amounts were translated into United States dollars using the spot rate as of December 31, 2017

Share-based Compensation - Additional Information (Details) $ / shares in Units, $ in Thousands	1 Months Ended		12 Months Ended
	Nov. 30, 2018 shares	Oct. 31, 2018 shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2018 £ / shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation expense \| $			$ 6,766	$ 1,019	$ 204
Exercise price of option \| £ / shares						£ 0.00001
Award vesting period			4 years
Maximum term of option			10 years
Expected dividend rate%			0.00%	0.00%	0.00%
Weighted Average Exercise Price, Granted \| $ / shares			$ 4.23
Exercise of share options, Shares			14,547	0	0
Weighted average grant date fair value of options granted \| $ / shares			$ 5.23	$ 2.70	$ 0.92
Compensation cost recognized \| $			$ 4,500
Unvested options outstanding			8,181,033
Total unrecognized compensation cost of stock option \| $			$ 33,300
Compensation cost expected to be recognized weighted average period			2 years 11 months 15 days
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
Restricted share units issued			219,922
Aggregate fair value of RSUs \| $			$ 4,500
Compensation cost recognized \| $			$ 100	$ 0
Performance-based Restricted Share Units ("RSUs") \| Chief Executive Officer
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Restricted share units issued	219,922
United States \| Employee
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted Average Exercise Price, Granted \| $ / shares			$ 5.74
2018 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares remaining available for future issuance		4,254,741
Shares available for future grant			3,953,726
Percentage of increase in number of ordinary shares reserved for issuance			5.00%
2018 ESPP
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of increase in number of ordinary shares reserved for issuance		1.00%
Number of shares increased in plan		1,500,000
Percentage of purchase price for each ordinary share		85.00%
Share-based compensation expense \| $			$ 100
2018 ESPP \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Employee contribution percentage to base compensation for purchase of ordinary shares		15.00%
Ordinary shares reserved for future issuance		850,948

Share-based Compensation - Assumptions Used in Black-Scholes Option Pricing Model (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate% - %, minimum	2.66%	1.52%	1.52%
Risk-free interest rate% - %, maximum	3.03%	2.30%	2.40%
Expected term (in years)		6 years 29 days
Expected volatility% - %, minimum	64.27%	77.80%	77.80%
Expected volatility% - %, maximum	68.58%	80.00%	79.70%
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 years 29 days
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	6 years 29 days		9 years 9 months

Share-based Compensation - Summary of Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Shares, Options outstanding at December 31, 2017	4,153,196
Shares, Granted	6,303,465
Shares, Exercised	(14,547)	0	0
Shares, Cancelled	(238,682)
Shares, Options outstanding at December 31, 2018	10,203,432	4,153,196
Shares, Vested as of December 31, 2018	2,022,399
Weighted Average Exercise Price, Options outstanding at December 31, 2017	$ 1.20
Weighted Average Exercise Price, Granted	4.23
Weighted Average Exercise Price, Exercised	1.96
Weighted Average Exercise Price, Cancelled	2.55
Weighted Average Exercise Price, Options outstanding at December 31, 2018	3.04	$ 1.20
Weighted Average Exercise Price, Vested as of December 31, 2018	$ 1.11
Weighted Average Remaining Contractual Life, Options outstanding at December 31, 2017	8 years 11 months 19 days	9 years 3 months 10 days
Weighted Average Remaining Contractual Life, Vested as of December 31, 2018	8 years 2 months 12 days
Aggregate Intrinsic Value, Options outstanding	$ 129,551	$ 10,483
Aggregate Intrinsic Value, Vested as of December 31, 2018	$ 29,568

Share-based Compensation - Summary of RSU Award Activity (Details) - RSU	12 Months Ended
	Dec. 31, 2018 $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares, Granted \| shares	219,922
Shares, Unvested at December 31, 2018 \| shares	219,922
Weighted Average Fair Value, Unvested at December 31, 2017
Weighted Average Fair Value, Granted	15.48
Weighted Average Fair Value, Unvested at December 31, 2018	$ 15.48

