Document and Entity Information	6 Months Ended
Document and Entity Information	Jun. 30, 2019
Document And Entity Information [Abstract]
Trading Symbol	ORTX
Entity Registrant Name	ORCHARD THERAPEUTICS PLC
Entity Central Index Key	0001748907
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q2
Document Type	6-K
Document Period End Date	Jun. 30, 2019
Amendment Flag	false
Entity File Number	001-38722
Entity Address, Address Line One	108 Cannon Street
Entity Address, City or Town	London
Entity Address, Country	United Kingdom
Entity Address, Postal Zip Code	EC4N 6EU

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 132,783	$ 335,844
Marketable securities	286,748
Trade and other receivables	1,956	2,153
Prepaid expenses and other assets	5,143	6,935
Research and development tax credit receivable, current	10,594	10,585
Total current assets	437,224	355,517
Property and equipment, net	5,549	5,476
Research and development tax credit receivable	9,731
Restricted cash	3,843	3,837
Other long-term assets	1,770	1,212
Total assets	458,117	366,042
Current liabilities:
Accounts payable	28,861	18,125
Accrued expenses and other current liabilities	26,574	29,780
Total current liabilities	55,435	47,905
Long-term debt, net	24,501
Other long-term liabilities	7,024	6,799
Total liabilities	86,960	54,704
Commitments and contingencies (see Note 12)
Shareholders’ equity:
Ordinary shares, £0.10 par value	12,234	10,924
Additional paid-in capital	725,552	587,490
Accumulated other comprehensive income	4,879	3,163
Accumulated deficit	(371,508)	(290,239)
Total shareholders’ equity	371,157	311,338
Total liabilities and shareholders’ equity	$ 458,117	$ 366,042

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Costs and operating expenses:
Research and development	$ 40,478	$ 150,991	$ 57,971	$ 160,162
Selling, general and administrative	13,674	7,421	24,464	11,948
Total costs and operating expenses	54,152	158,412	82,435	172,110
Loss from operations	(54,152)	(158,412)	(82,435)	(172,110)
Other income (expense):
Interest income	1,727		3,350
Interest expense	(245)		(245)
Other income (expense), net	1,368	2,097	(2,118)	401
Total other income (expense), net	2,850	2,097	987	401
Net loss before income tax	(51,302)	(156,315)	(81,448)	(171,709)
Income tax (expense) benefit	772	82	179	165
Net loss attributable to ordinary shareholders	(50,530)	(156,233)	(81,269)	(171,544)
Other comprehensive income (loss):
Foreign currency translation adjustment	(1,443)	(1,463)	1,608	1,970
Unrealized gain on marketable securities	108		108
Total other comprehensive income (loss):	(1,335)	(1,463)	1,716	1,970
Total comprehensive loss	$ (51,865)	$ (157,696)	$ (79,553)	$ (169,574)
Net loss per share attributable to ordinary shareholders, basic and diluted	$ (0.56)	$ (15.45)	$ (0.92)	$ (16.99)
Weighted average number of ordinary shares outstanding, basic and diluted	89,712,916	10,115,335	88,369,311	10,095,863

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flows from operating activities:
Net loss attributable to ordinary shareholders	$ (81,269)	$ (171,544)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	775	504
Non-cash share-based compensation	8,451	2,250
Non-cash interest expense	46
Non-cash consideration for licenses		93,391
Amortization of Strimvelis loss provision	(2,589)	(1,423)
Accretion of discount on marketable securities	(232)
Changes in operating assets and liabilities:
Trade and other receivables	370	(122)
Research and development tax credit receivable	(9,913)	(3,367)
Prepaid expenses and other assets	1,257	(2,662)
Accounts payable, accrued expenses and other current liabilities	9,784	34,103
Other Long-term liabilities	203	7,795
Net cash used in operating activities	(73,117)	(41,075)
Cash flows from investing activities:
Purchases of marketable securities	(286,399)
Purchases of property and equipment	(589)	(2,833)
Net cash used in investing activities	(286,988)	(2,833)
Cash flows from financing activities:
Issuance of debt from credit facility, net of debt issuance costs paid	24,695
Issuance of convertible preferred shares		2,250
Proceeds from share options and ESPP shares	1,270	25
Issuance of ADSs in follow-on offering, net of offering costs paid	130,163
Net cash provided by financing activities	156,128	2,275
Effect of exchange rate changes on cash, cash equivalents, and restricted cash	922	539
Net decrease in cash, cash equivalents and restricted cash	(203,055)	(41,094)
Cash, cash equivalents, and restricted cash, beginning of period	339,681	89,856
Cash, cash equivalents, and restricted cash, end of period	136,626	48,762
Supplemental disclosure of non-cash investing and financing activities
Deferred offering costs and debt issuance costs in accounts payable and accrued expenses	726
Property and equipment included in accounts payable and accrued expenses	262	357
Convertible preferred shares issued for licenses		$ 93,391
Supplemental disclosure of cash flow information:
Cash paid for interest	$ 199

Condensed Consolidated Statements of Convertible Preferred Shares and Shareholders' (Deficit) Equity (Unaudited) - USD ($) $ in Thousands	Total	Convertible Preferred Shares	Ordinary Shares	Additional Paid-in Capital	Accumulated Other Comprehensive Income	Accumulated Deficit
Stockholders Equity, Beginning Balance at Dec. 31, 2017	$ 86,405	$ 134,069	$ 1,145	$ 6,808	$ 4,127	$ (59,744)
Stockholders Equity, Beginning Balance, Shares at Dec. 31, 2017		33,277,678	8,927,121
Share-based compensation expense	1,104			1,104
Issuance of convertible preferred shares, Value	2,249	$ 2,249
Issuance of convertible preferred shares, Shares		493,496
Issuance of ordinary shares as part of license arrangement, Value			$ 45	(45)
Issuance of ordinary shares as part of license arrangement, Shares			349,770
Other comprehensive income (loss)	3,433				3,433
Net loss attributable to ordinary shareholders	(15,311)					(15,311)
Stockholders Equity, Ending Balance at Mar. 31, 2018	77,880	$ 136,318	$ 1,190	7,867	7,560	(75,055)
Stockholders Equity, Ending Balance, Shares at Mar. 31, 2018		33,771,174	9,276,891
Stockholders Equity, Beginning Balance at Dec. 31, 2017	86,405	$ 134,069	$ 1,145	6,808	4,127	(59,744)
Stockholders Equity, Beginning Balance, Shares at Dec. 31, 2017		33,277,678	8,927,121
Other comprehensive income (loss)	1,970
Net loss attributable to ordinary shareholders	(171,544)
Stockholders Equity, Ending Balance at Jun. 30, 2018	14,746	$ 229,709	$ 1,190	9,038	6,097	(231,288)
Stockholders Equity, Ending Balance, Shares at Jun. 30, 2018		46,226,426	9,438,220
Stockholders Equity, Beginning Balance at Mar. 31, 2018	77,880	$ 136,318	$ 1,190	7,867	7,560	(75,055)
Stockholders Equity, Beginning Balance, Shares at Mar. 31, 2018		33,771,174	9,276,891
Share-based compensation expense	1,146			1,146
Issuance of convertible preferred shares, Value	93,391	$ 93,391
Issuance of convertible preferred shares, Shares		12,455,252
Issuance of ordinary shares from exercise of share options	25			25
Issuance of ordinary shares from exercise of share options, Shares			10,503
Issuance of ordinary shares as part of license arrangement, Shares			150,826
Other comprehensive income (loss)	(1,463)				(1,463)
Net loss attributable to ordinary shareholders	(156,233)					(156,233)
Stockholders Equity, Ending Balance at Jun. 30, 2018	14,746	$ 229,709	$ 1,190	9,038	6,097	(231,288)
Stockholders Equity, Ending Balance, Shares at Jun. 30, 2018		46,226,426	9,438,220
Stockholders Equity, Beginning Balance at Dec. 31, 2018	311,338		$ 10,924	587,490	3,163	(290,239)
Stockholders Equity, Beginning Balance, Shares at Dec. 31, 2018			85,865,557
Share-based compensation expense	3,821			3,821
Issuance of ordinary shares from exercise of share options	4			4
Issuance of ordinary shares from exercise of share options, Shares			1,471
Other comprehensive income (loss)	3,051				3,051
Net loss attributable to ordinary shareholders	(30,739)					(30,739)
Stockholders Equity, Ending Balance at Mar. 31, 2019	287,475		$ 10,924	591,315	6,214	(320,978)
Stockholders Equity, Ending Balance, Shares at Mar. 31, 2019			85,867,028
Stockholders Equity, Beginning Balance at Dec. 31, 2018	$ 311,338		$ 10,924	587,490	3,163	(290,239)
Stockholders Equity, Beginning Balance, Shares at Dec. 31, 2018			85,865,557
Issuance of ordinary shares from exercise of share options, Shares	534,360
Unrealized gain on marketable securities	$ 108
Other comprehensive income (loss)	1,716
Net loss attributable to ordinary shareholders	(81,269)
Stockholders Equity, Ending Balance at Jun. 30, 2019	371,157		$ 12,234	725,552	4,879	(371,508)
Stockholders Equity, Ending Balance, Shares at Jun. 30, 2019			96,185,520
Stockholders Equity, Beginning Balance at Mar. 31, 2019	287,475		$ 10,924	591,315	6,214	(320,978)
Stockholders Equity, Beginning Balance, Shares at Mar. 31, 2019			85,867,028
Share-based compensation expense	4,630			4,630
Issuance of ordinary shares from exercise of share options	573		$ 69	504
Issuance of ordinary shares from exercise of share options, Shares			532,889
Issuance of ESPP shares	693		$ 8	685
Issuance of ESPP shares, Shares			60,335
Issuance of ADSs in follow-on offering, net of issuance costs	129,651		$ 1,233	128,418
Issuance of ADSs in follow-on offering, net of issuance costs, Shares			9,725,268
Unrealized gain on marketable securities	108
Other comprehensive income (loss)	(1,335)				(1,335)
Net loss attributable to ordinary shareholders	(50,530)					(50,530)
Stockholders Equity, Ending Balance at Jun. 30, 2019	$ 371,157		$ 12,234	$ 725,552	$ 4,879	$ (371,508)
Stockholders Equity, Ending Balance, Shares at Jun. 30, 2019			96,185,520

