Orchard Therapeutics (ORTX) 10-K 2 Mar 21 2020 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2020	Feb. 25, 2021
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Entity Registrant Name	ORCHARD THERAPEUTICS PLC
Current Fiscal Year End Date	--12-31
Entity Central Index Key	0001748907
Entity Filer Category	Non-accelerated Filer
Entity Well-known Seasoned Issuer	Yes
Entity Small Business	true
Entity Common Stock, Shares Outstanding		123,695,336
Entity Public Float	$ 508
Entity Shell Company	false
Document Transition Report	false
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Incorporation, State or Country Code	X0
Entity Address, Address Line One	108 Cannon Street
Entity Address, City or Town	London
Entity Address, Postal Zip Code	EC4N 6EU
City Area Code	44
Local Phone Number	(0) 203 808-8286
Document Annual Report	true
Entity File Number	001-38722
Entity Emerging Growth Company	false
Entity Tax Identification Number	00-0000000
Entity Address, Country	GB
ICFR Auditor Attestation Flag	false
Title of 12(b) Security	American Depositary Shares, each representing one ordinary share, nominal value £0.10 per share
Trading Symbol	ORTX
Security Exchange Name	NASDAQ
Documents Incorporated by Reference	Portions of the Registrant’s definitive proxy statement for its 2021 Annual General Meeting are incorporated by reference into Part III of this Annual Report on Form 10-K where indicated.

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 55,135	$ 19,053
Marketable securities	136,813	305,937
Trade receivables	878	1,442
Prepaid expenses and other current assets	13,365	8,530
Research and development tax credit receivable	17,344	14,934
Total current assets	223,535	349,896
Non-current assets:
Operating lease right-of-use-assets	29,815	19,415
Property and equipment, net	4,781	7,596
Research and development tax credit receivable, net of current portion		13,710
Restricted cash	4,266	4,264
Other assets	18,540	4,400
Total non-current assets	57,402	49,385
Total assets	280,937	399,281
Current liabilities:
Accounts payable	8,823	11,984
Accrued expenses and other current liabilities	28,943	37,980
Operating lease liabilities	8,934	5,892
Notes payable, current	4,861
Total current liabilities	51,561	55,856
Notes payable, long-term	20,204	24,699
Operating lease liabilities, net of current portion	24,168	15,320
Other long-term liabilities	6,570	4,213
Total liabilities	102,503	100,088
Commitments and contingencies (Note 16)
Shareholders’ equity:
Ordinary shares, £0.10 par value, authority to allot up to a maximum nominal value of £13,023,851.50 of shares at December 31, 2020 and 2019, respectively; 98,283,603 and 96,923,729 shares issued and outstanding at December 31, 2020 and 2019, respectively.	12,507	12,331
Additional paid-in capital	771,194	738,481
Accumulated other comprehensive income	373	2,042
Accumulated deficit	(605,640)	(453,661)
Total shareholders’ equity	178,434	299,193
Total liabilities and shareholders’ equity	$ 280,937	$ 399,281

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - GBP (£)	Dec. 31, 2020	Dec. 31, 2019
Statement Of Financial Position [Abstract]
Ordinary Shares, Par Value	£ 0.10	£ 0.10
Ordinary Shares, Authorized	£ 13,023,851.50	£ 13,023,851.50
Ordinary Shares, Issued	98,283,603	96,923,729
Ordinary Shares, Outstanding	98,283,603	96,923,729

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Statement [Abstract]
Product sales, net	$ 2,595	$ 2,513
Type of Revenue [Extensible List]	us-gaap:ProductMember	us-gaap:ProductMember
Costs and operating expenses
Cost of product sales	$ 857	$ 805
Type of Cost, Good or Service [Extensible List]	us-gaap:ProductMember	us-gaap:ProductMember
Research and development	$ 93,730	$ 117,363
Selling, general and administrative	64,986	57,218
Total costs and operating expenses	159,573	175,386
Loss from operations	(156,978)	(172,873)
Other income (expense):
Interest income	3,185	7,362
Interest expense	(2,328)	(1,538)
Other income, net	3,411	1,387
Total other income (expense), net	4,268	7,211
Net loss before income tax	(152,710)	(165,662)
Income tax benefit	731	2,240
Net loss attributable to ordinary shareholders	$ (151,979)	$ (163,422)
Net loss per share attributable to ordinary shareholders, basic and diluted	$ (1.53)	$ (1.75)
Weighted average number of ordinary shares outstanding, basic and diluted	99,445,874	93,240,355
Other comprehensive (loss) income
Foreign currency translation adjustment	$ (1,485)	$ (1,387)
Unrealized gain (loss) on marketable debt securities	(184)	266
Total other comprehensive loss	(1,669)	(1,121)
Total comprehensive loss	$ (153,648)	$ (164,543)

Consolidated Statements of Shar

Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Total	Ordinary Shares	Additional Paid-in Capital	Accumulated Other Comprehensive Income (loss)	Accumulated Deficit
Balance at Dec. 31, 2018	$ 311,338	$ 10,924	$ 587,490	$ 3,163	$ (290,239)
Balance, Shares at Dec. 31, 2018		85,865,557
Share-based compensation expense	19,424		19,424
Exercise of share options	2,003	$ 158	1,845
Exercise of share options, Shares		1,209,335
Issuance of ESPP shares	1,306	$ 16	1,290
Issuance of ESPP shares, Shares		123,569
Issuance of ADSs in follow-on offering, net of issuance costs of $605	129,665	$ 1,233	128,432
Issuance of ADSs in follow-on offering, net of issuance costs of $605, Shares		9,725,268
Foreign currency translation adjustment	(1,387)			(1,387)
Unrealized gain (loss) on marketable debt securities	266			266
Net loss	(163,422)				(163,422)
Balance at Dec. 31, 2019	299,193	$ 12,331	738,481	2,042	(453,661)
Balance, Shares at Dec. 31, 2019		96,923,729
Share-based compensation expense	27,962		27,962
Exercise of share options	$ 3,465	$ 149	3,316
Exercise of share options, Shares	1,154,441	1,154,441
Issuance of ESPP shares	$ 671	$ 14	657
Issuance of ESPP shares, Shares		107,262
Ordinary shares issued as part of license agreements	791	$ 13	778
Ordinary shares issued as part of license agreements, Shares		98,171
Foreign currency translation adjustment	(1,485)			(1,485)
Unrealized gain (loss) on marketable debt securities	(184)			(184)
Net loss	(151,979)				(151,979)
Balance at Dec. 31, 2020	$ 178,434	$ 12,507	$ 771,194	$ 373	$ (605,640)
Balance, Shares at Dec. 31, 2020		98,283,603

Consolidated Statements of Sh_2

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

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Cash flows from operating activities
Net loss attributable to ordinary shareholders	$ (151,979)	$ (163,422)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	2,004	1,675
Share-based compensation	27,962	19,424
Impairment of long-lived assets	5,650
Non-cash interest expense	500	311
Amortization of provision on loss contract	(2,413)	(3,855)
Non-cash consideration for licenses and milestones	791
Deferred income taxes	(2,257)	(2,942)
Amortization of (discount) premium on marketable securities	770	(676)
Unrealized foreign currency and other non-cash adjustments	(3,674)	(1,859)
Changes in operating assets and liabilities:
Trade receivables	582	715
Research and development tax credit receivable	11,674	(17,564)
Prepaids and other assets	(5,070)	(2,209)
Operating leases, right-of-use-assets	5,863	3,064
Accounts payable	(1,553)	(6,413)
Accrued expenses and other current liabilities	(10,725)	11,434
Other long-term liabilities	2,570	(1,424)
Operating lease liabilities	(6,969)	(2,390)
Net cash used in operating activities	(126,274)	(166,131)
Cash flows from investing activities
Proceeds from sales and maturities of marketable securities	281,433	109,019
Purchases of marketable securities	(113,262)	(414,010)
Payment of construction deposit	(10,000)
Receipt of funds from construction deposit	1,876
Purchases of property and equipment	(2,668)	(4,367)
Net cash used in investing activities	157,379	(309,358)
Cash flows from financing activities
Issuance of debt from credit facility, net of issuance costs		24,466
Issuance of ADRs in public offerings		130,270
Payment of offering costs		(605)
Proceeds from employee equity plans	3,936	3,322
Net cash provided by financing activities	3,936	157,453
Effect of exchange rate changes on cash	1,043	1,672
Net increase (decrease) in cash and restricted cash	36,084	(316,364)
Cash, cash equivalents, and restricted cash —beginning of year	23,317	339,681
Cash, cash equivalents, and restricted cash —end of year	59,401	23,317
Supplemental disclosure of non-cash investing and financing activities
Intangible assets and property and equipment in accounts payable and accrued expenses	3,096	647
Shares issued in consideration of license agreements	791
Employee equity plan proceeds received after year-end	200
Supplemental disclosure of cash flow information
Lease assets obtained in exchange for new operating lease liabilities	17,486
Cash paid for interest	1,828	1,227
Cash paid for taxes	$ 1,007	$ 1,474