Share-based Compensation - Share-based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation expense	$ 6,766	$ 1,019	$ 204
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation expense	2,740	615	181
Selling, General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation expense	$ 4,026	$ 404	$ 23

License and Research Arrangements - Additional Information (Details) £ / shares in Units, € in Millions, £ in Millions	1 Months Ended					12 Months Ended
	Jul. 31, 2018 USD ($)	Jun. 30, 2018 USD ($)	Apr. 30, 2018 USD ($) shares	Apr. 30, 2018 GBP (£) shares	Jan. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) shares	Nov. 30, 2018 shares	Sep. 30, 2018 shares	Apr. 30, 2018 GBP (£)	Apr. 30, 2018 EUR (€)	Feb. 28, 2017 £ / shares shares	Jan. 31, 2017 £ / shares shares	Aug. 31, 2016 £ / shares shares	Jul. 31, 2016 £ / shares shares	May 31, 2016 £ / shares shares	Feb. 29, 2016 £ / shares shares
Collaborative Arrangements and Noncollaborative Arrangement Transactions [Line Items]
Ordinary shares issued \| shares						85,865,557	8,927,121
Research and development						$ 205,319,000	$ 32,527,000	$ 16,206,000
Series A Convertible Preferred Shares
Collaborative Arrangements and Noncollaborative Arrangement Transactions [Line Items]
Preferred stock shares issued \| shares													266,766	5,335,333	266,766	5,335,333	266,767	5,335,333
Preferred stock par value per share \| £ / shares													£ 1.25	£ 1.25	£ 1.25	£ 1.25	£ 1.25	£ 1.25
GSK Asset Purchase and License Agreement
Collaborative Arrangements and Noncollaborative Arrangement Transactions [Line Items]
Total consideration			$ 133,601,000	£ 94.2
Upfront payment			14,186,000	10.0
Inventory purchase liability			6,893,000								£ 4.9
Transaction costs			780,000	0.6
In-process research and development expense			133,600,000	94.2
Strimveils liability recorded			$ 18,351,000	£ 12.9
Amortization as credit to research and development expense						6,300,000
Remaining liability						10,300,000
Decrease in liability in effect of foreign exchange translation						$ 1,700,000
Payment of tiered royalty, maximum percentage			20.00%								20.00%	20.00%
Milestone payments payable upon achievement of certain sales milestones											£ 90.0	€ 31.0
GSK Asset Purchase and License Agreement \| Series B-2 Convertible Preferred Shares
Collaborative Arrangements and Noncollaborative Arrangement Transactions [Line Items]
Shares issued for asset acquisition \| shares			12,455,252	12,455,252
Value of shares issued for asset acquisition			$ 93,391,000								£ 65.8
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
Research and development						$ 200,000	$ 1,800,000	$ 4,600,000
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						$ 38,900,000
UCLB/UCLA License Agreement \| Series A Convertible Preferred Shares
Collaborative Arrangements and Noncollaborative Arrangement Transactions [Line Items]
Preferred stock shares issued \| shares																		800,298
Preferred stock par value per share \| £ / shares																		£ 1.25
Oxford BioMedica License, Development and Supply Agreement
Collaborative Arrangements and Noncollaborative Arrangement Transactions [Line Items]
Ordinary shares issued \| shares						588,220			150,826	150,826
Research and development								500,000
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]
Research and development						3,000,000	5,000,000
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
Accrued expense and other liabilities						1,600,000	2,300,000
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
Other License and Research Agreements
Collaborative Arrangements and Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payments							$ 2,700,000	$ 400,000
Ordinary shares issued \| shares							1,030,786	375,380
Research and development							$ 1,400,000	$ 500,000
Clinical and regulatory milestone payments						$ 31,800,000