Nature of the Business

6 Months Ended

Jun. 30, 2019

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Nature of the Business

1. Nature of the Business

Orchard Therapeutics plc and its subsidiaries (the “Company”) is a commercial-stage fully-integrated biopharmaceutical company dedicated to transforming the lives of patients with serious and life threatening rare diseases through ex vivo autologous hematopoietic stem cell (“HSC”) based gene therapies. The Company’s gene therapy approach seeks to transform a patient’s own, or autologous, 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 ex vivo autologous HSC-based gene therapies focused on three franchises including primary immune deficiencies, neurometabolic disorders and hemoglobinopathies. The Company’s portfolio includes Strimvelis®, a gammaretroviral vector-based gene therapy and the first such treatment approved by the European Medicines Agency (“EMA”) for adenosine deaminase severe combined immunodeficiency (“ADA-SCID”), three clinical programs in advanced registrational studies in metachromatic leukodystrophy (“MLD”), Wiskott-Aldrich syndrome (“WAS”) and ADA-SCID, other clinical programs in X-linked chronic granulomatous disease (“X-CGD”), transfusion-dependent beta-thalassemia (“TDT”), and mucopolysaccharidosis type I (“MPS-I”), as well as an extensive preclinical pipeline.

The Company is a public limited company incorporated pursuant to the laws of England and Wales. In November 2018, the Company completed its initial public offering (“IPO”) of American Depositary Shares (“ADS”) in which the Company sold an aggregate of 16,103,572 ADSs representing the same number of ordinary shares at a public offering price of $14.00 per ADS. Net proceeds were $205.5 million, after deducting underwriting discounts and commissions of $15.8 million and offering expenses of $4.2 million paid by the Company. In June 2019, the Company completed a follow-on public offering of ADSs in which the Company sold an aggregate of 9,725,268 ADSs representing the same number of ordinary shares at a public offering price of $14.25 per ADS. Net proceeds were $129.7 million, after deducting underwriting discounts and commissions of $8.3 million and offering expenses of $0.6 million paid or accrued by the Company.

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. As part of a corporate reorganization in October 2018, 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. Upon completion of the reorganization, the historical consolidated financial statements of Orchard Therapeutics (Europe) Limited became the historical consolidated financial statements of Orchard Therapeutics plc because the reorganization was accounted for as a reorganization of entities under common control.

On November 1, 2018, the Company’s ordinary shares and different classes of preferred 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 condensed consolidated financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect the reverse stock split.

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 or 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 June 30, 2019, the Company funded its operations primarily with proceeds from the sale of convertible preferred shares and ADSs in the IPO and follow-on public offering. The Company has incurred recurring losses since inception. As of June 30, 2019, the Company had an accumulated deficit of $371.5 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 may 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 of raising additional capital to finance operations, 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, cash equivalents, marketable securities, and restricted cash on hand as of June 30, 2019 of $423.4 million, will be sufficient to fund its operations and capital expenditure requirements through at least twelve months from the issuance date of these condensed consolidated financial statements on August 8, 2019.

Summary of Significant Accounting Policies

6 Months Ended

Jun. 30, 2019

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of presentation

The condensed consolidated interim financial statements of the Company are unaudited and have been prepared in accordance with generally accepted accounting principles in the United States of America (“U.S. GAAP”) for interim financial reporting and in accordance with Regulation S-X, Rule 10-01. Accordingly, they do not include all of the information and footnotes required by U.S. GAAP for complete financial statements. Any reference in these notes to applicable guidance is meant to refer to the authoritative U.S. GAAP as found in the Accounting Standards Codification (“ASC”), and Accounting Standards Update (“ASU”), of the Financial Accounting Standards Board (“FASB”). All significant intercompany accounts and transactions between the Company and its subsidiaries have been eliminated upon consolidation.

The accompanying unaudited condensed consolidated interim financial statements should be read in conjunction with the audited consolidated financial statements and accompanying notes included in the Company’s Annual Report on Form 20-F filed with the SEC on March 22, 2019 (the “Annual Report”). The balance sheet as of December 31, 2018 was derived from audited consolidated financial statements included in the Company’s Annual Report but does not include all disclosures required by U.S. GAAP.

Certain information and footnote disclosures normally included in financial statements prepared in accordance with U.S. GAAP have been condensed or omitted from these interim financial statements. However, these interim financial statements include all adjustments, consisting only of normal recurring adjustments, which are, in the opinion of the Company’s management, necessary to fairly state the results of the interim period. The interim results are not necessarily indicative of results to be expected for the full year.

Amortization of the Stimvelis loss provision in the condensed consolidated statement of cash flows for the six months ended June 30, 2018 previously included in changes in accrued expenses and other liabilities has been presented as a separate line item within operating cash flows in the condensed consolidated statement of cash flows to conform to current period presentation.

Use of estimates

The preparation of condensed 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 condensed consolidated financial statements and the reported amounts of revenue and expenses during the reporting periods. Significant estimates and assumptions reflected in these condensed consolidated financial statements include, but are not limited to, the accrual for research and development expenses, the research and development tax credit receivable, the Strimvelis loss provision, 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 finances 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 an unrealized foreign currency transaction gain of $1.6 million and $2.1 million for the three months ended June 30, 2019 and 2018, respectively, which is included in other income (expense) in the condensed consolidated statements of operations and comprehensive loss. The Company recorded an unrealized foreign currency transaction loss of $1.9 million and gain of $0.4 million for the six months ended June 30, 2019 and 2018, respectively, which is included in other income (expense) in the condensed consolidated statements of operations and comprehensive loss.

The results of operations for subsidiaries, the functional currency of which 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.

Marketable securities

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

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 the Company’s condensed consolidated balance sheet. The Company has an outstanding letter of credit for $3.0 million associated with a lease, and is required to hold this amount in a standalone bank account, as of June 30, 2019 and 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 June 30, 2019 and December 31, 2018. 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 condensed consolidated statements of cash flows The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported in the condensed consolidated balance sheet that sum to the total of the amounts reported in the unaudited condensed consolidated statement of cash flows:.

	June 30,		December 31,
	2019		2018
	(in thousands)
Cash and cash equivalents	$	132,783	$	335,844
Restricted cash		3,843		3,837
Total cash, cash equivalents and restricted cash shown in the statement of cash flows	$	136,626	$	339,681

Property and equipment

Property and equipment are recorded at cost and depreciated or amortized using the straight-line method over the following estimated useful lives. Construction-in-process assets are not depreciated until they are placed into service.

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 June 30, 2019, the Company’s property and equipment consisted of furniture and fixtures, office and computer equipment, lab equipment, leasehold improvements, and construction-in-process. 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.

The Company evaluates assets for potential impairment when events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Recoverability is measured by comparing the book values of the assets to the expected future net undiscounted cash flows that the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the book values of the assets exceed their fair value. The Company has not recognized any impairment losses from inception through June 30, 2019.

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’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 had fixed assets of $2.0 million and $3.6 million located in the United Kingdom and United States, respectively, as of June 30, 2019, and $1.7 million and $3.8 million located in the United Kingdom and United States, respectively, as of December 31, 2018.

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, to the extent that reimbursement of the costs is deemed probable. Royalties associated with the Company’s research grants will be accrued when they become probable.

Research agreement costs and accruals

The Company has entered into various research and development-related agreements. 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.

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. The components of accumulated other comprehensive loss are detailed as follows (in thousands):

	Currency Translation		Unrealized Gain (Loss) on Investments		Accumulated Other Comprehensive Income (Loss)
Balance at December 31, 2018	$	3,163	$	—	$	3,163
Other comprehensive (loss) income, net of tax		1,608		108		1,716
Balance at June 30, 2019	$	4,771	$	108	$	4,879

Strimvelis loss provision

As part of the GSK transaction completed in April 2018 (see Note 9), the Company is required to use its best endeavors to make Strimvelis commercially available in the European Union until such time that an alternative gene therapy, such as the Company’s OTL-101 product candidate, is commercially available for patients in the European Union, 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 initially 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 June 30, 2019, the total Strimvelis loss provision liability was $7.8 million. During the three and six months ended June 30, 2019 the Company amortized $1.1 million and $2.6 million as a credit to research and development expense, respectively. The effects of foreign currency translation for the three months and six months ended June 30, 2019 decreased the liability by $0.2 million and increased the liability by $0.1 million, respectively. During the three months and six months ended June 30, 2018 the Company amortized $1.4 million as a credit to research and development expense. The effects of foreign currency translation for the three months and six months ended June 30, 2018 decreased the liability by $1.2 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 research and development tax credit (“SME”) program and the Research and Development Expenditure (“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.

Based on criteria established by HM Revenue and Customs (“HMRC”), management of the Company expects a proportion of expenditures being incurred in relation to its pipeline research, clinical trials management and manufacturing development activities to be eligible for research and development tax credits for the 2019 fiscal year. The Company has qualified under the more favorable SME regime for the year ended December 31, 2018 and expects to qualify under the SME regime for the year ending December 31, 2019.

The RDEC and SME credits are not dependent on the Company generating future taxable income or on the ongoing tax status or tax position of the Company. The Company has assessed its research and development activities and expenditures to determine which activities and expenditures are likely to be eligible under the research and development incentive program described above. At each period end, the Company estimates the reimbursement available to the Company based on available information at the time.