Nature of Business and Basis of

Nature of Business and Basis of Presentation	12 Months Ended
	Dec. 31, 2020
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of Business and Basis of Presentation	1. Nature of Business and Basis of Presentation Orchard Therapeutics plc (the “Company”) is a global gene therapy company dedicated to transforming the lives of people affected by rare diseases through the development of innovative, potentially curative gene therapies. The Company’s ex vivo The Company is a public limited company incorporated pursuant to the laws of England and Wales. The Company has American Depositary Shares (“ADSs”) registered with the U.S. Securities and Exchange Commission (the “SEC”) and has been listed on the Nasdaq Global Select Market since October 31, 2018. The Company’s ADSs each represent one ordinary share of the Company. In December 2020, the Company received full, or standard, marketing authorization from the European Commission for Libmeldy™ (autologous CD34+ cell enriched population that contains hematopoietic stem and progenitor cells transduced ex vivo arylsulfatase-A ARSA arylsulfatase-A ARSA The Company is subject to risks and uncertainties common to development-stage companies in the biotechnology industry. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s technology will be obtained, that any products developed will obtain necessary government regulatory approval or that any products, if approved, will be commercially viable. The Company operates in an environment of rapid technological innovation and substantial competition from pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees, consultants and service providers. Even if the Company’s product development efforts are successful in gaining regulatory approval, it is uncertain when, if ever, the Company will realize significant revenue from product sales. The accompanying consolidated financial statements have been prepared on the basis of continuity of operations, realization of assets and the satisfaction of liabilities and commitments in the ordinary course of business. Through December 31, 2020, the Company funded its operations primarily with proceeds from the sale of convertible preferred shares, and ADSs in the IPO and follow-on offering. The Company has incurred recurring losses since its inception, including net losses of $152.0 million and $163.4 million for the years ended December 31, 2020 and 2019, respectively. As of December 31, 2020, the Company had an accumulated deficit of $605.6 million. The Company expects to continue to generate operating losses for the foreseeable future. The Company expects that its cash, cash equivalents, and marketable securities on hand as of December 31, 2020 of $191.9 million, together with the proceeds from the Private Placement of $150.0 million of ordinary shares that closed in February 2021 (see Note 20), will be sufficient to fund its operations and capital expenditure requirements for at least the next twelve months. The Company will seek additional funding through private or public equity financings, debt financings, collaborations, strategic alliances and marketing, distribution or licensing arrangements. The Company may not be able to obtain financing on acceptable terms, or at all, and the Company may not be able to enter into collaborations or other arrangements. The terms of any financing may adversely affect the holdings or the rights of the Company's stockholders. Basis of presentation The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and include the accounts of the Company and its wholly owned subsidiaries, after elimination of all intercompany accounts and transactions. Deferred income taxes in the consolidated statement of cash flows for the year-ended December 31, 2019 previously included in changes in prepaid expenses and other assets has been presented as a separate line item as a non-cash item within adjustments to reconcile net loss to net cash used in operating activities in the consolidated statement of cash flows to conform to current period presentation. Amounts reported are computed based on thousands, except percentages, per share amounts or as otherwise noted. As a result, certain totals may not sum due to rounding.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Principles of consolidation The accompanying consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. Use of estimates The preparation of consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenue and expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual for research and development expenses, the research and development tax credit receivable, share-based compensation, operating lease assets and liabilities, and income taxes. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. The future developments of the COVID-19 pandemic also may directly or indirectly impact the Company’s business include quarantines, border closures, increased border controls, travel restrictions, shelter-in-place orders and shutdowns, business closures, cancellations of public gatherings and other measures. Actual results could differ from the Company’s 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, cash equivalents, marketable securities, and receivables. 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 The financial statements of the Company’s subsidiaries with functional currencies other than the U.S. dollar are translated into U.S. dollars using period-end exchange rates for assets and liabilities, historical exchange rates for stockholders’ equity and weighted average exchange rates for operating results. Translation gains and losses are included in accumulated other comprehensive income (loss) in shareholders’ equity. Foreign currency transaction gains and losses are included in other income (expense), net in the results of operations. The Company recorded realized and unrealized foreign currency transaction gains of $3.4 million, and $1.4 million for the years ended December 31, 2020 and 2019, respectively, which is included in other income (expense) in the statements of operations and comprehensive loss. Segment information The Company operates in a single segment, focusing on researching, developing and commercializing potentially curative gene therapies. Consistent with its operational structure, its chief operating decision maker manages and allocates resources at a global, consolidated level. Therefore, results of the Company's operations are reported on a consolidated basis for purposes of segment reporting. All material long-lived assets of the Company reside in the United States or United Kingdom. The Company had property and equipment of $3.7 million and $1.1 million located in the United Kingdom and United States, respectively, as of December 31, 2020. Cash and cash equivalents The Company considers all highly liquid investments purchased with original maturities of 90 days or less at the date of acquisition to be cash equivalents. Marketable securities Marketable securities consist of investments with original maturities greater than ninety days at the date of acquisition. The Company has classified its investments with maturities beyond one year as short term, based on their highly liquid nature and because such marketable securities represent the investment of cash that is available for current operations. The Company considers its investment portfolio of investments as available-for-sale. Accordingly, these investments are recorded at fair value, which is based on quoted market prices or other observable inputs. Unrealized gains and losses are recorded as a component of other comprehensive income (loss). Realized gains and losses are determined on a specific identification basis and are included in other income (loss). Amortization and accretion of discounts and premiums is also recorded in other income (loss). When the fair value is below the amortized cost of the asset an estimate of expected credit losses is made, and is limited to the amount by which fair value is less than amortized cost. The credit-related impairment amount is recognized in net income; the remaining impairment amount and unrealized gains are reported as a component of accumulated other comprehensive income (loss) in shareholders’ equity. Credit losses are recognized through the use of an allowance for credit losses account and subsequent improvements in expected credit losses are recognized as a reversal of the allowance account. If the Company has the intent to sell the security or it is more likely than not that the Company will be required to sell the security prior to recovery of its amortized cost basis the allowance for credit loss is written off and the excess of the amortized cost basis of the asset over its fair value is recorded in the consolidated statements of operation. Restricted cash Cash and cash equivalents that are restricted as to withdrawal or use under the terms of certain contractual agreements are recorded as restricted cash on our consolidated balance sheet. The Company has 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 at December 31, 2020 and 2019. The Company is also contractually required to maintain a cash collateral account associated with corporate credit cards and other leases in the amount of $1.3 The Company includes the restricted cash balance in cash and cash equivalents when reconciling beginning-of-period and end-of-period total amounts shown on the consolidated statements of cash flows. The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported in the consolidated balance sheet that sum to the total of the amounts reported in the consolidated statement of cash flows: As of December 31, 2020 2019 Cash and cash equivalents $ 55,135 $ 19,053 Restricted cash 4,266 4,264 Total cash, cash equivalents and restricted cash shown in the statement of cash flows $ 59,401 $ 23,317 Property and equipment Property and equipment are recorded at cost and depreciated or amortized using the straight-line method over the following estimated useful lives. Property and equipment: Estimated useful life Lab equipment 5-10 years Leasehold improvements Shorter of lease term or estimated useful life Furniture and fixtures 4 years Office and computer equipment 3-5 years Repairs and maintenance expenditures, which are not considered improvements and do not extend the useful life of property and equipment, are expensed as incurred. 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. Impairment of long-lived assets Long-lived assets consist of property and equipment and operating lease right-of-use assets. Long-lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset group for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset group to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset group are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset group over its fair value, as determined in accordance with the related accounting literature . Research and development costs Research and development costs are expensed as incurred. Research and development expenses consist of costs incurred in performing research and development activities, including salaries, share-based compensation and benefits, facilities costs, depreciation, third-party license fees, certain milestone payments, and external costs of outside vendors engaged to conduct clinical development activities and clinical trials, as well as costs to develop a manufacturing process, perform analytical testing and manufacture clinical trial materials. Non-refundable prepayments for goods or services that will be used or rendered for future research and development activities are recorded as prepaid expenses. Such amounts are recognized as an expense as the goods are delivered or the related services are performed, or until it is no longer expected that the goods will be delivered, or the services rendered. In addition, funding from research grants is recognized as an offset to research and development expense on the basis of costs incurred on the research program. Royalties to third parties associated with our research grants will be accrued when they become probable. Research contract costs and accruals The Company has entered into various research and development contracts. These agreements are cancelable, and related costs are recorded as research and development expenses as incurred. When billing terms under these contracts do not coincide with the timing of when the work is performed, the Company is required to make estimates of outstanding obligations as of period end to those third parties. Any accrual estimates are based on a number of factors, including the Company’s knowledge of the progress towards completion of the research and development activities, invoicing to date under the contracts, communication from the research institution or other companies of any actual costs incurred during the period that have not yet been invoiced, and the costs included in the contracts. Significant judgments and estimates may be made in determining the accrued balances at the end of any reporting period. Actual results could differ from the estimates made by the Company. The historical accrual estimates made by the Company have not been materially different from the actual costs. Share-based compensation The Company measures all stock options and other stock-based awards granted to employees at fair value on the date of grant. The Company uses the Black-Scholes option-pricing model in the valuation of its stock options. The fair value of performance-based share awards and restricted stock units is based on the fair value of the stock on the date of grant. The Company uses the Monte-Carlo model in order to calculate the fair value of the market-based awards. The fair value of options is recognized as stock-based compensation expense over the requisite service period, which is generally the vesting period of the respective award. The Company accounts for stock-based compensation expense related to forfeitures as the forfeitures occur. The straight-line method of expense recognition is applied to all awards with service-based and market-based conditions. The Company records stock-based compensation expense related to performance-based awards when the performance-based targets are probable of being achieved. The Company classifies stock-based compensation expense in the consolidated statements of operations in the same manner in which the award recipient’s payroll costs are classified. Comprehensive loss Comprehensive loss is composed of net loss and other comprehensive income (loss). Other comprehensive income (loss) consists of unrealized gains and losses on marketable debt securities and foreign currency translation. Leases The Company determines if an arrangement is a lease at contract inception. Operating lease assets represent a right to use an underlying asset for the lease term and operating lease liabilities represent an obligation to make lease payments arising from the lease. Operating lease liabilities with a term greater than one year and their corresponding right-of-use assets are recognized on the balance sheet at the commencement date of the lease based on the present value of lease payments over the expected lease term. Certain adjustments to the right-of-use asset may be required for items such as initial direct costs paid or incentives received. The Company made an accounting policy election to not record a right-of-use asset or lease liability for leases with a term of one year or less. To date, the Company has not identified any material short-term leases, either individually or in the aggregate. As the Company’s leases do not provide an implicit rate, the Company utilized the appropriate incremental borrowing rate, which is the rate incurred to borrow on a collateralized basis over a similar term as the lease an amount equal to the lease payments in a similar economic environment. The Company estimated the incremental borrowing rate based on the Company’s currently outstanding credit facility as inputs to the analysis to calculate a spread, adjusted for factors that reflect the profile of secured borrowing over the expected term of the lease. The components of a lease should be split into three categories: lease components (e.g., land, building, etc.), non-lease components (e.g., common area maintenance, utilities, performance of manufacturing services, purchase of inventory, etc.), and non-components (e.g., property taxes, insurance, etc.). Then the fixed contract consideration (including any related to non-components) must be allocated based on fair values to the lease components and non-lease components. Although separation of lease and non-lease components is required, certain practical expedients are available to entities. Entities electing the practical expedient would not separate lease and non-lease components. Rather, they would account for each lease component and the related non-lease component together as a single component. The Company has elected not to apply the practical expedient and with respect to its lease of manufacturing space at a contract manufacturing organization, the Company has allocated the consideration between the lease and non-lease components of the contract based on the respective fair values of the lease and non-lease components. The Company calculated the fair value of the lease and non-lease components using financial information readily available as part of its master services arrangement and other representative data indicative of fair value. The Company accounts for sublease income on a straight-line basis over the respective lease period and records an unbilled rent receivable for sublease income incurred but not yet paid. The Company periodically performs a collectability assessment associated with any unbilled rent receivables. The Company recognizes the sublease income as a reduction to the related operating expense associated with the head lease. Strimvelis loss provision As part of the GSK transaction, the Company is required to maintain commercial availability of Strimvelis in the European Union until such time that an alternative gene therapy is available (Note 13). 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 as part of the GSK transaction in 2018. The Company recognizes the amortization of the loss provision on a diminishing balance basis based on the actual net loss incurred associated with Strimvelis and the expected future net losses to be generated until such time as Strimvelis is no longer commercially available. The amortization of the provision is recorded as a credit to research and development expense. We have made an estimate of the expected future losses associated with Strimvelis and adjust this estimate as facts and circumstances change regarding the commercial availability and costs of maintaining and selling Strimvelis. The Company does not update the accrued loss provision for any subsequent adjustment of the future losses, however, the timing of recognizing the amortization of what was originally recorded is adjusted for updates to estimates of potential future losses. The Company paused treating new patients with Strimvelis in October 2020 upon learning that a patient treated with the drug in 2016 under a compassionate use program was diagnosed with lymphoid T cell leukemia, a known risk factor for gammaretroviral vector-based gene therapy. The EMA’s Committee for Medicinal Products for Human Use, or CHMP, reviewed the updated risk-benefit assessment of Strimvelis as part of its ongoing MAA renewal procedure, concluded that the risk-benefit balance remains favorable and recommended in February 2021 that the marketing authorization for Strimvelis be renewed for five years, allowing marketing of Strimvelis to resume. Year Ended December 31, 2020 2019 Balance at beginning of period $ 6,790 $ 10,339 Provisions — — Amortization of loss provision (2,413 ) (3,855 ) Foreign currency translation 105 306 Balance at end of period $ 4,482 $ 6,790 As of December 31, 2020, $0.9 million of the Strimvelis loss provision was classified as current, and $3.6 million was classified as non-current. As of December 31, 2019, $3.0 million of the Strimvelis loss provision was classified as current, and $3.8 million was classified as non-current. United Kingdom Research and development income tax credits As a company that carries out research and development activities, the Company is able to submit tax credit claims from two UK research and development tax relief programs, the Small and Medium-sized Enterprises research and development tax credit (“SME”) program and the Research and Development Expenditure Credit (“RDEC”) program depending on eligibility. Qualifying expenditures largely comprise employment costs for research staff, consumables and certain internal overhead costs incurred as part of research projects for which the Company does not receive income. Each reporting period, management evaluates which tax relief programs the Company is expected to be eligible for and records a reduction to research and development expense for the portion of the expense that it expects to qualify under the programs, that it plans to submit a claim for, and it has reasonable assurance that the amount will ultimately be realized. Based on criteria established by HM Revenue and Customs (“HMRC”), management of the Company expects a proportion of expenditures being carried in relation to its pipeline research, clinical trials management and manufacturing development activities to be eligible for the research and development tax relief programs for the year ended December 31, 2020. The Company has qualified under the more favorable SME regime for the year ended December 31, 2019 and expects to qualify under the SME regime for the year ending December 31, 2020. The RDEC and SME credits are not dependent on the Company generating future taxable income or on the ongoing tax status or tax position of the Company. The Company has assessed its research and development activities and expenditures to determine whether the nature of the activities and expenditures will qualify for credit under the tax relief programs and whether the claims will ultimately be realized based on the allowable reimbursable expense criteria established by the UK government which are subject to interpretation. At each period end, the Company estimates the reimbursement available to the Company based on available information at the time. The Company recognizes credits from the research and development incentives when the relevant expenditure has been incurred and there is reasonable assurance that the reimbursement will be received. Such credits are accounted for as reductions in research and development expense. The following table outlines the changes to the research and development tax credit receivable, including amount recognized as an offset to research and development expense during the years ended December 31, 2020 and 2019: Year Ended December 31, 2020 2019 Balance at beginning of period $ 28,644 $ 10,585 Recognition of credit claims as offset to research and development expense 21,130 17,564 Receipt of credit claims (33,771 ) (152 ) Foreign currency translation 1,341 647 Balance at end of period $ 17,344 $ 28,644 During the year ended December 31, 2020, the Company recorded $4.8 million of additional tax credits related to a change in estimate associated with its UK research and development tax credit receivable claim for fiscal year 2019. The change in estimate was based on the results of a tax credit analysis associated with the Company’s qualified projects and research and development expenditures completed during the third quarter to finalize the 2019 UK tax return. As of December 31, 2020, the Company’s tax credit receivable from the UK was $17.3 million, all of which was classified as current. As of December 31, 2019, the Company’s tax incentive receivable from the UK was $28.6 million, of which $14.9 million was classified as current and $13.7 million was classified as non-current. As of December 31, 2020, the Company has received all of its 2016-2019 tax credit claims from HMRC. Income taxes The Company is primarily subject to corporation taxes in the United Kingdom and the United States. The calculation of the Company’s tax provision involves the application of both United Kingdom and United States tax law and requires judgement and estimates. The Company accounts for income taxes in accordance with ASC Topic 740, Accounting for Income Taxes The Company accounts for uncertainty in income taxes by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed as the amount of benefit to recognize in the consolidated financial statements. The amount of benefits that may be used is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate, as well as the related net interest and penalties. Product sales The Company’s product sales of Strimvelis are currently distributed Revenue from Contracts with Customers Net income (loss) per share Basic net income (loss) per share is computed by dividing the net income (loss) by the weighted average number of ordinary shares outstanding for the period. Diluted net income (loss) is computed by adjusting net income (loss) based on the potential impact of dilutive securities. Diluted net income (loss) per share is computed by dividing the diluted net income (loss) 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 unvested restricted shares are considered potential dilutive ordinary shares. 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 potential ordinary share equivalents outstanding would have been anti-dilutive. The following securities, presented based on amounts outstanding at each period end, are considered to be ordinary share equivalents, but were not included in the computation of diluted net loss per ordinary share because to do so would have been anti-dilutive: December 31, 2020 2019 Share options 11,071,555 10,056,864 Unvested shares from share plan and consulting agreement 816,316 751,496 11,887,871 10,808,360 Recent accounting pronouncements In February 2016 and July 2018, the FASB issued ASU No. 2016-02, Leases (Topic 842) Leases Leases The new standard requires that all lessees (i) recognize, on the balance sheet, liabilities to remit lease payments and right-of- ASC 842 became effective for the Company in 2019. The Company adopted ASC 842 using the modified retrospective approach with an effective date of January 1, 2019 for leases that existed on that date. Prior period results continue to be presented under ASC 840 based on the accounting standards originally in effect for such periods. This standard provides a number of optional practical expedients in transition. The Company applied the package of practical expedients to leases that commenced prior to the effective date, whereby it elected not to reassess the following: (i) whether any expired or existing contracts contain leases; (ii) the lease classification for any expired or existing leases; and (iii) initial direct costs for any existing leases. The Company elected the short-term lease recognition exemption for all leases that qualify, where a right-of-use asset or lease liability will not be recognized for short term leases that have terms of one year or less. The operating lease right-of-use assets and corresponding liabilities relate to existing facility operating leases in London, UK, Boston, Massachusetts, and the San Francisco Bay Area, California, as well as an embedded operating lease for research and development space at a contract manufacturing organization. The most significant effects of adoption were the recognition of material new right-of-use assets and corresponding liabilities on its consolidated balance sheet related to its existing facility operating leases (see Note 10). The adoption of this standard had a material impact on the Company’s financial position but did not significantly affect the Company’s results of operations or cash flows. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments – Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments (“ASU 2016-13”), which requires entities to record expected credit losses for certain financial instruments, including trade receivables, as an allowance that reflects the entity's current estimate of credit losses expected to be incurred. For available-for-sale debt securities in unrealized loss positions, the new standard requires allowances to be recorded instead of reducing the amortized cost of the investment. The new standard became effective fo r us on January 1, 2020. This guidance did not have a significant impact on the Company’s consolidated financial statements and related disclosures. The Company has a UK research and development tax credit receivable and trade receivables that are subject to this guidance. The Company has assessed whether it believes there is a current estimate of credit loss expected to be recorded for these receivables and concluded that any amount would not be significant and therefore the Company has not recorded any credit loss allowance for these receivables. In December 2019, the FASB issued ASU No. 2019-12, Simplifying the Accounting for Income Taxes (Topic 740) , which removes certain exceptions to the general principles in Topic 740 – Income Taxes and improves consistent application of and simplifies GAAP for other areas of Topic 740 by clarifying and amending existing guidance. This ASU is effective for the Company beginning January 1, 202 1 and interim periods within that year, with early adoption permitted. The Company is currently evaluating the effect of adopting this new accounting guidance.