License and Research Arrangements - Summary of Consideration Transferred in Asset Acquisition (Details) - 1 months ended Apr. 30, 2018 - GSK Asset Purchase and License Agreement $ in Thousands, £ in Millions	USD ($)	GBP (£)	GBP (£)
Collaborative Arrangements and Noncollaborative Arrangement Transactions [Line Items]
Upfront cash paid for GSK Agreement	$ 14,186	£ 10.0
Transaction costs	780	0.6
Liabilities:
Strimvelis liability	18,351	12.9
Inventory purchase liability	6,893		£ 4.9
Total consideration transferred:	133,601	£ 94.2
Series B-2 Convertible Preferred Shares
Collaborative Arrangements and Noncollaborative Arrangement Transactions [Line Items]
Value of shares issued to GSK	$ 93,391		£ 65.8

Income Taxes - Schedule of Components of Loss from Operations Before Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
U.K.	$ (230,543)	$ (39,422)	$ (19,105)
Non-U.K.	1,018	(269)	40
Net loss before income tax	$ (229,525)	$ (39,691)	$ (19,065)

Income Taxes - Schedule of Provision for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current provision expense
Federal—United States	$ 607
State—United States	444	$ 16	$ 17
Total current provision expense	1,051	16	17
Deferred provision expense
Federal—United States	(31)
State—United States	(50)	37	3
Total deferred provision expense	(81)	37	3
Total provision for income taxes	$ 970	$ 53	$ 20

Income Taxes - Schedule of Reconciliation of Income Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Income taxes at United Kingdom statutory rate	$ (43,526)	$ (7,640)	$ (3,831)
State income taxes	370	41	14
Permanent differences	293	115	75
Tax credits		(286)	(99)
Foreign rate differential	20	(40)	6
Change in valuation allowance	43,562	7,827	3,855
Impact of United States tax reform	159	36
Other	92
Total provision for income taxes	$ 970	$ 53	$ 20

Income Taxes - Schedule of Deferred Tax Assets and Liabilities Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets
Net operating loss carryforwards	$ 29,436	$ 9,483
Research and development credits		356
Share-based compensation	1,297	147
Amortization	19,451	2,156
Accruals	184	28
Other	1,946
Total deferred tax assets	52,314	12,170
Valuation allowance	(51,281)	(11,882)	$ (3,503)
Net deferred tax assets	1,033	288
Deferred tax liabilities
Depreciation	(991)	(328)
Other non-current liabilities (net deferred tax assets and liabilities)	$ 42
Other non-current liabilities (net deferred tax assets and liabilities)		$ (40)

Income Taxes - Schedule of Changes in Valuation Allowance for Deferred Tax Assets Related Primarily to Increase in Net Operating Loss Carryforwards (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Valuation allowance as of beginning of year	$ (11,882)	$ (3,503)
Decreases recorded as benefit to income tax provision	604
Increases recorded to income tax provision	(44,166)	(7,827)	$ (3,855)
Effect of foreign currency translation	4,163	(552)	352
Valuation allowance as of end of year	$ (51,281)	$ (11,882)	$ (3,503)

Net Loss Per Share - Schedule of basic and diluted net loss per share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net loss	$ (230,495)	$ (39,744)	$ (19,085)
Net loss attributable to ordinary shareholders	$ (230,495)	$ (39,744)	$ (19,085)
Weighted average number of ordinary shares outstanding, basic and diluted	22,559,389	8,872,768	7,100,528
Net loss per share attributable to ordinary shareholders, basic and diluted	$ (10.22)	$ (4.48)	$ (2.69)

- Definition

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

+ References + Details

Net Loss Per Share - Securities Excluded in the Computation of Diluted Net Loss Per Ordinary Share (Details) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common shares attributable to anti-dilutive shares	9,399,169	36,889,966	13,013,641
Share options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common shares attributable to anti-dilutive shares	9,179,247	3,612,288	1,809,442
Performance-based Restricted Share Units ("RSUs")
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common shares attributable to anti-dilutive shares	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	11,204,199