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

The Company has recorded a United Kingdom research and development tax credit as an offset to research and development expense in the condensed consolidated statements of operations and comprehensive loss of $4.7 million and $2.2 million for the three months ended June 30, 2019 and 2018, respectively, and $9.9 million and $3.6 million for the six months ended June 30, 2019 and 2018, respectively. As of June 30, 2019, and December 31, 2018, the Company’s tax incentive receivable from the United Kingdom government was $20.3 million, of which $10.6 million was classified as current and $9.7 million was classified as long-term, and $10.6 million, respectively. The effects of foreign currency translation for the three months and six months ended June 30, 2019 decreased the receivable by $0.5 million and $0.2 million, respectively. As of June 30, 2019, these amounts have not yet been paid to the Company by HMRC.

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 that were outstanding in 2018 prior to conversion in the IPO contractually entitled the holders of such shares to participate in dividends but did not contractually require the holders of such shares to participate in losses of the Company. Accordingly, in periods in which there were convertible shares outstanding and the Company reported a net loss attributable to ordinary shareholders, such losses were 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 three months and six months ended June 30, 2019 and 2018.

Recent Accounting Pronouncements

Under the Jumpstart our Business Startups Act, or the JOBS Act, we qualify as an emerging growth company (“EGC”). However, we will no longer qualify as an EGC after December 31, 2019. While we maintain EGC status, we have elected to use the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act.

Recently adopted accounting pronouncements

In August 2018, the FASB issued ASU 2018-15, Intangibles–Goodwill and Other–Internal Use Software (Subtopic 350-40), Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement that is a Service Contract (“ASU 2018-15”) requiring a customer in a cloud computing arrangement that is a service contract to follow the internal use software guidance in ASC 350-402 to determine which implementation costs to capitalize as assets. The guidance is effective for the Company in annual periods beginning after December 15, 2019, and interim periods within those annual periods. The Company has the option to apply the guidance prospectively to all implementation costs incurred after the date of adoption or retrospectively. The new guidance requires certain disclosures in the interim and annual period of adoption. The Company adopted this standard as of January 1, 2019. The adoption of this guidance did not have a material impact on the condensed consolidated financial statements due to limited use in its operations of cloud computing arrangements that are service contracts.

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.

Recently issued accounting pronouncements not yet adopted

In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement (“ASU 2018-13”), which removes disclosures that are no longer considered cost beneficial, clarifies the specific requirements of disclosure and add disclosure requirements identified as relevant. This guidance is effective for annual and interim periods beginning after December 15, 2019 and early adoption is permitted. The Company does not expect that the adoption of this standard will have a material impact on the Company’s financial position, results of operations and cash flows.

In February 2016 and January 2018, the FASB issued ASU No. 2016-02, Leases (Topic 842) (“ASU 2016-02”) and ASU 2018-01, Leases (Topic 842), (“ASU 2018-01”), which requires that all lessees recognize the assets and liabilities that arise from leases on the condensed consolidated balance sheet and disclose qualitative and quantitative information about its leasing arrangements. When we no longer qualify as an EGC, this guidance will be effective for the Company for annual reporting periods beginning after December 15, 2018, including interim periods within those annual periods, and is required to be applied using a modified retrospective approach with an option to recognize the cumulative effect of applying the new standard as an adjustment to the opening balance of retained earnings on the date of adoption. Early adoption is permitted. The company expects that adoption of this standard will result in the recognition of material right-of-use assets and lease liabilities on the Company’s condensed consolidated balance sheets. While the Company is continuing to assess all potential impacts of this standard on its condensed consolidated financial statements and related disclosures, upon adoption the Company expects that the most significant impact of this standard on its condensed consolidated balance sheets will relate to the accounting for its lease agreements for laboratory, manufacturing, and office space, particularly with respect to the Company’s lease for office and manufacturing space in Fremont, California.

Fair Value Measurements and Marketable Securities

6 Months Ended

Jun. 30, 2019

Fair Value Disclosures [Abstract]

Fair Value Measurements and Marketable Securities

3. Fair Value Measurements and Marketable Securities

The following tables present information about the Company’s financial assets that have been measured at fair value as of June 30, 2019 and indicate the fair value of the hierarchy of the valuation inputs utilized to determine such fair value. In general, fair values determined by Level 1 inputs utilize quoted prices (unadjusted) in active markets for identical assets or liabilities. Fair value determined by Level 2 inputs utilize observable inputs other than Level 1 prices, such as quoted prices for similar assets or liabilities, quoted market prices in markets that are not active or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the related assets or liabilities. Fair values determined by Level 3 inputs are unobservable data points for the asset or liability, and include situations where there is little, if any, market activity for the asset or liability. During the three months and six months ended June 30, 2019, there were no transfers between Level 1 and Level 2 financial assets.

The following table summarizes the Company’s cash equivalents and marketable securities as of June 30, 2019, in thousands:

	Fair Value Measurements at June 30, 2019 Using:
	Level 1		Level 2		Level 3		Total
Cash Equivalents
Money market funds	$	575	$	—	$	—	$	—
U.S. government securities		—		22,587		—		—
Corporate bonds		—		5,806		—		—
Commercial paper				37,433
Total cash equivalents	$	575	$	65,826	$	—	$	—
Marketable securities
Corporate bonds				79,765
Commercial paper				206,983
Total marketable securities		—		286,748		—		—
Total	$	575	$	352,574	$	—	$	—

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

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

Marketable Securities

The following table summarizes the Company’s marketable securities as of June 30, 2019, in thousands:

	At June 30, 2019
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
U.S. government securities	$	22,587	$	—	$	—		$	22,587
Corporate bonds		212,708		171		(90	)		212,789
Commercial paper		117,171		60		(33	)		117,198
Total	$	352,466	$	231	$	(123	)	$	352,574

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

The following table summarizes the Company’s available-for-sale debt securities by contractual maturity, as of June 30, 2019, in thousands:

	At June 30, 2019
	Amortized Cost
Due in one year	$	295,232
Due after one year through three years		57,342
Total	$	352,574

- 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

Property and Equipment, Net

6 Months Ended

Jun. 30, 2019

Property Plant And Equipment [Abstract]

Property and Equipment, Net

4. Property and Equipment, Net

Property and equipment, net consisted of the following (in thousands):

	June 30,			December 31,
	2019			2018
Property and equipment:
Lab equipment	$	5,090		$	4,689
Leasehold improvements		1,759			1,487
Furniture and fixtures		505			403
Office and computer equipment		153			152
Construction-in-process		272			241
Property and equipment	$	7,779		$	6,972
Less: accumulated depreciation		(2,230	)		(1,496	)
Property and equipment, net	$	5,549		$	5,476

Depreciation expense was $0.4 million and $0.4 million for the three months ended June 30, 2019 and 2018, respectively. Depreciation expense was $0.8 million and $0.5 million for the six months ended June 30, 2019 and 2018, respectively.

Accrued Expenses and Other Liabilities

6 Months Ended

Jun. 30, 2019

Accrued Liabilities And Other Liabilities [Abstract]

Accrued Expenses and Other Liabilities

5. Accrued Expenses and Other Liabilities

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

	June 30,		December 31,
	2019		2018
Accrued external research and development expenses	$	11,893	$	12,738
Accrued payroll and related expenses		7,352		7,372
Accrued professional fees		1,316		1,186
Accrued other		1,814		2,762
Strimvelis liability - current portion		2,647		4,170
Due to UCLA		1,552		1,552
Total accrued expenses and other liabilities	$	26,574	$	29,780

Notes Payable

6 Months Ended

Jun. 30, 2019

Debt Disclosure [Abstract]

Notes Payable

6. Notes Payable

On May 24, 2019, the Company entered into a senior term facilities agreement (the “Credit Facility”) with MidCap Financial (Ireland) Limited (“MidCap Financial”), as agent, and additional lenders from time to time (together with MidCap Financial, the “Lenders”), to borrow up to $75.0 million in term loans. The Company concurrently borrowed $25.0 million under an initial term loan. The remaining $50.0 million under the Credit Facility may be drawn down in the form of a second and third term loan, the second term loan being a $25.0 million term loan available no earlier than September 30, 2019 and no later than December 31, 2020 upon submission of certain regulatory filings and evidence of the Company having $100 million in cash and cash equivalent investments; and the third term loan being a $25.0 million term loan available no earlier than July 1, 2020 and no later than September 30, 2021 upon certain regulatory approvals and evidence of the Company having $125 million in cash and cash equivalent investments.

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

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

As of June 30, 2019, notes payable consist of the following (in thousands):

	June 30,		December 31,
	2019		2018
Notes payable	$	24,477	$	—
Accretion related to final payment		24		—
Notes payable, long term	$	24,501	$	—

As of June 30, 2019, the estimated future principal payments due are as follows (in thousands):

	Aggregate Minimum Payments
2019	$	—
2020		—
2021		4,861
2022		8,333
2023		8,334
Thereafter		4,597
Total		26,125
Less unamortized portion of final payment		(1,101	)
Less unamortized debt issuance costs		(523	)
Notes payable, long term	$	24,501

During the three months and six months ended June 30, 2019, the Company recognized $0.2 million of interest expense related to the initial Term Loan. The effective annual interest rate as of June 30, 2019 on the outstanding debt under the Term Loan was approximately 12.0%.

Shareholders' Equity

6 Months Ended

Jun. 30, 2019

Stockholders Equity Note [Abstract]

Shareholders’ Equity

7. Shareholders’ Equity

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

In November 2018, the Company completed 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 of $15.8 million, and commissions and offering expenses paid by the Company of $4.2 million.

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

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

Additionally, as part of the corporate reorganization associated with 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 condensed consolidated financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect the effects of the redenomination of our ordinary shares.

Ordinary Shares

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. The Company has not declared any dividends since its inception. The Company has 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.