Fair Value Measurements and Mar

Fair Value Measurements and Marketable Securities	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Fair Value Measurements and Marketable Securities	3. Fair Value Measurements and Marketable Securities The following tables present information about the Company’s financial assets that have been measured at fair value as of December 31, 2020 and 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 years ended December 31, 2020 and 2019, there were no transfers between Level 1 and Level 2 financial assets. The following table summarizes the Company’s cash equivalents and marketable securities as of December 31, 2020: Fair Value Measurements as of December 31, 2020 Using: Level 1 Level 2 Level 3 Total Cash equivalents Money market funds $ 6,650 $ — $ — $ 6,650 Corporate bonds — 3,001 — 3,001 Commercial paper — 2,999 — 2,999 Total cash equivalents $ 6,650 $ 6,000 $ — $ 12,650 Marketable securities US government securities $ — 2,997 $ — 2,997 Corporate bonds — $ 93,358 — 93,358 Commercial paper — 40,458 — 40,458 Total marketable securities $ — $ 136,813 $ — $ 136,813 Total $ 6,650 $ 142,813 $ — $ 149,463 The following table summarizes the Company’s cash equivalents and marketable securities as of December 31, 2019: : Fair Value Measurements as of December 31, 2019 Using: Level 1 Level 2 Level 3 Total Cash equivalents Money market funds $ 202 $ — $ — $ 202 U.S. government securities — 3,159 — 3,159 Commercial paper — 9,792 — 9,792 Total cash equivalents $ 202 $ 12,951 $ — $ 13,153 Marketable securities Corporate bonds $ — $ 259,900 $ — 259,900 Commercial paper — 46,037 — 46,037 Total marketable securities $ — $ 305,937 $ — $ 305,937 Total $ 202 $ 318,888 $ — $ 319,090 The carrying amount reflected in the consolidated balance sheets for research and development tax incentive receivable, trade receivables, other receivables, accounts payable, and accrued expenses approximate fair value due to their short-term maturities. The carrying value of the Company’s outstanding notes payable approximates fair value (a Level 2 fair value measurement), reflecting interest rates currently available to the Company. Marketable Securities The following table summarizes the Company’s level 2 cash equivalents and marketable securities as of December 31, 2020: Fair Value Measurements as of December 31, 2020 Using: Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Credit Losses Fair Value U.S. government securities $ 3,000 $ — $ (4 ) $ — 2,996 Corporate bonds 96,259 133 (32 ) — 96,360 Commercial paper 43,469 1 (13 ) — 43,457 Total $ 142,728 $ 134 $ (49 ) $ — $ 142,813 The following table summarizes the Company’s level 2 cash equivalents and marketable securities as of December 31, 2019: Fair Value Measurements as of December 31, 2019 Using: Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value U.S. government securities $ 3,159 $ — $ — 3,159 Corporate bonds 259,669 285 (54 ) 259,900 Commercial paper 55,794 42 (7 ) 55,829 Total $ 318,622 $ 327 $ (61 ) $ 318,888 The following table summarizes the Company’s available-for-sale marketable debt securities by contractual maturity, as of December 31, 2020 and 2019: 2020 2019 Maturities in one year or less $ 132,056 $ 250,490 Maturities between one and three years 10,757 68,398 Total $ 142,813 $ 318,888

Revenue Recognition

Revenue Recognition	12 Months Ended
	Dec. 31, 2020
Revenue From Contract With Customer [Abstract]
Revenue Recognition	4. Revenue Recognition During the years ended December 31, 2020 and 2019 the Company recorded sales for one commercial-stage therapy, Strimvelis, for the treatment of ADA-SCID. Strimvelis is c urrently distributed exclusively at the San Raffaele Hospital in Milan, Italy. San Raffaele Hospital will purchase and pay for Strimvelis and submit a claim to the payer. The Company’s contracted sales with San Raffaele Hospital contain a single performance obligation and the Company recognizes revenue from product sales when the Company has satisfied its performance obligation by transferring control of Strimvelis to San Raffaele Hospital. Control of the product generally transfers upon the completion of the scheduled Strimvelis treatment. The Company’s product sales represent total net product sales of Strimvelis. The Company evaluated the variable consideration under Accounting Standards Codification (ASC) 606, Revenue from Contracts with Customers , and there is currently no variable consideration included in the transaction price for Strimvelis. Costs to manufacture and deliver the product and those associated with administering the therapy are included in cost of product sales. As the product is sold in direct relation to a scheduled treatment, the Company estimates that there is limited risk of product return, including the risk of product expiration. Costs to manufacture the product and those associated with administering the therapy are included in cost of product sales. As the product is sold in direct relation to a scheduled treatment, the Company estimates that there is minimal risk of product return, including the risk of product expiration. Payment terms and conditions generally require payment for Strimvelis sales within 60 days of treatment. Strimvelis is currently distributed exclusively at the San Raffaele Hospital, and there is currently no variable consideration included in the transaction price of Strimvelis.

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets	12 Months Ended
	Dec. 31, 2020
Prepaid Expense And Other Assets Current [Abstract]
Prepaid Expenses and Other Current Assets	5. Prepaid expenses and other current assets Prepaid expenses and other current assets consist of the following: December 31, 2020 2019 Prepaid external research and development expenses $ 1,421 $ 1,121 Inventories 665 — Other prepayments 4,930 2,800 VAT receivable 2,780 1,091 Construction deposit - current 1,552 — Non-trade receivables 2,017 3,518 Total prepaid expenses and other current assets $ 13,365 $ 8,530

Property and Equipment

Property and Equipment	12 Months Ended
	Dec. 31, 2020
Property Plant And Equipment [Abstract]
Property and Equipment	6. Property and equipment Property and equipment consist of the following: December 31, 2020 2019 Property and equipment: Lab equipment $ 5,114 $ 6,377 Leasehold improvements 2,522 1,839 Furniture and fixtures 304 508 Office and IT equipment 763 184 Construction-in-progress 302 1,848 Property and equipment $ 9,005 $ 10,756 Less: accumulated depreciation (4,224 ) (3,160 ) Property and equipment, net $ 4,781 $ 7,596 Depreciation expense for the years ended December 31, 2020 and 2019 was $2.0 million and $1.7 million, respectively.

Other Assets

Other Assets	12 Months Ended
	Dec. 31, 2020
Other Assets Noncurrent Disclosure [Abstract]
Other Assets	7. Other assets Other assets consist of the following: December 31, 2020 2019 Intangible assets - license milestones $ 3,076 $ — Deferred tax assets 5,219 2,985 Deposits 1,144 1,108 Deferred financing costs 975 307 Other non-current assets 1,554 — Construction deposits - long-term 6,572 — Total other assets $ 18,540 $ 4,400

Accrued Expenses and Other Liab

Accrued Expenses and Other Liabilities	12 Months Ended
	Dec. 31, 2020
Accrued Liabilities And Other Liabilities [Abstract]
Accrued Expenses and Other Liabilities	8. Accrued expenses and other liabilities Accrued expenses and other current liabilities consisted of the following: December 31, 2020 2019 Accrued external research and development expenses $ 8,878 $ 16,215 Accrued payroll and related expenses 11,881 12,381 Accrued professional fees 791 1,321 Accrued other 6,477 5,069 Strimvelis liability - current portion 916 2,994 Total accrued expenses and other current liabilities $ 28,943 $ 37,980

Restructuring Charges

Restructuring Charges	12 Months Ended
	Dec. 31, 2020
Restructuring And Related Activities [Abstract]
Restructuring Charges	9. Restructuring charges In May 2020 , the Company committed to a new strategic plan and restructuring intended to enable the Company to advance its corporate strategy while reducing overall operating expenses, including ceasing construction and build-out of its Fremont, California manufacturing facility, closing its office in Menlo Park, California, reducing its workforce by approximately 25% across the Company, eliminating a number of future positions expected to be recruited in 2020 and 2021, reducing its investment in the future development for certain programs, and other cost-saving measures (collectively, the “Restructuring”). The workforce reductions took place primarily during the second and third quarters of 2020, and concluded in the fourth quarter of 2020. Cash restructuring charges Accrued restructuring and severance costs are included in Accrued expenses and other current liabilities in the consolidated balance sheet. Activity for the fiscal year are summarized as follows: Year Ended December 31, 2020 Balance at beginning of period $ — Charged to expense 1,854 Payments made (1,848 ) Balance at end of period $ 6 There were no restructuring costs during the year ended December 31, 2019. Impairment of long-lived assets During the second quarter of 2020, the Company also took the following non-cash charges to research and development expense associated with the impairment of construction-in-process associated with the Fremont manufacturing facility, partial impairment of the right-of-use asset for the Fremont manufacturing facility lease (the “Fremont ROU asset”), and a write-down of laboratory equipment from the Company’s Menlo Park, CA facility: Asset write-down Operating lease right-of-use asset $ 2,605 Construction-in-progress 2,285 Laboratory equipment 760 Charge included in research and development expense $ 5,650 The Company assessed the Fremont construction-in-process for impairment in May 2020 upon the Restructuring. The construction-in-process related to design costs, and was determined to have no potential future value, and an impairment charge of $2.3 million was taken for the full value of the construction-in-process asset. The Company assessed the Fremont ROU asset for impairment in May 2020 upon the Restructuring when the carrying value of the asset was $13.8 million. The Fremont ROU asset represented the asset group for the impairment assessment. Upon failing the first step of the long-lived asset impairment model where the undiscounted cash flows were less than the carrying value of the Fremont ROU asset, the Company performed the second step by comparing the fair value of the Fremont ROU asset to its carrying value. The fair value of the Fremont ROU asset is a non-recurring fair value measurement that was measured using a probability-weighted discounted cash flow approach, which estimated the present value of potential sublease income to be generated by the facility, less costs incurred to sublease the facility. The significant assumptions inherent in estimating the various probability weighted scenarios included the undiscounted forecasted sublease income less costs incurred, which included assumptions of the expected income and timing of entering into a future sublease, and a market-participant discount rate that reflects a potential discount rate. The Company selected the assumptions used in the fair value estimate using current market data associated with the potential sublease income and market participant discount rates. The undiscounted cash flows utilized in the fair value estimate ranged from $ 11.7 million to $ 19.1 million to be generated over the remainder of the lease term. The market-participant discount rate utilized in the fair value estimate was 4.6 %. These assumptions represent level 3 inputs of the fair value hierarchy (see Note 3). As of the assessment date, the fair value of the Fremont ROU asset was $11.2 million, and the Company recorded a $2.6 million impairment charge related to the asset. The remaining carrying value of the Fremont ROU asset is being amortized over the remaining lease term on a straight-line basis. In December 2020, the Company executed a sublease for the Fremont manufacturing facility with an unrelated third-party for the remaining lease term (see Note 10). No further impairment was necessary as a result of the sublease. The occurrence of a triggering event for the Fremont ROU asset in future periods could result in additional impairment charges if the estimated fair value of the asset is determined to be lower than the carrying value.