Commitments and Contingencies - Additional Information (Details) - USD ($)	1 Months Ended					12 Months Ended			27 Months Ended
	Dec. 31, 2018	Mar. 31, 2018	Jan. 31, 2018	Nov. 30, 2017	Oct. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Mar. 31, 2021
Operating Leased Assets [Line Items]
Operating lease, term of contract			5 years		5 years
Operating leases expiration term	Jan. 07, 2023	Sep. 30, 2022		Nov. 30, 2020	Oct. 31, 2021	May 31, 2030
Annual rental expense	$ 100,000	$ 300,000		$ 800,000	$ 200,000	$ 2,400,000
Operating lease duration for free rent				1 month	1 month	8 months
Operating leases commenced term	Dec. 07, 2018
Proceeds from lines of credit						$ 3,000,000
Reduction of standby letter of credit amount percentage						25.00%
Rent expense						$ 2,400,000	$ 700,000	$ 200,000
Prepayments	$ 800,000					800,000	$ 0
Scenario, Forecast [Member]
Operating Leased Assets [Line Items]
Further payments									$ 10,100,000
Five Years Lease Term Terminates In January 2023 [Member]
Operating Leased Assets [Line Items]
Operating leases expiration term			Jan. 31, 2023
Annual rental expense			$ 800,000
Fremont Lease Agreement [Member]
Operating Leased Assets [Line Items]
Amortization of tenant improvements allowance						5,000,000
Escrow deposit	$ 10,000,000					10,000,000
Increase decrease in escrow deposit						$ 0

Commitments and Contingencies - Schedule of Future Minimum Lease Payments Due Under Operating 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	$ 39,499

Related-Party Transactions - Additional Information (Details) $ in Thousands, £ in Millions				1 Months Ended				12 Months Ended
	Nov. 02, 2018 shares	Apr. 30, 2018 GBP (£)	Feb. 06, 2016 shares	Dec. 31, 2018 USD ($)	Nov. 30, 2018 shares	Aug. 31, 2018 shares	Dec. 31, 2017 shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($)	Apr. 30, 2018 USD ($) shares	Feb. 29, 2016 shares
Related Party Transaction [Line Items]
Ordinary shares Issuable Upon Conversion							33,277,678		33,277,678
Research and development expense \| $								$ 205,319	$ 32,527	$ 16,206
Chief Executive Officer
Related Party Transaction [Line Items]
Shares issued						24,979
Chief Financial Officer
Related Party Transaction [Line Items]
Shares issued						9,294
Members of Board of Directors
Related Party Transaction [Line Items]
Shares issued						90,158
GSK
Related Party Transaction [Line Items]
License upfront fee \| £		£ 14.1
Inventory purchase liability \| $											$ 6,900
Series A Shares
Related Party Transaction [Line Items]
Ordinary shares Issuable Upon Conversion							16,806,298		16,806,298
Series B Preferred Stock \| GSK
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
Series A Convertible Preferred Shares \| F-Prime Capital
Related Party Transaction [Line Items]
Ordinary shares Issuable Upon Conversion												16,006,000
Series B Convertible Preferred Shares \| F-Prime Capital
Related Party Transaction [Line Items]
Shares issued							2,400,900
Series B Convertible Preferred Shares \| Scottish Mortgage Investment Trust
Related Party Transaction [Line Items]
Shares issued							3,201,200
Series B Convertible Preferred Shares
Related Party Transaction [Line Items]
Ordinary shares Issuable Upon Conversion							16,471,380		16,471,380
Series B Convertible Preferred Shares \| Chief Executive Officer
Related Party Transaction [Line Items]
Shares issued							39,825
Series B Convertible Preferred Shares \| Chief Medical Officer
Related Party Transaction [Line Items]
Shares issued							9,955
Series B Convertible Preferred Shares \| Senior Vice President
Related Party Transaction [Line Items]
Shares issued							3,982
American Depositary Shares \| Initial Public Offering
Related Party Transaction [Line Items]
Shares issued	16,103,572
American Depositary Receipts \| Chief Executive Officer \| Initial Public Offering
Related Party Transaction [Line Items]
Shares issued					18,500
American Depositary Receipts \| Chief Financial Officer \| Initial Public Offering
Related Party Transaction [Line Items]
Shares issued					5,000
American Depositary Receipts \| Members of Board of Directors \| Initial Public Offering
Related Party Transaction [Line Items]
Shares issued					13,000
UCL Business PLC
Related Party Transaction [Line Items]
Total program costs \| $										500
Costs and expenses \| $								200	$ 200	$ 400
UCL Business PLC \| Series A Shares
Related Party Transaction [Line Items]
Related party transactions shares purchased			800,298
G S K \| Transition Services Agreement
Related Party Transaction [Line Items]
Research and development expense \| $				$ 14,000
Royalty costs \| $				14,000
Accrued expenses and accounts payable \| $				$ 6,000				$ 6,000
Deerfield Management Company \| Minimum
Related Party Transaction [Line Items]
Percentage of share capital				5.00%				5.00%
Deerfield Management Company \| Series C Convertible Preferred Shares
Related Party Transaction [Line Items]
Shares issued						4,647,500
Deerfield Management Company \| American Depositary Shares \| Initial Public Offering
Related Party Transaction [Line Items]
Shares issued					3,376,100
Scottish Mortgage Investment Trust \| Series C Convertible Preferred Shares
Related Party Transaction [Line Items]
Shares issued						697,125
RA Capital Management Limited Liability Company \| Minimum
Related Party Transaction [Line Items]
Percentage of share capital				5.00%				5.00%
RA Capital Management Limited Liability Company \| American Depositary Shares \| Initial Public Offering
Related Party Transaction [Line Items]
Shares issued					2,057,432
Temasek Holdings (Private) Limited \| Minimum
Related Party Transaction [Line Items]
Percentage of share capital				5.00%				5.00%
Temasek Holdings (Private) Limited \| American Depositary Shares \| Initial Public Offering
Related Party Transaction [Line Items]
Shares issued					1,000,000
Scottish Mortgage Investment Trust \| Minimum
Related Party Transaction [Line Items]
Percentage of share capital				5.00%				5.00%
Scottish Mortgage Investment Trust \| American Depositary Shares \| Initial Public Offering
Related Party Transaction [Line Items]
Shares issued					925,000