Share-Based Compensation

6 Months Ended

Jun. 30, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Share-Based Compensation

8. Share-Based Compensation

The Company maintains three equity compensation plans; the Orchard Therapeutics Limited Employee Share Option Plan with Non-Employee Sub-Plan and U.S. Sub-Plan (the “2016 Plan”), the Orchard Therapeutics plc 2018 Share Option and Incentive Plan (the “2018 Plan”), and the 2018 Employee Share Purchase Plan (the “ESPP”). The board of directors has determined not to make any further awards under the 2016 plan following the Company’s IPO. On March 22, 2019, pursuant to the evergreen provisions in the 2018 Plan and the ESPP, the Company increased (i) the number of ordinary shares available for issuance under the 2018 Plan by 4,293,278, and (ii) the number of ordinary shares available for issuance under the ESPP by 858,656. As of June 30, 2019, 4,893,743 shares remained available for grant under the 2018 Plan, and 1,645,629 shares remained available for grant under the ESPP.

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

Share Options

The following table summarizes option activity under the plans for six months ended June 30, 2019 (in thousands except share and per share amounts):

	Number of Options			Weighted Average Exercise Price		Weighted Average Remaining Contractual Life (in years)		Aggregate Intrinsic Value
Outstanding at December 31, 2018		10,203,432		$	3.04		8.97	$	129,551
Granted		3,619,587			13.82
Exercised		(534,360	)		1.08
Forfeited		(641,826	)		7.20
Outstanding at June 30, 2019		12,646,833		$	5.92		8.79	$	105,528
Vested and expected to vest, as of June 30, 2019		3,569,306		$	2.56		8.23	$	40,781

The aggregate intrinsic value of share options is calculated as the difference between the exercise price of the share options and the fair value of the Company’s ordinary shares for those share options that had exercise prices lower than the fair value of the Company’s ordinary shares at the reporting date.

The weighted-average grant date fair value of share options granted during the six month period ended June 30, 2019 was $8.85 per share. All share options granted have a term of 10 years.

Restricted Share Units

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

The maximum aggregate total fair value of the performance-based RSUs is $11.0 million. The fair value associated with the shares that could vest based on the market-based condition is being recognized as expense over an average derived service period of 1.4 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 June 30, 2019, none of the regulatory and research and development milestones were deemed probable.

The following table summarizes award activity for the six months ended June 30, 2019:

	Shares			Weighted Average Grant Date Fair Value
Unvested and outstanding at December 31, 2018		219,922		$	15.48
Granted		394,250			12.30
Vested		—			—
Forfeited		(18,750	)		11.17
Unvested and outstanding at June 30, 2019		595,422		$	13.51

The amount of compensation cost recognized for the six months ended June 30, 2019 and 2018 for the market condition associated with the performance-based RSUs was $0.6 million and nil, respectively.

Share-based compensation

Share-based compensation expense recorded as research and development and general and administrative expenses is as follows (in thousands):

	June 30,		June 30,
	2019		2018
Research and development	$	2,970	$	674
General and administrative		5,481		1,576
Total share-based compensation	$	8,451	$	2,250

The Company had 9,077,527 unvested options outstanding as of June 30, 2019. As of June 30, 2019, total unrecognized compensation cost related to unvested stock option grants was approximately $53.6 million. This amount is expected to be recognized over a weighted average period of approximately 2.9 years. As of June 30, 2019, the total unrecognized compensation cost related to performance-based RSUs is a maximum of $10.3 million, dependent upon achievement of the milestones.

License Agreements

6 Months Ended

Jun. 30, 2019

License Agreements [Abstract]

License Agreements

9. License Agreements

GSK asset purchase and license agreement

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

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

•	One earlier stage clinical gene therapy program for TDT;

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

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

The Company accounted for the GSK Agreement as an asset acquisition, since the asset purchase and licensing arrangement did not meet the definition of a business pursuant to ASC 805, Business Combinations. Total consideration of £94.2 million ($133.6 million 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), a loss contract on the Strimvelis program valued at £12.9 million ($18.4 million), an inventory purchase liability valued at £4.9 million ($6.9 million) and transaction costs of £0.6 million ($0.8 million). The Company allocated £94.2 million ($133.6 million) to in-process research and development expense (based on the fair value of the underlying programs in development). The 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 Priority Review Voucher (“PRV”) from the United States Food and Drug Administration for each of the programs for MLD, WAS and TDT, the first of which GSK retained beneficial ownership over. GSK also has an option to acquire, at a price pursuant to an agreed upon formula, any PRV granted to the Company thereafter for MLD, WAS and TDT. If GSK does not exercise this option to purchase any PRV, the Company may sell the PRV to a third party and must share any proceeds in excess of a specified sale price equally with GSK. For accounting purposes, as of June 30, 2019, the Company does not consider the attainment of a PRV from the United States Food and Drug Administration to be probable.

As part of the GSK Agreement the Company is required to use its best endeavors to make Strimvelis commercially available in the European Union until such time as an alternative gene therapy, such as 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. As of June 30, 2019, the total Strimvelis loss provision liability was $7.8 million. During the three and six months ended June 30, 2019 the Company amortized $1.1 million and $2.6 million as a credit to research and development expense, respectively. The effects of foreign currency translation for the three months and six months ended June 30, 2019 decreased the liability by $0.2 million and increased the liability by $0.1 million, respectively

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

The Company and GSK also separately executed a Transition Services Agreement (“TSA”) as well as an Inventory Sale Agreement, both effective April 11, 2018. The TSA outlined several activities that the Company had requested GSK to assist with during the transition period, including but not limited to utilizing GSK to sell, market and distribute Strimvelis, and assist with regulatory, clinical and non-clinical activities for the other non-commercialized products which were ongoing at the date of the GSK Agreement. The TSA expired in December 2018.

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

Telethon-OSR research and development collaboration and license agreements

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

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

In May 2019, the Company entered into a license agreement with Telethon-OSR, under which Telethon-OSR granted to the Company an exclusive worldwide license for the research, development, manufacture and commercialization of Telethon-OSR’s ex vivo autologous HSC lentiviral based gene therapy for the treatment of mucopolysaccharidosis type I (“MPS-I”), including the Hurler variant. Under the terms of the agreement, Telethon-OSR is entitled to receive €15.0 million in upfront and milestone payments from the Company. The Company is also required to make milestone payments contingent upon certain development, regulatory and commercial milestones are achieved. Additionally, the Company will be required to pay Telethon a tiered mid-single to low-double digit royalty percentage on annual net sales of licensed products. For the three and six months ended June 30, 2019, the Company has recorded $17.2 million as in-process research and development expense associated with the upfront and milestone payments.

UCLB/UCLA License Agreement

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

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

Under the UCLB/UCLA License Agreement, the Company is also obligated to pay an annual administration fee of $0.1 million on the first, second and third anniversary of the agreement date. Additionally, the Company 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.

The Company recorded $0.1 million of research and development costs in respect of the UCLB/UCLA license agreement, which comprise the upfront payments, issuance of ordinary shares and payments for clinical data, for the six months ended June 30, 2019 and 2018, respectively.

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

Oxford BioMedica license, development and supply agreement

In November 2016, and amended in September 2018, the Company entered into an arrangement with Oxford BioMedica whereby Oxford BioMedica granted an exclusive intellectual property license to the Company for the purposes of research, development, and commercialization of collaboration products, and will provide process development services, and manufacture clinical and commercial GMP-grade lentiviral vectors 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. No milestones were met during the three months and six months ended June 30, 2019 and 2018.

The Company recorded $0.5 million to research and development expense upon execution of the Oxford BioMedica Agreement in 2016 and $0.1 million upon achievement of the first development milestone in 2017. The Company recorded $1.4 million upon achievement of the second and third development milestones in 2018. The expense recognized in 2016 and 2017 was determined based on the ordinary shares’ fair value as of the time the agreement was executed. The expense recognized in 2018 was determined based on the ordinary shares’ fair value as of the time the agreement was modified in September 2018. There was no expense recorded in the three months and six months ended June 30, 2019 as no milestones were met during the period.

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 a low single digit percentage royalties 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 interim 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 six months ended June 30, 2019 and 2018, the Company recorded nil and $2.4 million as a reduction of research and development expenses related to the UCLA Research Agreements. As of June 30, 2019, and 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.

Income Taxes

6 Months Ended

Jun. 30, 2019

Income Tax Disclosure [Abstract]

Income Taxes

10. Income Taxes

Deferred tax assets and deferred tax liabilities are determined based on temporary differences between the financial reporting and tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. A valuation allowance is recorded against deferred tax assets if it is more likely than not that some portion or all of the deferred tax assets will not be realized.

The Company does not recognize a tax benefit for uncertain tax positions unless it is more likely than not that the position will be sustained upon examination by tax authorities, including resolution of any related appeals or litigation processes, based on the technical merits of the position. The tax benefit that is recorded for these positions is measured at the largest amount of cumulative benefit that has greater than a 50 percent likelihood of being realized upon ultimate settlement. Deferred tax assets that do not meet these recognition criteria are not recorded and the Company recognizes a liability for uncertain tax positions that may result in tax payments. If such unrecognized tax benefits were realized and not subject to valuation allowances, the entire amount would impact the tax provision. The Company has not recorded any amounts for unrecognized tax positions as of June 30, 2019 or December 31, 2018.

The Company recognized an income tax benefit of $0.8 million and $0.2 million for the three months and six months ended June 30, 2019, respectively. The Company recognized an income tax benefit of $0.1 million and $0.2 million for the three months and six months ended June 30, 2018, respectively. The benefit for income taxes consists of current and deferred tax expenses, which relates primarily to the Company’s subsidiary operations in the U.S.