Leases

Leases	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Leases	10. Leases Operating leases In November 2017 and January 2019, the Company entered into lease agreements for office and laboratory space in Menlo Park, California, United States. The leases terminated in December 2020 In January 2018 and December 2018, the Company entered into lease agreements for office space in London, United Kingdom, both of which terminate in January 2023 In March 2018, the Company entered into a lease agreement for office space in Boston, Massachusetts, United States, which terminates in September 2022 In July 2019, the Company entered into a lease agreement for office space in Boston, Massachusetts, United States, which commences for accounting purposes in January 2020 September 2026 As of December 31, 2020, the carrying value of the operating lease right-of-use assets in Boston and London was $ 5.4 5.7 Fremont operating lease and sublease agreements In December 2018, the Company leased manufacturing, laboratory, and office space in Fremont, California (the “Fremont facility” and the “Head Lease”) which terminates in May 2030 n December 2020, the Company entered into a sublease agreement (the “Sublease”) with an unrelated third-party (the “subtenant”) whereby the Company subleased the entire Fremont facility to the subtenant. The Company accounts for the Head Lease and Sublease as two separate contracts. Both the Head Lease and Sublease were determined to be operating leases. The Head Lease annual rental payments, including variable payments, were $3.1 million in 2020. The Head Lease includes annual rent escalation provisions. The Company was provided with 8 months of free rent. Subject to the terms of the Head Lease agreement, the Company executed a $3.0 million letter of credit upon signing the lease, which may be reduced by 25% subject to reduction requirements specified therein. This amount is classified as restricted cash on the consolidated balance sheet. As of December 31, 2020, the carrying value of the Fremont Head Lease right-of-use asset was $10.5 million and the lease liability was $14.4 million related to the Fremont facility. The Head Lease provides for up to $5.3 million in tenant improvement allowances to be reimbursed to the Company by the landlord. These tenant improvement allowances have been included in the calculation of the operating lease liability and is currently expected to be received in 2021 and 2022. The Company continues to assess the expected receipt of the tenant improvement allowances any may remeasure the right-of-use asset and liability from time to time as facts and circumstances may change The Sublease commenced in December 2020 and is in force for the remainder of the Head Lease term, through May 2030. The Sublease provides for 12 months of free rent until December 2021. The sublease provides for an initial annual cash base rent of $2.1 million, with annual rent escalation provisions. The subtenant is also responsible for paying all operating expenses associated with the Head Lease. The Sublease also includes pass-through of up to $5.3 million in tenant improvement allowances to the subtenant, subject to the Company being reimbursed for the allowances per the terms of the Head Lease. The Subtenant provided the Company with a $2.6 million security deposit, which may be converted to a letter of credit upon providing evidence of $2.6 million in construction expenditures. The Company accounts for the security deposit within other long-term liabilities. The Company has $8.1 million in an escrow account associated with construction on the Fremont facility, for which the Company has ceased construction and build-out. Subject to the terms of the Head Lease and reduction provisions, this amount may be returned to the Company upon qualifying construction expenditure, or will be returned in late 2022 (the “Sunset Date”) to the extent construction expenses have not been incurred. The Company deposited $10.0 million into the account in the first quarter of 2020 and has received $1.9 million in receipts from the escrow funds for costs incurred to date. Of the $8.1 million remaining in the escrow account, $1.6 million is classified within prepaid expenses and other current assets and $6.5 million is classified within other assets on the consolidated balance sheet based on the timing of when the Company expects funds to be returned from the escrow agent. Future receipts from the escrow deposit will be dependent upon the timing of the subtenant construction spend through the Sunset Date. Embedded operating lease arrangement The Company is party to a manufacturing agreement for research and development and commercial production with AGC Biologics, S.p.A. (formerly MolMed S.p.A.) (“AGC”) pursuant to which AGC will develop, manufacture and supply certain viral vectors and conduct cell processing activities for certain Company development and commercial programs. A manufacturing agreement with AGC was novated to the Company as part of the GSK Agreement (see Note 16). On July 2, 2020 (the “Effective Date”), the Company entered into a new manufacturing and technology development master agreement with AGC (the “AGC Agreement”) which superseded the novated agreement. The Company determined that the AGC Agreement contains an embedded lease as it includes provision of manufacturing suites designated for the Company’s exclusive use during the term of the agreement. The AGC Agreement has an initial term of five years, beginning on the Effective Date and ending July 2, 2025. The agreement may be extended for an additional two years by mutual agreement of the Company and AGC As of December 31, 2020, the carrying value of the embedded operating lease right-of-use asset was $13.9 million and the lease liability was $13.1 million. The Company may terminate the AGC Agreement and the use of the exclusive manufacturing suites, with 12-months’ notice, and beginning no earlier than July 2, 2022. AGC may terminate the AGC Agreement with 24-months’ notice. The AGC Agreement provides for an option to reserve one additional exclusive manufacturing suite any time prior to January 1, 2022 for a one-time option fee plus annual rental fee. The AGC Agreement extends until July 2, 2025. Summary of all lease costs recognized under ASC 842 Our facility leases described above generally contain customary provisions allowing the landlords to terminate the leases if we fail to remedy a breach of any of our obligations under any such lease within specified time periods, or upon our bankruptcy or insolvency. The leases do not include any restrictions or covenants that had to be accounted for under the lease guidance. The following table contains a summary of the lease-related costs recognized within operating expenses, and other information pertaining to the Company’s operating leases as of December 31, 2020 and 2019: 2020 2019 Fixed lease cost $ 7,593 $ 5,589 Impairment of right-of-use assets $ 2,781 — Variable lease cost 2,131 1,436 Sublease income (181 ) — Total lease cost $ 12,324 $ 7,025 Other information Operating cash flows used for operating leases 8,447 5,738 Weighted-average remaining lease term (years) 6.6 8.2 Weighted-average discount rate 8.6 % 9.3 % Fixed lease cost represents the ASC 842 rent expense associated with the amortization of our right-of-use assets and lease liabilities. Impairment of right-of-use assets relates to discrete impairment charges taken when, in the Company’s estimation, the fair value of a right-of-use asset is below the carrying value. Variable lease cost are the amounts owed by the Company to a lessor that are not fixed, such as reimbursement for common area maintenance and utilities costs, and are not included in the calculation of the Company’s operating lease right of use assets or operating lease liabilities and are expensed when incurred. Sublease income represents the straight-line recognition of base rent sublease income over the term of the Sublease, and recognition of pass-through operating expense costs per the terms of the Sublease. During the year ended December 31, 2020, the Company obtained right of use assets valued at $17.5 million in exchange for lease liabilities of $17.5 million. During the year ended December 31, 2019 there were no material right of use assets obtained in exchange for material new lease obligations. As of December 31, 2020, future minimum base rent commitments under ASC 842 under the Company’s property leases were as follows: Due in: Gross lease payments Gross sublease receipts Net lease payments 2021 $ 8,941 $ (181 ) $ 8,760 2022 8,093 (2,180 ) 5,913 2023 7,043 (2,245 ) 4,798 2024 7,067 (2,312 ) 4,755 2025 4,657 (2,382 ) 2,275 Thereafter 15,053 (11,413 ) 3,640 Total future minimum lease payments 50,854 (20,713 ) 30,141 Less: imputed interest (17,752 ) Total operating lease payments $ 33,102 *Tabular disclosure above for leases denominated in GBP have been translated at a rate of £1.00 to $1.36, and leases denominated in Euro have been translated at a rate of €1.00 to $1.23.

Notes Payable

Notes Payable	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
Notes Payable	11. Notes Payable In May 2019 form of a second and third term loan, the second term loan being a $ 25.0 million term loan available no earlier than July 1, 2020 and no later than March 31, 2021 upon submission of certain regulatory filings and evidence of the Company having $ 100.0 million in cash and cash equivalent investments ; and the third term loan being a $ 25.0 million term loan available no earlier than July 1, 2020 and no later than September 30, 2021 upon certain regulatory approvals and evidence of the Company having $ 125.0 million in cash and cash equivalent investments . As of December 31, 20 20 , the Company had met the criteria to draw down the second and third term loan s totaling $ million. The term loans under the Credit Facility will terminate on the fifth anniversary of the Closing Date (the “Loan Maturity Date”). Each term loan under the Credit Facility bears interest at an annual rate equal to 6% plus LIBOR. The Company is required to make interest-only payments on the term loan for all payment dates prior to 24 months following the date of the Credit Facility, unless the third tranche is drawn, in which case the Company is required to make interest-only payments for all payment dates prior to 36 months following the date of the Credit Facility. The term loans under the Credit Facility will begin amortizing on either the 24-month or the 36-month anniversary of the Credit Facility (as applicable), with equal monthly payments of principal plus interest to be made by the Company to the Lenders in consecutive monthly installments until the Loan Maturity Date. In addition, a final payment of 4.5% is due on the Loan Maturity Date. The Credit Facility includes affirmative and negative covenants. The affirmative covenants include, among others, covenants requiring the Company to maintain their legal existence and governmental approvals, deliver certain financial reports, maintain insurance coverage, maintain property, pay taxes, satisfy certain requirements regarding accounts and comply with laws and regulations. The negative covenants include, among others, restrictions on the Company transferring collateral, incurring additional indebtedness, engaging in mergers or acquisitions, paying dividends or making other distributions, making investments, creating liens, amending material agreements and organizational documents, selling assets, changing the nature of the business and undergoing a change in control, in some cases subject to certain exceptions. The Company is also subject to an ongoing minimum cash financial covenant in which the Company must maintain unrestricted cash in an amount not less than $20.0 million following the utilization of the second term loan and not less than $35.0 million following the utilization of the third term loan. As of December 31, 2020 and 2019, notes payable consist of the following: December 31, 2020 2019 Notes payable, net of issuance costs $ 24,659 $ 24,541 Less: current portion $ (4,861 ) — Notes payable, net of current portion 19,798 24,541 Accretion related to final payment 406 158 Notes payable, long term $ 20,204 $ 24,699 As of December 31, 2020, the future principal payments due are as follows: Aggregate Minimum Payments 2021 4,861 2022 8,333 2023 8,334 2024 4,597 2025 — Thereafter — Total 26,125 Less current portion (4,861 ) Less unamortized portion of final payment (719 ) Less unamortized debt issuance costs (341 ) Notes payable, long term $ 20,204 During the years ended December 31, 2020 and 2019, the Company recognized $2.3 million and $1.5 million of interest expense related to the term loan, respectively. The effective annual interest rate as of December 31, 2020 on the outstanding debt under the Term Loan was approximately 9.3%.

Shareholders' Equity and Conver

Shareholders' Equity and Convertible Preferred Shares	12 Months Ended
	Dec. 31, 2020
Shareholders Equity And Convertible Preferred Shares [Abstract]
Shareholders' Equity and Convertible Preferred Shares	12. Shareholders’ Equity and Convertible Preferred Shares Ordinary shares As of December 31, 2020, and 2019, each holder of ordinary shares and ADSs is entitled to one vote per ordinary share and to receive dividends when and if such dividends are recommended by the board of directors and declared by the shareholders. As of December 31, 2020, and 2019, the Company has not declared any dividends. As of December 31, 2020, and 2019, the Company had authority to allot ordinary shares up to a maximum nominal value of £13,023,851.50 with a nominal value of £0.10 per share. Ordinary share issuances In June 2019, the Company completed its follow-on public offering of ADSs. The Company sold an aggregate of 9,725,268 ADSs representing the same number of ordinary shares at a public offering price of $14.25 per ADS, including a partial exercise by the underwriters of their option to purchase additional ADSs. Net proceeds were $129.7 million, after deducting underwriting discounts of $8.3 million, and commissions and offering expenses paid by the Company of $0.6 million. In April 2020, the Company issued 75,413 ordinary shares to Oxford BioMedica pursuant to the terms of our license agreement (see Note 14). In December 2020, the Company issued 22,758 ordinary shares pursuant to a consulting agreement (see Note 16) with a non-employee advisor.