Subsequent Events - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions						12 Months Ended
	Mar. 13, 2019	Mar. 01, 2019	Feb. 01, 2019	Jan. 16, 2019	Jan. 02, 2019	Dec. 31, 2018	Dec. 31, 2021
Subsequent Event [Line Items]
Shares, Granted						6,303,465
Weighted Average Exercise Price, Granted						$ 4.23
Award vesting period						4 years
Performance-based Restricted Share Units ("RSUs")
Subsequent Event [Line Items]
Award vesting period						1 year 3 months 18 days
Shares, Granted						219,922
Aggregate fair value of RSUs						$ 4.5
Subsequent Event \| Share Options and Performance-Based Restricted Share Units \| 2018 Plan
Subsequent Event [Line Items]
Shares, Granted		24,700	95,800	2,470,423	117,280
Weighted Average Exercise Price, Granted		$ 16.89	$ 12.86	$ 12.54	$ 14.98
Aggregate grant-date fair value of the options		$ 0.3	$ 0.8	$ 19.8	$ 1.1
Award vesting period		4 years	4 years	4 years	4 years
Subsequent Event \| Performance-based Restricted Share Units ("RSUs") \| 2018 Plan
Subsequent Event [Line Items]
Shares, Granted	108,000			219,500
Vesting date	Dec. 31, 2021			Dec. 31, 2021
Aggregate fair value of RSUs	$ 1.9						$ 3.3

Income Taxes - Additional Information (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
United Kingdom
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	$ 155.2	$ 48.4

Consolidated Statement of Convertible Preferred Shares and Shareholders' Equity (Parenthetical) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)
Statement Of Stockholders Equity [Abstract]
Issuance of ordinary shares in initial public offering, issuance costs	$ 4,200

Fair Values of Assets and Liabilities - Additional Information (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Assets, fair value	$ 0	$ 0
Liabilities, fair value	$ 0	$ 0