Net Loss Per Share

6 Months Ended

Jun. 30, 2019

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 (in thousands, except per share and share amounts):

	Six Months Ended June 30,						Three Months Ended June 30,
	2019			2018			2019			2018
Net loss	$	(81,269	)	$	(171,544	)	$	(50,530	)	$	(156,233	)
Net loss attributable to ordinary shareholders	$	(81,269	)	$	(171,544	)	$	(50,530	)	$	(156,233	)
Weighted average ordinary shares outstanding, basic and diluted		88,369,311			10,095,863			89,712,916			10,115,335
Net loss per share attributable to ordinary shareholders, basic and diluted	$	(0.92	)	$	(16.99	)	$	(0.56	)	$	(15.45	)

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:

	Six Months Ended June 30,
	2019		2018
Convertible preferred shares		—		46,226,426
Share options		10,972,582		7,258,870
Unvested performance-based restricted share units		595,422		—
		11,568,004		53,485,296

Commitments and Contingencies

6 Months Ended

Jun. 30, 2019

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 had a term of 5 years and originally terminated in October 2021. The annual rental expense approximated $0.2 million. The Company was provided with one month of free rent upon inception of the lease. In June 2019, the Company assigned the lease to a third-party and were relieved of future payment obligations under the lease. Costs associated with the termination of the lease were not material.

In November 2017, the Company entered into a lease agreement for laboratory space in Menlo Park, California, United States. The lease terminates 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 office space in London, United Kingdom, which 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, which terminates in January 2023. The annual rental expense approximates $0.1 million.

In January 2019, the Company leased additional office and laboratory space in Menlo Park, California, United States, which terminates in December 2020. The annual rental expense approximates $0.1 million.

The Company recorded rent expense totaling $1.3 million and $2.5million for the three months and six months ended June 30, 2019, respectively. The Company recorded rent expense totaling $0.7 million and $1.2 million for the three months and six months ended June 30, 2018, respectively.

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 condensed 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 June 30, 2019, the Company has not broken ground or incurred significant soft costs associated with the construction. 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 June 30, 2019, the Company has no funds deposited in the escrow account.

Other funding commitments

The Company has entered into several license agreements (Note 9). The Company’s obligations in connection with these agreements include requirements to pay royalties on future sales of specified products, make annual license maintenance payments and make payments upon the achievement of certain milestones not met as of June 30, 2019 and December 31, 2018. As of June 30, 2019, the Company had $16.9 million in accounts payable for upfront and milestone payments associated with our licensing agreements.

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 June 30, 2019, and December 31, 2018, there was nil and $0.8 million included within prepayments related to prepaid instalments against these minimum commitments. The Company is committed to make further payments totaling $7.8 million between April 2019 and March 2021.

Legal proceedings

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

Employee Benefit Plans

6 Months Ended

Jun. 30, 2019

Compensation And Retirement Disclosure [Abstract]

Employee Benefit Plans

13. Employee Benefit Plans

The Company makes contributions to private defined contribution employee benefit plans on behalf of its employees. The Company provides employee contributions up to six percent of each employee’s annual salary based on the jurisdiction the employees are located. The Company paid $0.3 million and $0.6 million in matching contributions for the three and six months ended June 30, 2019, respectively. The Company paid $0.1 million and $0.2 million in matching contributions for the three and six months ended June 30, 2018, respectively.

Related Party Transactions

6 Months Ended

Jun. 30, 2019

Related Party Transactions

14. Related Party Transactions

GSK

In April 2018, the Company completed 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 7). As consideration for the agreement the Company paid an upfront fee of $14.2 million, incurred an inventory purchase liability of $6.9 million, paid $0.8 million in transaction costs, 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 agreement, as the costs to maintain Strimvelis are expected to significantly exceed revenues. The issuance of the convertible preferred shares made GSK a principal shareholder in the Company.

During the six months ended June 30, 2019, the Company made $7.2 million in payments to settle accounts payable due to GSK associated with the TSA and royalties associated with sales of Strimvelis incurred during 2018. Additionally, during the six months ended June 30, 2019, the Company made a $1.7 million payment associated with the inventory purchase liability incurred upon entering into the agreement. As of June 30, 2019, and December 31, 2018, the Company had inventory purchase liability in accrued research and development expenses of $3.2 million and $6.2 million, respectively. During the three months and six months ended June 30, 2019 there were no sales of Strimvelis and incurred no royalties due to GSK.

Summary of Significant Accounting Policies (Policies)

6 Months Ended

Jun. 30, 2019

Accounting Policies [Abstract]

Basis of presentation

Use of estimates

Concentration of credit risk

Foreign currency translation

Cash and cash equivalents

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

Marketable securities

Restricted cash

	June 30,		December 31,
	2019		2018
	(in thousands)
Cash and cash equivalents	$	132,783	$	335,844
Restricted cash		3,843		3,837
Total cash, cash equivalents and restricted cash shown in the statement of cash flows	$	136,626	$	339,681

Property and equipment

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

Segment information

Research and development costs

Research agreement costs and accruals

Share-based compensation

Comprehensive loss

Comprehensive Loss

	Currency Translation		Unrealized Gain (Loss) on Investments		Accumulated Other Comprehensive Income (Loss)
Balance at December 31, 2018	$	3,163	$	—	$	3,163
Other comprehensive (loss) income, net of tax		1,608		108		1,716
Balance at June 30, 2019	$	4,771	$	108	$	4,879

Strimvelis loss provision

Research and development income tax credit

Net income (loss) per share

The Company reported a net loss attributable to ordinary shareholders for the three months and six months ended June 30, 2019 and 2018.

Recent Accounting Pronouncements

Recently adopted accounting pronouncements

Recently issued accounting pronouncements not yet adopted

Summary of Significant Accounting Policies (Tables)

6 Months Ended

Jun. 30, 2019

Accounting Policies [Abstract]

Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash

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

	June 30,		December 31,
	2019		2018
	(in thousands)
Cash and cash equivalents	$	132,783	$	335,844
Restricted cash		3,843		3,837
Total cash, cash equivalents and restricted cash shown in the statement of cash flows	$	136,626	$	339,681

Schedule of Estimated Useful Lives of Property and Equipment

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

Components of Accumulated Other Comprehensive Loss

The components of accumulated other comprehensive loss are detailed as follows (in thousands):

	Currency Translation		Unrealized Gain (Loss) on Investments		Accumulated Other Comprehensive Income (Loss)
Balance at December 31, 2018	$	3,163	$	—	$	3,163
Other comprehensive (loss) income, net of tax		1,608		108		1,716
Balance at June 30, 2019	$	4,771	$	108	$	4,879

Fair Value Measurements and Marketable Securities (Tables)

6 Months Ended

Jun. 30, 2019

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

Schedule of Cash Equivalents and Marketable Securities

The following table summarizes the Company’s cash equivalents and marketable securities as of June 30, 2019, in thousands:

	Fair Value Measurements at June 30, 2019 Using:
	Level 1		Level 2		Level 3		Total
Cash Equivalents
Money market funds	$	575	$	—	$	—	$	—
U.S. government securities		—		22,587		—		—
Corporate bonds		—		5,806		—		—
Commercial paper				37,433
Total cash equivalents	$	575	$	65,826	$	—	$	—
Marketable securities
Corporate bonds				79,765
Commercial paper				206,983
Total marketable securities		—		286,748		—		—
Total	$	575	$	352,574	$	—	$	—

Schedule of Marketable Securities

The following table summarizes the Company’s marketable securities as of June 30, 2019, in thousands:

	At June 30, 2019
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
U.S. government securities	$	22,587	$	—	$	—		$	22,587
Corporate bonds		212,708		171		(90	)		212,789
Commercial paper		117,171		60		(33	)		117,198
Total	$	352,466	$	231	$	(123	)	$	352,574

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

The following table summarizes the Company’s available-for-sale debt securities by contractual maturity, as of June 30, 2019, in thousands:

	At June 30, 2019
	Amortized Cost
Due in one year	$	295,232
Due after one year through three years		57,342
Total	$	352,574

Property and Equipment, Net (Tables)

6 Months Ended

Jun. 30, 2019

Property Plant And Equipment [Abstract]

Schedule of Property and Equipment, Net

Property and equipment, net consisted of the following (in thousands):

	June 30,			December 31,
	2019			2018
Property and equipment:
Lab equipment	$	5,090		$	4,689
Leasehold improvements		1,759			1,487
Furniture and fixtures		505			403
Office and computer equipment		153			152
Construction-in-process		272			241
Property and equipment	$	7,779		$	6,972
Less: accumulated depreciation		(2,230	)		(1,496	)
Property and equipment, net	$	5,549		$	5,476

Accrued Expenses and Other Liabilities (Tables)

6 Months Ended

Jun. 30, 2019

Accrued Liabilities And Other Liabilities [Abstract]

Schedule of Accrued Expenses and Other Liabilities

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

	June 30,		December 31,
	2019		2018
Accrued external research and development expenses	$	11,893	$	12,738
Accrued payroll and related expenses		7,352		7,372
Accrued professional fees		1,316		1,186
Accrued other		1,814		2,762
Strimvelis liability - current portion		2,647		4,170
Due to UCLA		1,552		1,552
Total accrued expenses and other liabilities	$	26,574	$	29,780

Notes Payable (Tables)

6 Months Ended

Jun. 30, 2019

Debt Disclosure [Abstract]

Summary of Notes Payable

As of June 30, 2019, notes payable consist of the following (in thousands):

	June 30,		December 31,
	2019		2018
Notes payable	$	24,477	$	—
Accretion related to final payment		24		—
Notes payable, long term	$	24,501	$	—

Summary of Estimated Future Principal Payments Due

As of June 30, 2019, the estimated future principal payments due are as follows (in thousands):

	Aggregate Minimum Payments
2019	$	—
2020		—
2021		4,861
2022		8,333
2023		8,334
Thereafter		4,597
Total		26,125
Less unamortized portion of final payment		(1,101	)
Less unamortized debt issuance costs		(523	)
Notes payable, long term	$	24,501

Share-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Option Activity

The following table summarizes option activity under the plans for six months ended June 30, 2019 (in thousands except share and per share amounts):

	Number of Options			Weighted Average Exercise Price		Weighted Average Remaining Contractual Life (in years)		Aggregate Intrinsic Value
Outstanding at December 31, 2018		10,203,432		$	3.04		8.97	$	129,551
Granted		3,619,587			13.82
Exercised		(534,360	)		1.08
Forfeited		(641,826	)		7.20
Outstanding at June 30, 2019		12,646,833		$	5.92		8.79	$	105,528
Vested and expected to vest, as of June 30, 2019		3,569,306		$	2.56		8.23	$	40,781