Share-based Compensation

Share-based Compensation	12 Months Ended
	Dec. 31, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Share-based Compensation	13. Share-based Compensation The Company maintains four 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”), the 2018 Employee Share Purchase Plan (the “ESPP”), and the 2020 Inducement Equity Plan (the “Inducement Plan”). The number of shares of common stock that may be issued under the 2018 Plan is subject to increase by the number of shares forfeited under any options forfeited and not exercised under the 2018 Plan or 2016 Plan. 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 fair value of each stock option award is determined on the date of grant using the Black-Scholes option-pricing model. The risk-free interest rate is based on a U.S. treasury instrument whose term is consistent with the expected term of the stock options. The expected term of the Company’s options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. The expected volatility is based on the historical volatility of a representative group of companies with similar characteristics to the Company, including those in the early stages of product development with a similar and therapeutic focus. For these analyses, the Company selects companies with comparable characteristics to its own including enterprise value, risk profiles, position within the industry, and with historical share price information sufficient to meet the expected term of the options. Year Ended December 31, 2020 2019 Risk-free interest rate 0.3 - 1.7% 1.4 - 2.6% Expected term (in years) 5.5 - 6.1 5.5 - 6.1 Expected volatility 70.7 - 75.2% 70.1 - 72.1% Expected dividend rate 0.00% 0.00% The following table summarizes option activity under the plans for the year ended December 31, 2020: Shares Weighted Average Exercise Price per Share Weighted Average Remaining Contractual Life (Years) Aggregate Intrinsic Value Outstanding at December 31, 2019 12,216,140 $ 6.61 8.31 $ 91,133 Granted 5,846,152 11.37 Exercised (1,154,441 ) 2.99 Forfeited (3,012,208 ) 10.99 Outstanding and expected to vest at December 31, 2020 13,895,643 $ 7.96 7.16 $ 15,473 Exercisable, as of December 31, 2020 7,120,307 $ 5.98 5.73 $ 12,318 The aggregate intrinsic value of options is calculated as the difference between the exercise price of the options and the fair value of the Company’s ordinary shares for those options that had exercise prices lower than the fair value of the Company’s ordinary shares. During the years ended December 31, 2020 and 2019, the total intrinsic value of share options exercised was $5.0 million and $17.2 million, respectively. During the years ended December 31, 2020 and 2019, the total proceeds to the Company from share options exercised was $3.9 million and $2.0 million, respectively. As of December 31, 2020, and 2019, there was $0.2 million and nil The weighted average grant date fair value of the options granted during the years ended December 31, 2020 and 2019 was $7.22 per shares and $8.67 per share, respectively. Restricted Share Units Performance-based 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 RSUs will vest, and the award will become fully vested upon achievement of three of the four performance conditions. No performance-based share units vested during the years ended December 31, 2020 or 2019. The fair value associated with the performance-based conditions is recognized when achievement of the milestones becomes probable, if at all. In the fourth quarter of 2020, the Company determined that a performance milestone was probable upon approval of Libmeldy by the European Commission in December 2020, and recognized $1.2 million in compensation cost. The shares associated with recognition of this performance milestone vested and were issued in January 2021. The amount of compensation cost recognized for the years ended December 31, 2020 and 2019 for the market condition associated with the performance-based RSUs was $ 0.3 million and $ 1.2 million, respectively. CEO Award The Company granted 195,000 performance-based RSUs with a total grant date fair value of $1.4 million to its Chief Executive Officer, Bobby Gaspar, M.D., Ph.D., in April 2020. The award vests on January 2, 2024 as to 1/3 of the award for each of the first three to occur of four milestones, if each such milestone is achieved by the Company on or before December 31, 2023 and Dr. Gaspar remains continuously employed with the Company through January 2, 2024. The milestones relate to achievement of specific clinical and regulatory milestones. No performance-based share unit performance conditions associated with the CEO award were deemed probable and none vested during the year ended December 31, 2020. Time-based restricted share units Time-based restricted share units general vest in equal annual installments over a three-year The following table summarizes restricted share unit award activity for the year-end December 31, 2020: Shares Weighted Average Fair Value per Share Unvested at December 31, 2019 556,422 $ 13.58 Granted 426,750 6.42 Vested — — Forfeited (339,172 ) 13.75 Unvested at December 31, 2020 644,000 $ 8.75 Share-based compensation Share-based compensation expense related to share options, restricted share unit awards, and the employee stock purchase plan was classified in the consolidated statements of operations and comprehensive loss as follows: Year Ended December 31, 2020 2019 Research and development $ 11,679 $ 7,425 Selling, general and administrative 16,283 11,999 Total $ 27,962 $ 19,424 The Company had 6,775,336 unvested options outstanding as of December 31, 2020. As of December 31, 2020, total unrecognized compensation cost related to unvested stock option grants and time-based RSUs was approximately $46.4 million. This amount is expected to be recognized over a weighted average period of approximately 2.52 years. As of December 31, 2020, the total unrecognized compensation cost related to performance-based RSUs is a maximum of $4.0 million, dependent upon achievement of milestones.

License and Research Agreements

License and Research Agreements	12 Months Ended
	Dec. 31, 2020
License And Research Arrangements [Abstract]
License and Research Arrangements	14. License and Research Arrangements GSK asset purchase and license agreement In April 2018, the Company completed an asset purchase and license agreement (the “GSK Agreement”) with subsidiaries of GSK to acquire a portfolio of autologous ex vivo • 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 was £94.2 million ($133.6 million at the acquisition date), which included an upfront payment of £10.0 million ($14.2 million at the acquisition date) and 12,455,252 convertible preferred shares of the Company issued to GSK at an aggregate value of £65.8 million ($93.4 million at the acquisition date), a loss contract on the Strimvelis program valued at £12.9 million ($18.4 million), an inventory purchase liability valued at £4.9 million ($6.9 million) and transaction costs of £0.6 million ($0.8 million). The Company allocated £94.2 million ($133.6 million) to in-process research and development expense (based on the fair value of the underlying programs in development). The convertible preferred shares were converted to ordinary shares as part of our IPO in November 2018. The Company is required to use commercially reasonable efforts to obtain a Priority Review Voucher (“PRV”) from the United States Food and Drug Administration for each of the programs for MLD, WAS and TDT, the first of which GSK retained beneficial ownership over. GSK also has an option to acquire, at a price pursuant to an agreed upon formula, any PRV granted to the Company thereafter for MLD, WAS and TDT. If GSK does not exercise this option to purchase any PRV, the Company may sell the PRV to a third party and must share any proceeds in excess of a specified sale price equally with GSK. For accounting purposes, as of December 31, 2020, the Company does not consider the attainment of a PRV from the United States Food and Drug Administration to be probable. As part of the GSK Agreement the Company is required to use its best endeavors to make Strimvelis commercially available in the European Union until such time as an alternative gene therapy, such as the Company’s OTL-101 product candidate, is commercially available for patients in Italy, and at all times at the San Raffaele Hospital in Milan, provided that a minimum number of patients continue to be treated at this site. Strimvelis is not currently expected to generate sufficient cash flows to overcome the costs of maintaining the product and certain regulatory commitments; therefore, the Company recorded a liability associated with the loss contract of £12.9 million ($18.4 million at the acquisition date) associated with the loss expected due to this obligation. This liability is being amortized over the remaining period of expected sales of Strimvelis as a credit to research and development expenses (see Note 2). The Company will pay GSK non-refundable royalties and milestone payments in relation to the gene therapy programs acquired and OTL-101. The Company will pay a flat mid-single digit percentage royalty on the combined annual net sales of ADA-SCID products, which includes Strimvelis and the Company-developed product candidate, OTL-101. The Company will also pay tiered royalty rates at a percentage beginning in the mid-teens up to twenty percent for the MLD and WAS products, upon marketing approval, calculated as percentages of aggregate cumulative net sales of the MLD and WAS products, respectively. The Company will pay a tiered royalty at a percentage from the high single-digits to low double-digit for the TDT product, upon marketing approval, calculated as percentages of aggregate annual net sales of the TDT product. These royalties owed to GSK are in addition to any royalties owed to other third parties under various license agreements for the GSK programs. In aggregate, the Company may pay up to £90.0 million in milestone payments upon achievement of certain sales milestones applicable to GSK. The Company’s royalty obligations with respect to MLD and WAS may be deferred for a certain period in the interest of prioritizing available capital to develop each product. The Company’s royalty obligations are subject to reduction on a product-by-product basis in the event of market control by biosimilars and will expire in April 2048. Other than Strimvelis, these royalty and milestone payments were not determined to be probable and estimable at the date of the acquisition and are not included as part of consideration. The Company and GSK also separately executed a Transition Services Agreement (“TSA”) as well as an Inventory Sale Agreement, in April 2018. The TSA outlined several activities that the Company had requested GSK to assist with during the transition period, including but not limited to utilizing GSK to sell, market and distribute Strimvelis, and assist with regulatory, clinical and non-clinical activities for the other non-commercialized products which were ongoing at the date of the GSK Agreement. The TSA expired in December 2018. In connection with the Company’s entering into the GSK Agreement, GSK assigned rights and obligations to certain contracts, which include among others, the original license agreement with Telethon-OSR and an ongoing manufacturing agreement (see Note 16). 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 As consideration for the licenses, the Company will be required to make payments to Telethon-OSR upon achievement of certain product development milestones. Additionally, the Company will be required to pay to Telethon-OSR a tiered mid-single to low-double digit royalty percentage on annual sales of licensed products covered by patent rights on a country-by-country basis, as well as a low double-digit percentage of sublicense income received from any certain third-party sublicenses of the collaboration programs. These royalties are in addition to those payable to GSK under the GSK Agreement. The Company may pay up to an aggregate of approximately €31.0 million ($38.1 million at December 31, 2020) in milestone payments upon achievement of certain product development milestones for the program. In May 2019, the Company entered into a license agreement with Telethon-OSR, under which Telethon-OSR granted to the Company an exclusive worldwide license for the research, development, manufacture and commercialization of Telethon-OSR’s ex vivo UCLB/UCLA License Agreement In February 2016, and amended in July 2017, the Company completed the UCLB/UCLA license agreement, under which the Company has been granted exclusive and non-exclusive, sublicensable licenses under certain intellectual property rights controlled by UCLB and UCLA to develop and commercialize gene therapy products in certain fields and territories. In exchange for these rights, in 2016, the Company made upfront cash payments consisting of $0.8 million for the license to the joint UCLB/UCLA technology and $1.1 million for the license to the UCLB technology and manufacturing technology. The Company also issued an aggregate of 4,665,384 ordinary shares to UCLB, of which 1,224,094, and 3,441,290 ordinary shares were issued in 2017 and 2016, respectively. The Company recorded research and development expense based on the fair value of the ordinary shares as of the time the agreement was executed or modified. The Company was also obligated to make an additional cash payment for clinical data. In 2017, the Company paid $0.8 million in relation to clinical data acquired. The Company recorded the payments to research and development expense. Under the UCLB/UCLA License Agreement, the Company is also obligated to pay an annual administration fee of $0.1 million on the first, second and third anniversary of the agreement date. Additionally, the Company may become obligated to make payments to the parties of up to an aggregate of £19.9 million upon the achievement of specified regulatory milestones as well as royalties ranging from low to mid-single-digit percentage on net sales of the applicable gene therapy product. The Company recorded $0.1 million of research and development costs in respect of the UCLB/UCLA license agreement, which comprise the upfront payments, issuance of ordinary shares and payments for clinical data, for each of the years ended December 31, 2020 and 2019. Unless terminated earlier by either party, the UCLB/UCLA license agreement will expire on the 25 th Oxford BioMedica license, development and supply agreement In November 2016, and as amended in June 2017, May 2018, July 2018, September 2018, May 2019 and April 2020 , the Company entered into an arrangement with Oxford BioMedica whereby Oxford BioMedica granted an exclusive intellectual property license to the Company for the purposes of research, development, and commercialization of collaboration products, and will provide process development services, and manufacture clinical and commercial GMP-grade lentiviral vectors to the Company (“Oxford BioMedica Agreement”). As part of the consideration to rights and licenses granted under the Oxford BioMedica Agreement, the Company issued 588,220 ordinary shares to Oxford BioMedica. The Company is also obligated to make certain development milestone payments in the form of issuance of additional ordinary shares if the milestones are achieved. In November 2017, the first milestone was achieved, and the Company was committed to issue another 150,826 ordinary shares, and issued these shares in 2018. In September 2018, the second and fourth milestones were achieved, and the Company issued 150,826 ordinary shares. In April 2020, the fifth milestone was deemed to have been met upon execution of the amended agreement in April 2020, and the Company issued 75,413 ordinary shares to Oxford BioMedica with a total value of $0.8 million, which was expensed to research and development expense. No milestones were met during the year ended December 31, 2019. The Company may also pay low single-digit percentage royalties on annual net sales of collaborated product generated under the Oxford BioMedica Agreement.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Income Taxes	15. Income Taxes The components of income (loss) from operations before income taxes for the years ended December 31, 2020 and 2019 are as follows: December 31, 2020 2019 UK $ (155,614 ) $ (173,118 ) Non-UK 2,904 7,456 Loss before taxes $ (152,710 ) $ (165,662 ) The (benefit from) provision for income taxes for the years ended December 31, 2020 and 2019 are as follows: December 31, 2020 2019 Current (benefit) provision Federal—United States $ 1,107 $ 888 State—United States 189 (275 ) Other foreign 230 89 Total current (benefit) provision 1,526 702 Deferred (benefit) provision Federal—United States (1,774 ) (2,820 ) State—United States (103 ) (122 ) Other foreign (380 ) — Total deferred (benefit) provision (2,257 ) (2,942 ) Total (benefit) provision for income taxes $ (731 ) $ (2,240 ) The following table presents a reconciliation of income tax (benefit) expense computed at the UK statutory income tax rate to the effective income tax rate as reflected in the consolidated financial statements (in thousands): December 31, 2020 2019 Income taxes at United Kingdom statutory rate $ (29,015 ) $ (31,475 ) Change in valuation allowance 29,302 16,507 Reduction in research expense for credits granted 8,435 9,787 Change in tax rates (8,105 ) 8,109 Tax credits (1,369 ) (3,372 ) U.S. Deduction for foreign derived intangible income (1,254 ) (2,058 ) Permanent differences, including share-based compensation deduction shortfalls 1,265 344 U.S. state income taxes 68 (238 ) Foreign rate differential (58 ) 156 Total (benefit) provision for income taxes $ (731 ) $ (2,240 ) The Company’s income tax benefit for the year ended December 31, 2020 compared to the year ended December 31, 2019 decreased primarily related to shortfall of tax deduction from share-based compensation and reduction of U.S. deduction for foreign derived intangible income (“FDII”) and U.S. federal research and development tax credits. The Company accounts for income taxes in accordance with ASC Topic 740. Deferred income tax assets and liabilities are determined based upon temporary differences between the financial reporting and tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. The following table presents the principal components of the Company’s deferred tax assets and liabilities as of December 31, 2020 and 2019: December 31, 2020 2019 Deferred tax assets Net operating loss carryforwards $ 75,502 $ 45,358 Amortization 22,599 21,741 Research and development credits 1,564 1,244 Share-based compensation 7,400 3,604 Accruals 1,001 1,286 Lease Liability 6,805 4,406 Other 3 1 Total deferred tax assets 114,874 77,640 Valuation allowance (103,890 ) (70,153 ) Fixed assets and right-of-use asset (5,765 ) (4,502 ) Other non-current assets (net deferred tax assets and liabilities) $ 5,219 $ 2,985 For the years ended December 31, 2020 and 2019, the Company had cumulative UK net operating loss carryforwards of approximately $390.1 million and $266.8 million, respectively. Unsurrendered UK losses may be carried forward indefinitely, subject to numerous utilization criteria and restrictions and are fully offset by a valuation allowance. For the year ended December 31, 2020, the Company had cumulative U.S. federal general business and U.S. state research and development tax credit carryforwards of approximately $2.0 million available to reduce future U.S. state tax liabilities. The U.S. state tax credit carryforwards can be carried forward indefinitely and are fully offset by a valuation allowance. In measuring the Company’s deferred tax assets, the Company considers all available evidence, both positive and negative, to determine whether, based on the weight of that evidence, a valuation allowance is needed for all or some portion of the deferred tax assets. Significant judgment is required in considering the relative impact of the negative and positive evidence, and weight given to each category of evidence is commensurate with the extent to which it can be objectively verified. The more negative evidence that exists, the more positive evidence is necessary, and the more difficult it is to support a conclusion that a valuation allowance is not needed. Additionally, the Company utilizes the "more likely than not" criteria established in FASB ASC Topic 740 to determine whether the future tax benefit from the deferred tax assets should be recognized. As a result, the Company has established valuation allowances on the deferred tax assets in jurisdictions that have incurred net operating losses and in which it is more likely than not that such losses will not be utilized in the foreseeable future. As of each reporting date, we consider new evidence, both positive and negative, that could impact our view with regard to future realization of our deferred tax assets. Management has considered the Company’s history of cumulative net losses in the UK, along with estimated future taxable income and has concluded that it is more likely than not that the Company will not realize the benefits of its UK deferred tax assets and U.S. state research and development tax credits. Accordingly, the Company has maintained a full valuation allowance against these net deferred tax assets as of December 31, 2020 and 2019, respectively. Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2020 and 2019 related primarily to the increase in UK net operating loss carryforwards and UK amortization of intangible assets and were as follows: December 31, 2020 2019 Valuation allowance as of beginning of year $ (70,153 ) $ (51,281 ) Decreases recorded as benefit to income tax provision — — Increases recorded to income tax provision (29,302 ) (16,507 ) Effect of foreign currency translation (4,435 ) (2,365 ) Valuation allowance as of end of year $ (103,890 ) $ (70,153 ) The Company applies the authoritative guidance on accounting for and disclosure of uncertainty in tax positions, which requires the Company to determine whether a tax position of the Company is more likely than not to be sustained upon examination, including resolution of any related appeals of litigation processes, based on the technical merits of the position. For tax positions meeting the more likely than not threshold, the tax amount recognized in the financial statements is reduced by the largest benefit that has a greater than fifty percent likelihood of being realized upon the ultimate settlement with the relevant taxing authority. There were no material uncertain tax positions as of December 31, 2020 and 2019. The Company will recognize interest and penalties related to uncertain tax positions in income tax expense when in a taxable income position. As of December 31, 2020, and 2019, the Company had no accrued interest or penalties related to uncertain tax positions and no amounts have been recognized in the Company’s statement of operations. The Company and its subsidiaries file income tax returns in the UK, the U.S., and various foreign jurisdictions. Generally, the tax years 2017 through 2020 remain open to examination by the major taxing jurisdictions to which the Company is subject. To the extent the Company has tax attribute carryforwards, the tax years in which the attribute was generated may still be adjusted upon examination by the federal, state, or foreign tax authorities, if such tax attributes are utilized in a future period.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2020
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	16. Commitments and Contingencies Lease commitments The Company leases office and laboratory space and has an embedded lease at AGC. Refer to Note 10, Leases Manufacturing and technology development master agreement with AGC As discussed in Note 10 , on July 2, 2020, the Company entered into the AGC Agreement, pursuant to which AGC will develop, manufacture and supply certain viral vectors and conduct cell processing activities for certain Company development and commercial programs. Under the terms of the AGC Agreement, the Company is obligated to pay AGC for a minimum product manufacturing commitment, dedicated manufacturing and development resources, and for a lease component associated with the right of use of exclusive manufacturing suites within AGC’s existing facilities The following table outlines the annual commitments associated with the contract, as of December 31, 2020: Due in: Product manufacturing commitments (1) Dedicated manufacturing and development resources (2) Exclusive transduction suites (3) Total AGC Commitment 2021 $ 2,491 $ 8,524 $ 4,190 $ 15,205 2022 3,321 8,524 3,352 15,197 2023 3,321 8,524 3,352 15,197 2024 3,321 8,524 3,352 15,197 2025 1,661 4,262 838 6,761 Thereafter — — — — Total manufacturing commitments $ 14,115 $ 38,358 $ 15,084 $ 67,557 *Tabular disclosure above has been translated to U.S. Dollar, from Euro, using an exchange rate of €1.00 to $1.23. (1) The minimum product manufacturing commitments may be increased to the mid-seven figures per contract year upon achievement of certain milestones. (2) The Company may increase or decrease the usage of dedicated development services on a rolling basis with between six and 12-months’ prior written notice to AGC. The above table assumes continued usage of dedicated development services at current rates. (3) Refer to Note 10 for further information on the embedded operating lease agreement The AGC Agreement has an initial term of five years, beginning on the Effective Date and ending July 2, 2025. The AGC Agreement may be extended for an additional two years by mutual agreement of the Company and AGC. The Company has the right to terminate the AGC Agreement at its discretion upon 12-month’s prior written notice to AGC, and beginning no earlier than July 2, 2022, AGC has the right to terminate the AGC Agreement at its discretion upon 24-month’s prior written notice to the Company. Each party may terminate the AGC Agreement upon prior notice to the other party for an uncured material breach that the breaching party does not cure within the notice period. Other funding commitments The Company has entered into several license agreements (see Note 14). In connection with these agreements the Company is required to make milestone payments and annual license maintenance payments or royalties on future sales of specified products. Consulting Agreement In December 2019, the Company entered into a consulting agreement with non-employee advisor whereby the Company is obligated to make cash payments of $0.1 million per year and to issue up to 91,034 ordinary shares, which vest annually over a four year period, and 92,035 ordinary shares upon attainment of certain clinical development and regulatory milestones. In December 2020, the Company issued 22,758 ordinary shares associated with the service condition. During the years ended December 31, 2020 and 2019, the Company recorded $0.3 million and nil in research and development expense associated with the share-based awards with service conditions. During the years ended December 31, 2020 and 2019, Legal proceedings The Company is not a party to any litigation and does not have contingency reserves established for any litigation liabilities.

Benefit Plans

Benefit Plans	12 Months Ended
	Dec. 31, 2020
Compensation And Retirement Disclosure [Abstract]
Benefit Plans	17. Benefit Plans The Company makes contributions to private defined contribution pension plans on behalf of its employees. The Company matches its employee contributions up to six percent of each employee’s annual salary based on the jurisdiction the employees are located. The Company paid $1.6 million and $1.3 million, in matching contributions for the years ended December 31, 2020 and 2019, respectively.

Related-Party Transactions

Related-Party Transactions	12 Months Ended
	Dec. 31, 2020
Related Party Transactions [Abstract]
Related Party Transactions	18. Related-party Transactions GSK In April 2018, the Company completed the GSK Agreement with subsidiaries of GSK (See Note 14). As consideration under the agreement the Company paid an upfront fee of $14.2 million, purchased inventory of $6.9 million, paid $0.8 million in transaction costs, and issued 12,455,252 convertible preferred shares valued at $93.4 million. Additionally, as part of the GSK Agreement, the Company obtained, and is responsible for maintaining the commercial availability of Strimvelis. The Company recorded a loss provision of $18.4 million associated with the agreement, as the costs to maintain Strimvelis are expected to significantly exceed revenues. The issuance of the convertible preferred shares made GSK a principal shareholder in the Company. As of December 31, 2020, the Company had accounts payable and accrued expenses due to GSK of $0.1 During the year-ended December 31, 2019, the Company made $7.2 million in payments to settle accounts payable due to GSK associated with the TSA and royalties associated with sales of Strimvelis incurred during 2018. Additionally, during 2019, the Company made a $3.6 million payment associated with the inventory purchase liability incurred upon entering into the agreement, and $0.1 million in royalties associated with Strimvelis sales during the year. As of December 31, 2019, the Company had inventory purchase liability in accrued research and development expenses of $3.3 million.

Selected Quarterly Financial In

Selected Quarterly Financial Information (unaudited)	12 Months Ended
	Dec. 31, 2020
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Information (unaudited)	19. Selected Quarterly Financial Information (unaudited) The following tables summarizes the unaudited quarterly financial data for the last two fiscal years: 2020 First Quarter Second Quarter Third Quarter Fourth Quarter Full year Total revenues $ — $ 597 $ 1,998 $ — $ 2,595 Total costs and operating expenses 44,981 47,418 28,301 38,873 159,573 Loss from operations (44,981 ) (46,821 ) (26,303 ) (38,873 ) (156,978 ) Net loss attributable to ordinary shareholders (50,569 ) (47,500 ) (20,290 ) (33,620 ) (151,979 ) Weighted average ordinary shares outstanding - basic and diluted 98,713,126 99,251,314 99,664,616 100,013,246 99,445,874 Earnings per share $ (0.51 ) $ (0.48 ) $ (0.20 ) $ (0.34 ) $ (1.53 ) 2019 First Quarter Second Quarter (2) Third Quarter Fourth Quarter Full year Total revenues $ — $ — $ 1,918 $ 595 $ 2,513 Total costs and operating expenses 28,283 54,152 43,330 49,621 175,386 Loss from operations (28,283 ) (54,152 ) (41,412 ) (49,026 ) (172,873 ) Net loss attributable to ordinary shareholders (30,739 ) (50,530 ) (36,737 ) (45,416 ) (163,422 ) Weighted average ordinary shares outstanding - basic and diluted 87,010,596 89,712,916 97,817,847 98,243,915 93,240,355 Earnings per share $ (0.35 ) $ (0.56 ) $ (0.38 ) $ (0.46 ) $ (1.75 )