Summary of Award Activity

The following table summarizes award activity for the six months ended June 30, 2019:

	Shares			Weighted Average Grant Date Fair Value
Unvested and outstanding at December 31, 2018		219,922		$	15.48
Granted		394,250			12.30
Vested		—			—
Forfeited		(18,750	)		11.17
Unvested and outstanding at June 30, 2019		595,422		$	13.51

Share-based Compensation Expense

Share-based compensation expense recorded as research and development and general and administrative expenses is as follows (in thousands):

	June 30,		June 30,
	2019		2018
Research and development	$	2,970	$	674
General and administrative		5,481		1,576
Total share-based compensation	$	8,451	$	2,250

Net Loss Per Share (Tables)

6 Months Ended

Jun. 30, 2019

Earnings Per Share [Abstract]

Computation of Basic and Diluted Net Loss Per Share

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

	Six Months Ended June 30,						Three Months Ended June 30,
	2019			2018			2019			2018
Net loss	$	(81,269	)	$	(171,544	)	$	(50,530	)	$	(156,233	)
Net loss attributable to ordinary shareholders	$	(81,269	)	$	(171,544	)	$	(50,530	)	$	(156,233	)
Weighted average ordinary shares outstanding, basic and diluted		88,369,311			10,095,863			89,712,916			10,115,335
Net loss per share attributable to ordinary shareholders, basic and diluted	$	(0.92	)	$	(16.99	)	$	(0.56	)	$	(15.45	)

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

	Six Months Ended June 30,
	2019		2018
Convertible preferred shares		—		46,226,426
Share options		10,972,582		7,258,870
Unvested performance-based restricted share units		595,422		—
		11,568,004		53,485,296

Nature of the Business - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended		3 Months Ended	6 Months Ended
	Nov. 02, 2018	Jun. 30, 2019	Nov. 30, 2018	Jun. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Payments of stock issuance costs				$ 620
Reverse stock split					one-for-0.8003
Accumulated deficit		$ 371,508		371,508	$ 371,508	$ 290,239
Cash, cash equivalents, marketable securities and restricted cash		423,400		$ 423,400	$ 423,400
American Depositary Shares \| Initial Public Offering
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Shares issued	16,103,572		16,103,572
Issue price per share	$ 14.00		$ 14.00
Proceeds from shares issued	$ 205,500
Payments of stock issuance costs	4,200		$ 4,200
American Depositary Shares \| Underwriter Discounts and Commissions
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Payments of stock issuance costs	$ 15,800	$ 600
American Depositary Shares \| Follow on Public Offering
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Shares issued		9,725,268
Issue price per share		$ 14.25		$ 14.25	$ 14.25
Proceeds from shares issued		$ 129,700
Payments of stock issuance costs		$ 8,300

Summary of Significant Accounting Policies - Additional Information (Details) £ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Apr. 30, 2018 USD ($)	Apr. 30, 2018 GBP (£)
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.
Unrealized foreign currency transaction gain (loss)	$ 1,600,000	$ 2,100,000	$ (1,900,000)	$ 400,000
Proceeds from lines of credit			3,000,000		$ 3,000,000
Cash collateral associated with corporate credit card accounts	900,000		900,000		900,000
Impairment losses			0
Fixed assets	5,549,000		5,549,000		5,476,000
Tax incentive receivable, current	10,594,000		10,594,000		10,585,000
Tax incentive receivable, long term	9,731,000		9,731,000
Increase (decrease) in tax receivable	(500,000)		(200,000)
GSK Asset Purchase and License Agreement
Summary Of Significant Accounting Policies [Line Items]
Liability associated with the loss contract	18,400,000		18,400,000
Remaining liability	7,800,000		7,800,000			$ 18,400,000	£ 12.9
Amortization as credit to research and development expense	1,100,000	1,400,000	2,600,000	1,400,000
Increase (decrease) in liability in effect of foreign exchange translation	(200,000)	(1,200,000)	100,000	(1,200,000)
United Kingdom
Summary Of Significant Accounting Policies [Line Items]
Fixed assets	2,000,000		2,000,000		1,700,000
Tax incentive receivable	20,300,000		20,300,000		10,600,000
Tax incentive receivable, current	10,600		10,600
Tax incentive receivable, long term	9,700		9,700
United Kingdom \| Research and Development Expense
Summary Of Significant Accounting Policies [Line Items]
Tax credit	4,700,000	$ 2,200,000	9,900,000	$ 3,600,000
United States
Summary Of Significant Accounting Policies [Line Items]
Fixed assets	$ 3,600,000		$ 3,600,000		$ 3,800,000

Summary of Significant Accounting Policies - Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Summary Of Significant Accounting Policies [Abstract]
Cash and cash equivalents	$ 132,783	$ 335,844
Restricted cash	3,843	3,837
Total cash, cash equivalents and restricted cash shown in the statement of cash flows	$ 136,626	$ 339,681	$ 48,762	$ 89,856

Summary of Significant Accounting Policies - Schedule of Estimated Useful Lives of Property and Equipment (Details)	6 Months Ended
	Jun. 30, 2019
Lab Equipment \| Minimum
Property Plant And Equipment [Line Items]
Property and equipment useful life	5 years
Lab Equipment \| Maximum
Property Plant And Equipment [Line Items]
Property and equipment useful life	10 years
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment useful life, term	Shorter of lease term or estimated useful life
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment useful life	4 years
Office and Computer Equipment \| Minimum
Property Plant And Equipment [Line Items]
Property and equipment useful life	3 years
Office and Computer Equipment \| Maximum
Property Plant And Equipment [Line Items]
Property and equipment useful life	5 years

Summary of Significant Accounting Policies - Components of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Accumulated Other Comprehensive Income Loss [Line Items]
Stockholders Equity, Beginning Balance	$ 287,475	$ 311,338	$ 77,880	$ 86,405	$ 311,338	$ 86,405
Other comprehensive (loss) income, net of tax	(1,335)	3,051	(1,463)	3,433	1,716	1,970
Stockholders Equity, Ending Balance	371,157	287,475	$ 14,746	$ 77,880	371,157	$ 14,746
Currency Translation
Accumulated Other Comprehensive Income Loss [Line Items]
Stockholders Equity, Beginning Balance		3,163			3,163
Other comprehensive (loss) income, net of tax					1,608
Stockholders Equity, Ending Balance	4,771				4,771
Unrealized Gain (Loss) on Investments
Accumulated Other Comprehensive Income Loss [Line Items]
Other comprehensive (loss) income, net of tax					108
Stockholders Equity, Ending Balance	108				108
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income Loss [Line Items]
Stockholders Equity, Beginning Balance		$ 3,163			3,163
Other comprehensive (loss) income, net of tax					1,716
Stockholders Equity, Ending Balance	$ 4,879				$ 4,879

Fair Value Measurements and Marketable Securities - Additional Information (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Fair value of financial assets transfers between level 1 to level 2	$ 0
Fair value of financial assets transfers between level 2 to level 1	0
Cash equivalents		$ 0
Marketable securities	$ 352,574	$ 0

Fair Value Measurements and Marketable Securities - Schedule of Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Cash Equivalents
Total cash equivalents		$ 0
Marketable securities
Total marketable securities	$ 352,574	$ 0
Level 1
Cash Equivalents
Total cash equivalents	575
Marketable securities
Total	575
Level 2
Cash Equivalents
Total cash equivalents	65,826
Marketable securities
Total marketable securities	286,748
Total	352,574
Money Market Funds \| Level 1
Cash Equivalents
Total cash equivalents	575
U.S. Government Securities
Marketable securities
Total marketable securities	22,587
U.S. Government Securities \| Level 2
Cash Equivalents
Total cash equivalents	22,587
Corporate Bonds
Marketable securities
Total marketable securities	212,789
Corporate Bonds \| Level 2
Cash Equivalents
Total cash equivalents	5,806
Marketable securities
Total marketable securities	79,765
Commercial Paper
Marketable securities
Total marketable securities	117,198
Commercial Paper \| Level 2
Cash Equivalents
Total cash equivalents	37,433
Marketable securities
Total marketable securities	$ 206,983

Fair Value Measurements and Marketable Securities - Schedule of Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 352,466
Gross Unrealized Gains	231
Gross Unrealized Losses	(123)
Fair Value	352,574	$ 0
U.S. Government Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	22,587
Fair Value	22,587
Corporate Bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	212,708
Gross Unrealized Gains	171
Gross Unrealized Losses	(90)
Fair Value	212,789
Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	117,171
Gross Unrealized Gains	60
Gross Unrealized Losses	(33)
Fair Value	$ 117,198

Fair Value Measurements and Marketable Securities - Schedule of Available-for-Sale Debt Securities by contractual Maturity (Details) $ in Thousands	Jun. 30, 2019 USD ($)
Amortized Cost
Due in one year	$ 295,232
Due after one year through three years	57,342
Total	$ 352,574

Property and Equipment, Net - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Property and equipment	$ 7,779	$ 6,972
Less: accumulated depreciation	(2,230)	(1,496)
Property and equipment, net	5,549	5,476
Lab Equipment
Property Plant And Equipment [Line Items]
Property and equipment	5,090	4,689
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment	1,759	1,487
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment	505	403
Office and Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment	153	152
Construction-In-Process
Property Plant And Equipment [Line Items]
Property and equipment	$ 272	$ 241

Property and Equipment, Net - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Property Plant And Equipment [Abstract]
Depreciation expense	$ 400	$ 400	$ 775	$ 504