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2020
Subsequent Events [Abstract]
Subsequent Events	20. Subsequent Events Securities Purchase Agreement On February 9, 2021, the Company issued and sold (i) 20,900,321 ordinary shares, nominal value £0.10 per share, at a purchase price of $6.22 per share (the “Purchase Price”), which was the closing sale price of the Company’s ADSs on the Nasdaq Global Select Market on February 4, 2021, and (ii) 3,215,434 non-voting ordinary shares, nominal value £0.10 per share, at the Purchase Price (the “Private Placement”). The Private Placement resulted in gross proceeds to the Company of $150.0 million before deducting placement agent fees of $6.0 million. The ordinary shares and non-voting ordinary shares were sold pursuant to a securities purchase agreement entered into between the Company and the purchasers named therein on February 4, 2021.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Principles of consolidation	Principles of consolidation The accompanying consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation.
Use of estimates	Use of estimates The preparation of consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenue and expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual for research and development expenses, the research and development tax credit receivable, share-based compensation, operating lease assets and liabilities, and income taxes. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. The future developments of the COVID-19 pandemic also may directly or indirectly impact the Company’s business include quarantines, border closures, increased border controls, travel restrictions, shelter-in-place orders and shutdowns, business closures, cancellations of public gatherings and other measures. Actual results could differ from the Company’s estimates.
Concentration of credit risk	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, cash equivalents, marketable securities, and receivables. 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	Foreign currency The financial statements of the Company’s subsidiaries with functional currencies other than the U.S. dollar are translated into U.S. dollars using period-end exchange rates for assets and liabilities, historical exchange rates for stockholders’ equity and weighted average exchange rates for operating results. Translation gains and losses are included in accumulated other comprehensive income (loss) in shareholders’ equity. Foreign currency transaction gains and losses are included in other income (expense), net in the results of operations. The Company recorded realized and unrealized foreign currency transaction gains of $3.4 million, and $1.4 million for the years ended December 31, 2020 and 2019, respectively, which is included in other income (expense) in the statements of operations and comprehensive loss.
Segment information	Segment information The Company operates in a single segment, focusing on researching, developing and commercializing potentially curative gene therapies. Consistent with its operational structure, its chief operating decision maker manages and allocates resources at a global, consolidated level. Therefore, results of the Company's operations are reported on a consolidated basis for purposes of segment reporting. All material long-lived assets of the Company reside in the United States or United Kingdom. The Company had property and equipment of $3.7 million and $1.1 million located in the United Kingdom and United States, respectively, as of December 31, 2020.
Cash and cash equivalents	Cash and cash equivalents The Company considers all highly liquid investments purchased with original maturities of 90 days or less at the date of acquisition to be cash equivalents.
Marketable securities	Marketable securities Marketable securities consist of investments with original maturities greater than ninety days at the date of acquisition. The Company has classified its investments with maturities beyond one year as short term, based on their highly liquid nature and because such marketable securities represent the investment of cash that is available for current operations. The Company considers its investment portfolio of investments as available-for-sale. Accordingly, these investments are recorded at fair value, which is based on quoted market prices or other observable inputs. Unrealized gains and losses are recorded as a component of other comprehensive income (loss). Realized gains and losses are determined on a specific identification basis and are included in other income (loss). Amortization and accretion of discounts and premiums is also recorded in other income (loss). When the fair value is below the amortized cost of the asset an estimate of expected credit losses is made, and is limited to the amount by which fair value is less than amortized cost. The credit-related impairment amount is recognized in net income; the remaining impairment amount and unrealized gains are reported as a component of accumulated other comprehensive income (loss) in shareholders’ equity. Credit losses are recognized through the use of an allowance for credit losses account and subsequent improvements in expected credit losses are recognized as a reversal of the allowance account. If the Company has the intent to sell the security or it is more likely than not that the Company will be required to sell the security prior to recovery of its amortized cost basis the allowance for credit loss is written off and the excess of the amortized cost basis of the asset over its fair value is recorded in the consolidated statements of operation.
Restricted cash	Restricted cash Cash and cash equivalents that are restricted as to withdrawal or use under the terms of certain contractual agreements are recorded as restricted cash on our consolidated balance sheet. The Company has 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 at December 31, 2020 and 2019. The Company is also contractually required to maintain a cash collateral account associated with corporate credit cards and other leases in the amount of $1.3 The Company includes the restricted cash balance in cash and cash equivalents when reconciling beginning-of-period and end-of-period total amounts shown on the consolidated statements of cash flows. The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported in the consolidated balance sheet that sum to the total of the amounts reported in the consolidated statement of cash flows: As of December 31, 2020 2019 Cash and cash equivalents $ 55,135 $ 19,053 Restricted cash 4,266 4,264 Total cash, cash equivalents and restricted cash shown in the statement of cash flows $ 59,401 $ 23,317
Property and equipment	Property and equipment Property and equipment are recorded at cost and depreciated or amortized using the straight-line method over the following estimated useful lives. Property and equipment: Estimated useful life Lab equipment 5-10 years Leasehold improvements Shorter of lease term or estimated useful life Furniture and fixtures 4 years Office and computer equipment 3-5 years Repairs and maintenance expenditures, which are not considered improvements and do not extend the useful life of property and equipment, are expensed as incurred. 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.
Impairment of long-lived assets	Impairment of long-lived assets Long-lived assets consist of property and equipment and operating lease right-of-use assets. Long-lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset group for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset group to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset group are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset group over its fair value, as determined in accordance with the related accounting literature .
Research and development costs	Research and development costs Research and development costs are expensed as incurred. Research and development expenses consist of costs incurred in performing research and development activities, including salaries, share-based compensation and benefits, facilities costs, depreciation, third-party license fees, certain milestone payments, and external costs of outside vendors engaged to conduct clinical development activities and clinical trials, as well as costs to develop a manufacturing process, perform analytical testing and manufacture clinical trial materials. Non-refundable prepayments for goods or services that will be used or rendered for future research and development activities are recorded as prepaid expenses. Such amounts are recognized as an expense as the goods are delivered or the related services are performed, or until it is no longer expected that the goods will be delivered, or the services rendered. In addition, funding from research grants is recognized as an offset to research and development expense on the basis of costs incurred on the research program. Royalties to third parties associated with our research grants will be accrued when they become probable.
Research contract costs and accruals	Research contract costs and accruals The Company has entered into various research and development contracts. These agreements are cancelable, and related costs are recorded as research and development expenses as incurred. When billing terms under these contracts do not coincide with the timing of when the work is performed, the Company is required to make estimates of outstanding obligations as of period end to those third parties. Any accrual estimates are based on a number of factors, including the Company’s knowledge of the progress towards completion of the research and development activities, invoicing to date under the contracts, communication from the research institution or other companies of any actual costs incurred during the period that have not yet been invoiced, and the costs included in the contracts. Significant judgments and estimates may be made in determining the accrued balances at the end of any reporting period. Actual results could differ from the estimates made by the Company. The historical accrual estimates made by the Company have not been materially different from the actual costs.
Share-based compensation	Share-based compensation The Company measures all stock options and other stock-based awards granted to employees at fair value on the date of grant. The Company uses the Black-Scholes option-pricing model in the valuation of its stock options. The fair value of performance-based share awards and restricted stock units is based on the fair value of the stock on the date of grant. The Company uses the Monte-Carlo model in order to calculate the fair value of the market-based awards. The fair value of options is recognized as stock-based compensation expense over the requisite service period, which is generally the vesting period of the respective award. The Company accounts for stock-based compensation expense related to forfeitures as the forfeitures occur. The straight-line method of expense recognition is applied to all awards with service-based and market-based conditions. The Company records stock-based compensation expense related to performance-based awards when the performance-based targets are probable of being achieved. The Company classifies stock-based compensation expense in the consolidated statements of operations in the same manner in which the award recipient’s payroll costs are classified.
Comprehensive loss	Comprehensive loss Comprehensive loss is composed of net loss and other comprehensive income (loss). Other comprehensive income (loss) consists of unrealized gains and losses on marketable debt securities and foreign currency translation.
Leases	Leases The Company determines if an arrangement is a lease at contract inception. Operating lease assets represent a right to use an underlying asset for the lease term and operating lease liabilities represent an obligation to make lease payments arising from the lease. Operating lease liabilities with a term greater than one year and their corresponding right-of-use assets are recognized on the balance sheet at the commencement date of the lease based on the present value of lease payments over the expected lease term. Certain adjustments to the right-of-use asset may be required for items such as initial direct costs paid or incentives received. The Company made an accounting policy election to not record a right-of-use asset or lease liability for leases with a term of one year or less. To date, the Company has not identified any material short-term leases, either individually or in the aggregate. As the Company’s leases do not provide an implicit rate, the Company utilized the appropriate incremental borrowing rate, which is the rate incurred to borrow on a collateralized basis over a similar term as the lease an amount equal to the lease payments in a similar economic environment. The Company estimated the incremental borrowing rate based on the Company’s currently outstanding credit facility as inputs to the analysis to calculate a spread, adjusted for factors that reflect the profile of secured borrowing over the expected term of the lease. The components of a lease should be split into three categories: lease components (e.g., land, building, etc.), non-lease components (e.g., common area maintenance, utilities, performance of manufacturing services, purchase of inventory, etc.), and non-components (e.g., property taxes, insurance, etc.). Then the fixed contract consideration (including any related to non-components) must be allocated based on fair values to the lease components and non-lease components. Although separation of lease and non-lease components is required, certain practical expedients are available to entities. Entities electing the practical expedient would not separate lease and non-lease components. Rather, they would account for each lease component and the related non-lease component together as a single component. The Company has elected not to apply the practical expedient and with respect to its lease of manufacturing space at a contract manufacturing organization, the Company has allocated the consideration between the lease and non-lease components of the contract based on the respective fair values of the lease and non-lease components. The Company calculated the fair value of the lease and non-lease components using financial information readily available as part of its master services arrangement and other representative data indicative of fair value. The Company accounts for sublease income on a straight-line basis over the respective lease period and records an unbilled rent receivable for sublease income incurred but not yet paid. The Company periodically performs a collectability assessment associated with any unbilled rent receivables. The Company recognizes the sublease income as a reduction to the related operating expense associated with the head lease.
Strimvelis loss provision	Strimvelis loss provision As part of the GSK transaction, the Company is required to maintain commercial availability of Strimvelis in the European Union until such time that an alternative gene therapy is available (Note 13). 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 as part of the GSK transaction in 2018. The Company recognizes the amortization of the loss provision on a diminishing balance basis based on the actual net loss incurred associated with Strimvelis and the expected future net losses to be generated until such time as Strimvelis is no longer commercially available. The amortization of the provision is recorded as a credit to research and development expense. We have made an estimate of the expected future losses associated with Strimvelis and adjust this estimate as facts and circumstances change regarding the commercial availability and costs of maintaining and selling Strimvelis. The Company does not update the accrued loss provision for any subsequent adjustment of the future losses, however, the timing of recognizing the amortization of what was originally recorded is adjusted for updates to estimates of potential future losses. The Company paused treating new patients with Strimvelis in October 2020 upon learning that a patient treated with the drug in 2016 under a compassionate use program was diagnosed with lymphoid T cell leukemia, a known risk factor for gammaretroviral vector-based gene therapy. The EMA’s Committee for Medicinal Products for Human Use, or CHMP, reviewed the updated risk-benefit assessment of Strimvelis as part of its ongoing MAA renewal procedure, concluded that the risk-benefit balance remains favorable and recommended in February 2021 that the marketing authorization for Strimvelis be renewed for five years, allowing marketing of Strimvelis to resume. Year Ended December 31, 2020 2019 Balance at beginning of period $ 6,790 $ 10,339 Provisions — — Amortization of loss provision (2,413 ) (3,855 ) Foreign currency translation 105 306 Balance at end of period $ 4,482 $ 6,790 As of December 31, 2020, $0.9 million of the Strimvelis loss provision was classified as current, and $3.6 million was classified as non-current. As of December 31, 2019, $3.0 million of the Strimvelis loss provision was classified as current, and $3.8 million was classified as non-current.
United Kingdom Research and development income tax credits	United Kingdom Research and development income tax credits As a company that carries out research and development activities, the Company is able to submit tax credit claims from two UK research and development tax relief programs, the Small and Medium-sized Enterprises research and development tax credit (“SME”) program and the Research and Development Expenditure Credit (“RDEC”) program depending on eligibility. Qualifying expenditures largely comprise employment costs for research staff, consumables and certain internal overhead costs incurred as part of research projects for which the Company does not receive income. Each reporting period, management evaluates which tax relief programs the Company is expected to be eligible for and records a reduction to research and development expense for the portion of the expense that it expects to qualify under the programs, that it plans to submit a claim for, and it has reasonable assurance that the amount will ultimately be realized. Based on criteria established by HM Revenue and Customs (“HMRC”), management of the Company expects a proportion of expenditures being carried in relation to its pipeline research, clinical trials management and manufacturing development activities to be eligible for the research and development tax relief programs for the year ended December 31, 2020. The Company has qualified under the more favorable SME regime for the year ended December 31, 2019 and expects to qualify under the SME regime for the year ending December 31, 2020. The RDEC and SME credits are not dependent on the Company generating future taxable income or on the ongoing tax status or tax position of the Company. The Company has assessed its research and development activities and expenditures to determine whether the nature of the activities and expenditures will qualify for credit under the tax relief programs and whether the claims will ultimately be realized based on the allowable reimbursable expense criteria established by the UK government which are subject to interpretation. At each period end, the Company estimates the reimbursement available to the Company based on available information at the time. The Company recognizes credits from the research and development incentives when the relevant expenditure has been incurred and there is reasonable assurance that the reimbursement will be received. Such credits are accounted for as reductions in research and development expense. The following table outlines the changes to the research and development tax credit receivable, including amount recognized as an offset to research and development expense during the years ended December 31, 2020 and 2019: Year Ended December 31, 2020 2019 Balance at beginning of period $ 28,644 $ 10,585 Recognition of credit claims as offset to research and development expense 21,130 17,564 Receipt of credit claims (33,771 ) (152 ) Foreign currency translation 1,341 647 Balance at end of period $ 17,344 $ 28,644 During the year ended December 31, 2020, the Company recorded $4.8 million of additional tax credits related to a change in estimate associated with its UK research and development tax credit receivable claim for fiscal year 2019. The change in estimate was based on the results of a tax credit analysis associated with the Company’s qualified projects and research and development expenditures completed during the third quarter to finalize the 2019 UK tax return. As of December 31, 2020, the Company’s tax credit receivable from the UK was $17.3 million, all of which was classified as current. As of December 31, 2019, the Company’s tax incentive receivable from the UK was $28.6 million, of which $14.9 million was classified as current and $13.7 million was classified as non-current. As of December 31, 2020, the Company has received all of its 2016-2019 tax credit claims from HMRC.
Income taxes	Income taxes The Company is primarily subject to corporation taxes in the United Kingdom and the United States. The calculation of the Company’s tax provision involves the application of both United Kingdom and United States tax law and requires judgement and estimates. The Company accounts for income taxes in accordance with ASC Topic 740, Accounting for Income Taxes The Company accounts for uncertainty in income taxes by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed as the amount of benefit to recognize in the consolidated financial statements. The amount of benefits that may be used is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate, as well as the related net interest and penalties.
Product sales	Product sales The Company’s product sales of Strimvelis are currently distributed Revenue from Contracts with Customers
Net income (loss) per share	Net income (loss) per share Basic net income (loss) per share is computed by dividing the net income (loss) by the weighted average number of ordinary shares outstanding for the period. Diluted net income (loss) is computed by adjusting net income (loss) based on the potential impact of dilutive securities. Diluted net income (loss) per share is computed by dividing the diluted net income (loss) 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 unvested restricted shares are considered potential dilutive ordinary shares. 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 potential ordinary share equivalents outstanding would have been anti-dilutive. The following securities, presented based on amounts outstanding at each period end, are considered to be ordinary share equivalents, but were not included in the computation of diluted net loss per ordinary share because to do so would have been anti-dilutive: December 31, 2020 2019 Share options 11,071,555 10,056,864 Unvested shares from share plan and consulting agreement 816,316 751,496 11,887,871 10,808,360
Recent accounting pronouncements	Recent accounting pronouncements In February 2016 and July 2018, the FASB issued ASU No. 2016-02, Leases (Topic 842) Leases Leases The new standard requires that all lessees (i) recognize, on the balance sheet, liabilities to remit lease payments and right-of- ASC 842 became effective for the Company in 2019. The Company adopted ASC 842 using the modified retrospective approach with an effective date of January 1, 2019 for leases that existed on that date. Prior period results continue to be presented under ASC 840 based on the accounting standards originally in effect for such periods. This standard provides a number of optional practical expedients in transition. The Company applied the package of practical expedients to leases that commenced prior to the effective date, whereby it elected not to reassess the following: (i) whether any expired or existing contracts contain leases; (ii) the lease classification for any expired or existing leases; and (iii) initial direct costs for any existing leases. The Company elected the short-term lease recognition exemption for all leases that qualify, where a right-of-use asset or lease liability will not be recognized for short term leases that have terms of one year or less. The operating lease right-of-use assets and corresponding liabilities relate to existing facility operating leases in London, UK, Boston, Massachusetts, and the San Francisco Bay Area, California, as well as an embedded operating lease for research and development space at a contract manufacturing organization. The most significant effects of adoption were the recognition of material new right-of-use assets and corresponding liabilities on its consolidated balance sheet related to its existing facility operating leases (see Note 10). The adoption of this standard had a material impact on the Company’s financial position but did not significantly affect the Company’s results of operations or cash flows. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments – Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments (“ASU 2016-13”), which requires entities to record expected credit losses for certain financial instruments, including trade receivables, as an allowance that reflects the entity's current estimate of credit losses expected to be incurred. For available-for-sale debt securities in unrealized loss positions, the new standard requires allowances to be recorded instead of reducing the amortized cost of the investment. The new standard became effective fo r us on January 1, 2020. This guidance did not have a significant impact on the Company’s consolidated financial statements and related disclosures. The Company has a UK research and development tax credit receivable and trade receivables that are subject to this guidance. The Company has assessed whether it believes there is a current estimate of credit loss expected to be recorded for these receivables and concluded that any amount would not be significant and therefore the Company has not recorded any credit loss allowance for these receivables. In December 2019, the FASB issued ASU No. 2019-12, Simplifying the Accounting for Income Taxes (Topic 740) , which removes certain exceptions to the general principles in Topic 740 – Income Taxes and improves consistent application of and simplifies GAAP for other areas of Topic 740 by clarifying and amending existing guidance. This ASU is effective for the Company beginning January 1, 202 1 and interim periods within that year, with early adoption permitted. The Company is currently evaluating the effect of adopting this new accounting guidance.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash	The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported in the consolidated balance sheet that sum to the total of the amounts reported in the consolidated statement of cash flows: As of December 31, 2020 2019 Cash and cash equivalents $ 55,135 $ 19,053 Restricted cash 4,266 4,264 Total cash, cash equivalents and restricted cash shown in the statement of cash flows $ 59,401 $ 23,317
Schedule of Estimated Useful Lives of Property and Equipment	Property and equipment are recorded at cost and depreciated or amortized using the straight-line method over the following estimated useful lives. Property and equipment: Estimated useful life Lab equipment 5-10 years Leasehold improvements Shorter of lease term or estimated useful life Furniture and fixtures 4 years Office and computer equipment 3-5 years
Schedule of Changes in Loss Provision	The following table below outlines the changes to the Strimvelis loss provision for the periods ended December 31, 2020 and 2019: Year Ended December 31, 2020 2019 Balance at beginning of period $ 6,790 $ 10,339 Provisions — — Amortization of loss provision (2,413 ) (3,855 ) Foreign currency translation 105 306 Balance at end of period $ 4,482 $ 6,790
Summary of Amounts Recognized to Offset Research and Development Expense	The following table outlines the changes to the research and development tax credit receivable, including amount recognized as an offset to research and development expense during the years ended December 31, 2020 and 2019: Year Ended December 31, 2020 2019 Balance at beginning of period $ 28,644 $ 10,585 Recognition of credit claims as offset to research and development expense 21,130 17,564 Receipt of credit claims (33,771 ) (152 ) Foreign currency translation 1,341 647 Balance at end of period $ 17,344 $ 28,644
Securities Excluded in the Computation of Diluted Net Loss Per Ordinary Share	The following securities, presented based on amounts outstanding at each period end, are considered to be ordinary share equivalents, but were not included in the computation of diluted net loss per ordinary share because to do so would have been anti-dilutive: December 31, 2020 2019 Share options 11,071,555 10,056,864 Unvested shares from share plan and consulting agreement 816,316 751,496 11,887,871 10,808,360