Accrued Expenses and Other Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Accrued Liabilities And Other Liabilities [Abstract]
Accrued external research and development expenses	$ 11,893	$ 12,738
Accrued payroll and related expenses	7,352	7,372
Accrued professional fees	1,316	1,186
Accrued other	1,814	2,762
Strimvelis liability - current portion	2,647	4,170
Due to UCLA	1,552	1,552
Total accrued expenses and other liabilities	$ 26,574	$ 29,780

Notes Payable - Additional Information (Details) - USD ($) $ in Millions		3 Months Ended	6 Months Ended
	May 24, 2019	Jun. 30, 2019	Jun. 30, 2019
Debt Instrument [Line Items]
Line of credit facility agreement date	May 24, 2019
Credit facility maximum borrowings	$ 75.0
Credit facility, remaining borrowings	$ 50.0
Line of credit facility interest payments term	24 months
Line of credit facility amortization term	36 months
Line of credit facility, frequency of payments	monthly payments
Line of credit facility, percentage of final payment	4.50%
Final payment amount	$ 1.1
Line of credit facility interest rate description	Each term loan under the Credit Facility bears interest at an annual rate equal to 6% plus LIBOR. The Borrower is required to make interest-only payments on the term loan for all payment dates prior to 24 months following the date of the Credit Facility, unless the third tranche is drawn, in which case for all payment dates prior to 36 months following the date of the Credit Facility. The term loans under the Credit Facility will begin amortizing on either the 24-month or the 36-month anniversary of the Credit Facility (as applicable), with equal monthly payments of principal plus interest to be made by the Borrower to the Lenders in consecutive monthly installments until the Loan Maturity Date. In addition, a final payment of 4.5% is due on the Loan Maturity Date.
Line of credit facility covenants description	The Company is also subject to an ongoing minimum cash financial covenant in which the Company must maintain unrestricted cash in an amount not less than $20.0 million following the utilization of the second term loan and not less than $35.0 million following the utilization of the third term loan
Term Loan
Debt Instrument [Line Items]
Debt instrument, interest expense		$ 0.2	$ 0.2
Debt instrument, effective annual interest rate		12.00%
LIBOR
Debt Instrument [Line Items]
Debt instrument, annual interest rate	6.00%
Minimum \| Second Tranche (Term Loan Available from September 30, 2019 to December 31, 2020)
Debt Instrument [Line Items]
Line of credit facility minimum cash financial covenant	$ 20.0
Minimum \| Third Tranche (Term Loan Available from July 1, 2020 to September 30, 2021)
Debt Instrument [Line Items]
Line of credit facility minimum cash financial covenant	35.0
Initial Term Loan
Debt Instrument [Line Items]
Credit facility maximum borrowings	25.0
Second Tranche (Term Loan Available from September 30, 2019 to December 31, 2020)
Debt Instrument [Line Items]
Credit facility maximum borrowings	$ 25.0
Line of Credit Facility, Covenant Terms	upon submission of certain regulatory filings and evidence of the Company having $100 million in cash and cash equivalent investments
Second Tranche (Term Loan Available from September 30, 2019 to December 31, 2020) \| Minimum
Debt Instrument [Line Items]
Cash and cash equivalent investments	$ 100.0
Third Tranche (Term Loan Available from July 1, 2020 to September 30, 2021)
Debt Instrument [Line Items]
Credit facility maximum borrowings	$ 25.0
Line of Credit Facility, Covenant Terms	upon certain regulatory approvals and evidence of the Company having $125 million in cash and cash equivalent investments
Third Tranche (Term Loan Available from July 1, 2020 to September 30, 2021) \| Minimum
Debt Instrument [Line Items]
Cash and cash equivalent investments	$ 125.0

Notes Payable - Summary of Notes Payable (Details) $ in Thousands	Jun. 30, 2019 USD ($)
Long Term Notes And Loans [Abstract]
Notes payable	$ 24,477
Accretion related to final payment	24
Notes payable, long term	$ 24,501

Notes Payable - Summary of Estimated Future Principal Payments Due (Details) $ in Thousands	Jun. 30, 2019 USD ($)
Long Term Debt By Maturity [Abstract]
2021	$ 4,861
2022	8,333
2023	8,334
Thereafter	4,597
Total	26,125
Less unamortized portion of final payment	(1,101)
Less unamortized debt issuance costs	(523)
Notes payable, long term	$ 24,501

Shareholders' Equity - Additional Information (Details)		1 Months Ended		3 Months Ended	6 Months Ended
Shareholders' Equity - Additional Information (Details)	Nov. 02, 2018 USD ($) $ / shares shares	Jun. 30, 2019 USD ($) $ / shares shares	Nov. 30, 2018 USD ($) $ / shares shares	Jun. 30, 2019 USD ($) $ / shares	Jun. 30, 2019 USD ($) VotingRight $ / shares	Jun. 30, 2019 GBP (£) £ / shares	Dec. 31, 2018 £ / shares	Dec. 31, 2017 £ / shares
Class Of Stock [Line Items]
Payment of offering expenses				$ 620,000
Common Stock, Conversion Basis					one-for-0.8003
Ordinary Shares, Par Value \| £ / shares						£ 0.10	£ 0.10
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
Maximum
Class Of Stock [Line Items]
Ordinary shares maximum nominal value \| £						£ 13,023,851.50
Redesignation
Class Of Stock [Line Items]
Common Stock, Conversion Basis					one-for-one
Corporate Reorganization
Class Of Stock [Line Items]
Ordinary Shares, Par Value \| (per share)		$ 0.10		$ 0.10	$ 0.10			£ 0.00001
American Depositary Shares \| Initial Public Offering
Class Of Stock [Line Items]
Issuance of ordinary shares in initial public offering net of issuance, Shares \| shares	16,103,572		16,103,572
Issue price per share \| $ / shares	$ 14.00		$ 14.00
Issuance of ADRs in initial public offering, net of issuance costs			$ 205,500,000
Underwriting discounts and commissions			15,800,000
Payment of offering expenses	$ 4,200,000		$ 4,200,000
American Depositary Shares \| Follow-On Public Offering
Class Of Stock [Line Items]
Issuance of ordinary shares in initial public offering net of issuance, Shares \| shares		9,725,268
Issue price per share \| $ / shares		$ 14.25		$ 14.25	$ 14.25
Issuance of ADRs in initial public offering, net of issuance costs		$ 129,700,000
Underwriting discounts and commissions		8,300,000
Payment of offering expenses		8,300,000
Offering expenses including accrued		600,000
American Depositary Shares \| Underwriter Discounts and Commissions
Class Of Stock [Line Items]
Payment of offering expenses	$ 15,800,000	$ 600,000
Convertible Preferred Shares
Class Of Stock [Line Items]
Preferred stock conversion basis					one-for-0.8003

Share-Based Compensation - Additional Information (Details) $ / shares in Units, $ in Millions		6 Months Ended
	Mar. 22, 2019 shares	Jun. 30, 2019 USD ($) $ / shares shares	Jun. 30, 2019 £ / shares	Jun. 30, 2018 USD ($)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Exercise price of option \| £ / shares			£ 0.00001
Award vesting period		4 years
Maximum term of option		10 years
Weighted average grant date fair value of share options granted \| $ / shares		$ 8.85
Weighted average grant date fair value of share options granted, term		10 years
Compensation cost recognized \| $		$ 10.3
Unvested options outstanding		9,077,527
Total unrecognized compensation cost of stock option \| $		$ 53.6
Compensation cost expected to be recognized weighted average period		2 years 10 months 24 days
Performance-based Restricted Share Units ("RSUs")
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting period		1 year 4 months 24 days
Aggregate fair value of RSUs \| $		$ 11.0
Compensation cost recognized \| $		$ 0.6		$ 0.0
2018 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Increase in ordinary shares available for issuance	4,293,278
Shares available for grant		4,893,743
2018 ESPP
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Increase in ordinary shares available for issuance	858,656
Shares available for grant		1,645,629

Share-Based Compensation - Summary of Option Activity (Details) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Options, Outstanding at December 31, 2018 \| shares	10,203,432
Number of Options, Granted \| shares	3,619,587
Number of Options, Exercised \| shares	(534,360)
Number of Options, Forfeited \| shares	(641,826)
Number of Options, Outstanding at June 30, 2019 \| shares	12,646,833	10,203,432
Number of Options, Vested and expected to vest, as of June 30, 2019 \| shares	3,569,306
Weighted Average Exercise Price, Outstanding at December 31, 2018 \| $ / shares	$ 3.04
Weighted Average Exercise Price, Granted \| $ / shares	13.82
Weighted Average Exercise Price, Exercised \| $ / shares	1.08
Weighted Average Exercise Price, Forfeited \| $ / shares	7.20
Weighted Average Exercise Price, Outstanding at June 30,2019 \| $ / shares	5.92	$ 3.04
Weighted Average Exercise Price, Vested and expected to vest, as of June 30, 2019 \| $ / shares	$ 2.56
Weighted Average Remaining Contractual Life (in years), Outstanding at December 31, 2018	8 years 9 months 14 days	8 years 11 months 19 days
Weighted Average Remaining Contractual Life (in years), Vested and expected to vest, as of June 30, 2019	8 years 2 months 23 days
Aggregate Intrinsic Value, Outstanding \| $	$ 105,528	$ 129,551
Aggregate Intrinsic Value, Vested and expected to vest, as of June 30, 2019 \| $	$ 40,781

Share-Based Compensation - Summary of Award Activity (Details) - Performance-based Restricted Share Units ("RSUs")	6 Months Ended
	Jun. 30, 2019 $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares, Unvested and outstanding at December 31, 2018 \| shares	219,922
Shares, Granted \| shares	394,250
Shares, Forfeited \| shares	(18,750)
Shares, Unvested and outstanding at June 30, 2019 \| shares	595,422
Weighted Average Grant Date Fair Value, Unvested and outstanding at December 31, 2018 \| $ / shares	$ 15.48
Weighted Average Grant Date Fair Value, Granted \| $ / shares	12.30
Weighted Average Grant Date Fair Value, Forfeited \| $ / shares	11.17
Weighted Average Grant Date Fair Value, Unvested and outstanding at June 30, 2019 \| $ / shares	$ 13.51

Share-Based Compensation - Share-based Compensation Expense (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation	$ 8,451	$ 2,250
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation	2,970	674
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation	$ 5,481	$ 1,576

License Agreements - Additional Information (Details) € in Millions, £ in Millions	1 Months Ended						3 Months Ended		6 Months Ended		12 Months Ended
	May 31, 2019 EUR (€)	Jul. 31, 2018 USD ($)	Jun. 30, 2018 USD ($)	Apr. 30, 2018 USD ($) shares	Apr. 30, 2018 GBP (£) shares	Jan. 31, 2017 USD ($)	Jun. 30, 2019 USD ($) Milestone shares	Jun. 30, 2018 USD ($) Milestone	Jun. 30, 2019 USD ($) Milestone shares	Jun. 30, 2018 USD ($) Milestone	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) shares	Jun. 30, 2019 GBP (£) shares	Jun. 30, 2019 EUR (€) shares	Nov. 30, 2018 shares	Sep. 30, 2018 shares	Apr. 30, 2018 GBP (£)
Collaborative Arrangements and Noncollaborative Arrangement Transactions [Line Items]
Research and development							$ 40,478,000	$ 150,991,000	$ 57,971,000	$ 160,162,000
GSK Asset Purchase and License Agreement
Collaborative Arrangements and Noncollaborative Arrangement Transactions [Line Items]
Total consideration				$ 133,600,000	£ 94.2
Upfront payment				14,200,000	10.0
Remaining liability				18,400,000			7,800,000		7,800,000									£ 12.9
Inventory purchase liability				6,900,000														4.9
Transaction costs				800,000	0.6
In-process research and development expense				$ 133,600,000	£ 94.2
Amortization as credit to research and development expense							1,100,000	1,400,000	2,600,000	1,400,000
Increase (Decrease) in liability, effects of foreign exchange translation							$ (200,000)	$ (1,200,000)	$ 100,000	(1,200,000)
Payment of tiered royalty, maximum percentage							20.00%		20.00%					20.00%	20.00%
Milestone payments payable upon achievement of certain sales milestones														£ 90.0	€ 31.0
Upfront payment description									In May 2019, the Company entered into a license agreement with Telethon-OSR, under which Telethon-OSR granted to the Company an exclusive worldwide license for the research, development, manufacture and commercialization of Telethon-OSR’s ex vivo autologous HSC lentiviral based gene therapy for the treatment of mucopolysaccharidosis type I (“MPS-I”), including the Hurler variant. Under the terms of the agreement, Telethon-OSR is entitled to receive €15.0 million in upfront and milestone payments from the Company.
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				$ 93,400,000														£ 65.8
Telethon O S R License Agreements
Collaborative Arrangements and Noncollaborative Arrangement Transactions [Line Items]
Upfront and milestone payments \| €	€ 15.0
In-process research and development expense for upfront and milestone payments							$ 17,200,000		$ 17,200,000
UCLB/UCLA Technology
Collaborative Arrangements and Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payments													$ 800,000
UCLB Technology and Manufacturing Technology
Collaborative Arrangements and Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payments													$ 1,100,000
UCLB License Agreement
Collaborative Arrangements and Noncollaborative Arrangement Transactions [Line Items]
Ordinary shares issued \| shares											4,665,384	1,224,094	3,441,290
UCLB/UCLA License Agreement
Collaborative Arrangements and Noncollaborative Arrangement Transactions [Line Items]
Payments to acquire clinical data												$ 800,000
Annual administration fee									100,000
Payments upon achievement of specified regulatory milestones									38,900,000
Research and development									$ 100,000	$ 100,000
Oxford BioMedica License, Development and Supply Agreement
Collaborative Arrangements and Noncollaborative Arrangement Transactions [Line Items]
Ordinary shares issued \| shares							588,220		588,220					588,220	588,220	150,826	150,826
Research and development							$ 0		$ 0				$ 500,000
Number of milestones met \| Milestone							0	0	0	0
Oxford BioMedica License, Development and Supply Agreement \| Achievement Of Second And Third Milestone
Collaborative Arrangements and Noncollaborative Arrangement Transactions [Line Items]
Research and development											$ 1,400,000
Oxford BioMedica License, Development and Supply Agreement \| Achievement Of First Milestone
Collaborative Arrangements and Noncollaborative Arrangement Transactions [Line Items]
Research and development												$ 100,000
UCLA/CIRM Research Agreement
Collaborative Arrangements and Noncollaborative Arrangement Transactions [Line Items]
Total Reimbursements						$ 10,400,000
Reimbursements received			$ 7,300,000
Total reimbursement under new award		$ 8,500,000
Loan payable term period						10 days
Accrued royalties											0
Research and Development Expense										$ (2,400,000)
Accrued Liabilities and Other Liabilities							$ 1,600,000		$ 1,600,000		$ 1,600,000
UCLA/CIRM Research Agreement \| Maximum
Collaborative Arrangements and Noncollaborative Arrangement Transactions [Line Items]
Obligation to reimburse research activities upon achievement of certain milestones		$ 5,500,000

Income Taxes - Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Unrecognized tax positions	$ 0		$ 0		$ 0
Income tax benefit	$ (772,000)	$ (82,000)	$ (179,000)	$ (165,000)

Net Loss Per Share - Schedule of basic and diluted net loss per share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Earnings Per Share [Abstract]
Net loss	$ (50,530)	$ (30,739)	$ (156,233)	$ (15,311)	$ (81,269)	$ (171,544)
Net loss attributable to ordinary shareholders	$ (50,530)		$ (156,233)		$ (81,269)	$ (171,544)
Weighted average ordinary shares outstanding, basic and diluted	89,712,916		10,115,335		88,369,311	10,095,863
Net loss per share attributable to ordinary shareholders, basic and diluted	$ (0.56)		$ (15.45)		$ (0.92)	$ (16.99)

- 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	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common shares attributable to anti-dilutive shares	11,568,004	53,485,296
Share options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common shares attributable to anti-dilutive shares	10,972,582	7,258,870
Performance-based Restricted Share Units ("RSUs")
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common shares attributable to anti-dilutive shares	595,422
Convertible Preferred Shares
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common shares attributable to anti-dilutive shares		46,226,426

Commitments and Contingencies - Additional Information (Details) - USD ($)	1 Months Ended						3 Months Ended		6 Months Ended		12 Months Ended	24 Months Ended
	Jan. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Jan. 31, 2018	Nov. 30, 2017	Oct. 31, 2016	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Mar. 31, 2021
Operating Leased Assets [Line Items]
Operating lease, term of contract						5 years
Operating leases expiration term					Nov. 30, 2020	Oct. 31, 2021
Operating lease duration for free rent					1 month	1 month
Proceeds from lines of credit									$ 3,000,000		$ 3,000,000
Prepayments		$ 800,000									$ 800,000
Scenario, Forecast [Member]
Operating Leased Assets [Line Items]
Further payments												$ 7,800,000
Licensing Agreements [Member]
Operating Leased Assets [Line Items]
Accounts payable for upfront and milestone payments							16,900,000		16,900,000
Foster City [Member]
Operating Leased Assets [Line Items]
Annual rental expense						$ 200,000
Menlo Park [Member]
Operating Leased Assets [Line Items]
Annual rental expense					$ 800,000
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
Boston [Member]
Operating Leased Assets [Line Items]
Operating leases expiration term			Sep. 30, 2022
Annual rental expense			$ 300,000
London [Member]
Operating Leased Assets [Line Items]
Operating leases expiration term		Jan. 31, 2023
Annual rental expense		$ 100,000
Menlo Park Additional Lease [Member]
Operating Leased Assets [Line Items]
Operating leases expiration term	Dec. 31, 2020
Annual rental expense	$ 100,000
Rent expense							1,300,000	$ 700,000	2,500,000	$ 1,200,000
Freemont Lease Agreement [Member]
Operating Leased Assets [Line Items]
Operating leases expiration term											May 31, 2030
Annual rental expense											$ 2,400,000
Operating lease duration for free rent											8 months
Proceeds from lines of credit											$ 3,000,000
Reduction of standby letter of credit amount percentage											25.00%
Tenant improvements allowance									5,000,000
Escrow deposit							$ 10,000,000		10,000,000
Increase decrease in escrow deposit									$ 0

Employee Benefit Plans - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Compensation And Retirement Disclosure [Abstract]
Contribution expenses	$ 0.3	$ 0.1	$ 0.6	$ 0.2
Maximum annual contribution employer matches per employee, percent			6.00%

Related Party Transactions - Additional Information (Details) - G S K - USD ($)		3 Months Ended	6 Months Ended	12 Months Ended
	Apr. 30, 2018	Jun. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Related Party Transaction [Line Items]
License upfront fee	$ 14,200,000
Inventory purchase liability	6,900,000
Transaction costs	$ 800,000
Payment on inventory purchase liability			$ 3,200,000	$ 6,200,000
Accrued royalties		$ 0	0
Strimvelis
Related Party Transaction [Line Items]
Revenues		$ 0	0
Transition Services Agreement
Related Party Transaction [Line Items]
Payments to settle accounts payable			7,200,000
Payment on inventory purchase liability			$ 1,700,000
Series B Preferred Stock
Related Party Transaction [Line Items]
Ordinary shares Issuable Upon Conversion	12,455,252
Preferred stock, Value	$ 93,400,000
Loss provision associated with contract	$ 18,400,000

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - £ / shares	Jun. 30, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Ordinary Shares, Par Value	£ 0.10	£ 0.10

Condensed Consolidated Statements of Convertible Preferred Shares and Shareholders' (Deficit) Equity (Parenthetical) (Details) $ in Thousands	3 Months Ended
	Jun. 30, 2019 USD ($)
Statement Of Stockholders Equity [Abstract]
Issuance of ADSs in follow-on offering, issuance costs	$ 620