Fair Value Measurements and M_2

Fair Value Measurements and Marketable Securities (Tables)	12 Months Ended
	Dec. 31, 2020
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract]
Schedule of Cash Equivalents and Marketable Securities	The following table summarizes the Company’s cash equivalents and marketable securities as of December 31, 2020: Fair Value Measurements as of December 31, 2020 Using: Level 1 Level 2 Level 3 Total Cash equivalents Money market funds $ 6,650 $ — $ — $ 6,650 Corporate bonds — 3,001 — 3,001 Commercial paper — 2,999 — 2,999 Total cash equivalents $ 6,650 $ 6,000 $ — $ 12,650 Marketable securities US government securities $ — 2,997 $ — 2,997 Corporate bonds — $ 93,358 — 93,358 Commercial paper — 40,458 — 40,458 Total marketable securities $ — $ 136,813 $ — $ 136,813 Total $ 6,650 $ 142,813 $ — $ 149,463 The following table summarizes the Company’s cash equivalents and marketable securities as of December 31, 2019: : Fair Value Measurements as of December 31, 2019 Using: Level 1 Level 2 Level 3 Total Cash equivalents Money market funds $ 202 $ — $ — $ 202 U.S. government securities — 3,159 — 3,159 Commercial paper — 9,792 — 9,792 Total cash equivalents $ 202 $ 12,951 $ — $ 13,153 Marketable securities Corporate bonds $ — $ 259,900 $ — 259,900 Commercial paper — 46,037 — 46,037 Total marketable securities $ — $ 305,937 $ — $ 305,937 Total $ 202 $ 318,888 $ — $ 319,090
Schedule of Cash Equivalents and Marketable Securities	The following table summarizes the Company’s level 2 cash equivalents and marketable securities as of December 31, 2020: Fair Value Measurements as of December 31, 2020 Using: Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Credit Losses Fair Value U.S. government securities $ 3,000 $ — $ (4 ) $ — 2,996 Corporate bonds 96,259 133 (32 ) — 96,360 Commercial paper 43,469 1 (13 ) — 43,457 Total $ 142,728 $ 134 $ (49 ) $ — $ 142,813 The following table summarizes the Company’s level 2 cash equivalents and marketable securities as of December 31, 2019: Fair Value Measurements as of December 31, 2019 Using: Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value U.S. government securities $ 3,159 $ — $ — 3,159 Corporate bonds 259,669 285 (54 ) 259,900 Commercial paper 55,794 42 (7 ) 55,829 Total $ 318,622 $ 327 $ (61 ) $ 318,888
Schedule of Available-for-Sale Debt Marketable Securities by contractual Maturity	The following table summarizes the Company’s available-for-sale marketable debt securities by contractual maturity, as of December 31, 2020 and 2019: 2020 2019 Maturities in one year or less $ 132,056 $ 250,490 Maturities between one and three years 10,757 68,398 Total $ 142,813 $ 318,888

Prepaid Expenses and Other Cu_2

Prepaid Expenses and Other Current Assets (Tables)	12 Months Ended
	Dec. 31, 2020
Prepaid Expense And Other Assets Current [Abstract]
Schedule of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets consist of the following: December 31, 2020 2019 Prepaid external research and development expenses $ 1,421 $ 1,121 Inventories 665 — Other prepayments 4,930 2,800 VAT receivable 2,780 1,091 Construction deposit - current 1,552 — Non-trade receivables 2,017 3,518 Total prepaid expenses and other current assets $ 13,365 $ 8,530

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2020
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment consist of the following: December 31, 2020 2019 Property and equipment: Lab equipment $ 5,114 $ 6,377 Leasehold improvements 2,522 1,839 Furniture and fixtures 304 508 Office and IT equipment 763 184 Construction-in-progress 302 1,848 Property and equipment $ 9,005 $ 10,756 Less: accumulated depreciation (4,224 ) (3,160 ) Property and equipment, net $ 4,781 $ 7,596

Other Assets (Tables)

Other Assets (Tables)	12 Months Ended
	Dec. 31, 2020
Other Assets Noncurrent Disclosure [Abstract]
Schedule of Other Assets	Other assets consist of the following: December 31, 2020 2019 Intangible assets - license milestones $ 3,076 $ — Deferred tax assets 5,219 2,985 Deposits 1,144 1,108 Deferred financing costs 975 307 Other non-current assets 1,554 — Construction deposits - long-term 6,572 — Total other assets $ 18,540 $ 4,400

Accrued Expenses and Other Li_2

Accrued Expenses and Other Liabilities (Tables)	12 Months Ended
	Dec. 31, 2020
Accrued Liabilities And Other Liabilities [Abstract]
Schedule of Accrued Expenses and Other Liabilities	Accrued expenses and other current liabilities consisted of the following: December 31, 2020 2019 Accrued external research and development expenses $ 8,878 $ 16,215 Accrued payroll and related expenses 11,881 12,381 Accrued professional fees 791 1,321 Accrued other 6,477 5,069 Strimvelis liability - current portion 916 2,994 Total accrued expenses and other current liabilities $ 28,943 $ 37,980

Restructuring Charges (Tables)

Restructuring Charges (Tables)	12 Months Ended
	Dec. 31, 2020
Restructuring And Related Activities [Abstract]
Summary of Cash Restructuring Charges	Accrued restructuring and severance costs are included in Accrued expenses and other current liabilities in the consolidated balance sheet. Activity for the fiscal year are summarized as follows: Year Ended December 31, 2020 Balance at beginning of period $ — Charged to expense 1,854 Payments made (1,848 ) Balance at end of period $ 6
Summary of Impairment of Long Lived Assets	During the second quarter of 2020, the Company also took the following non-cash charges to research and development expense associated with the impairment of construction-in-process associated with the Fremont manufacturing facility, partial impairment of the right-of-use asset for the Fremont manufacturing facility lease (the “Fremont ROU asset”), and a write-down of laboratory equipment from the Company’s Menlo Park, CA facility: Asset write-down Operating lease right-of-use asset $ 2,605 Construction-in-progress 2,285 Laboratory equipment 760 Charge included in research and development expense $ 5,650

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Summary of Lease Cost Under Operating Leases	The following table contains a summary of the lease-related costs recognized within operating expenses, and other information pertaining to the Company’s operating leases as of December 31, 2020 and 2019: 2020 2019 Fixed lease cost $ 7,593 $ 5,589 Impairment of right-of-use assets $ 2,781 — Variable lease cost 2,131 1,436 Sublease income (181 ) — Total lease cost $ 12,324 $ 7,025 Other information Operating cash flows used for operating leases 8,447 5,738 Weighted-average remaining lease term (years) 6.6 8.2 Weighted-average discount rate 8.6 % 9.3 %
Schedule of Future Minimum Base Rent Commitments Under Operating Leases	As of December 31, 2020, future minimum base rent commitments under ASC 842 under the Company’s property leases were as follows: Due in: Gross lease payments Gross sublease receipts Net lease payments 2021 $ 8,941 $ (181 ) $ 8,760 2022 8,093 (2,180 ) 5,913 2023 7,043 (2,245 ) 4,798 2024 7,067 (2,312 ) 4,755 2025 4,657 (2,382 ) 2,275 Thereafter 15,053 (11,413 ) 3,640 Total future minimum lease payments 50,854 (20,713 ) 30,141 Less: imputed interest (17,752 ) Total operating lease payments $ 33,102

Notes Payable (Tables)

Notes Payable (Tables)	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
Summary of Notes Payable	As of December 31, 2020 and 2019, notes payable consist of the following: December 31, 2020 2019 Notes payable, net of issuance costs $ 24,659 $ 24,541 Less: current portion $ (4,861 ) — Notes payable, net of current portion 19,798 24,541 Accretion related to final payment 406 158 Notes payable, long term $ 20,204 $ 24,699
Summary of Future Principal Payments Due	As of December 31, 2020, the future principal payments due are as follows: Aggregate Minimum Payments 2021 4,861 2022 8,333 2023 8,334 2024 4,597 2025 — Thereafter — Total 26,125 Less current portion (4,861 ) Less unamortized portion of final payment (719 ) Less unamortized debt issuance costs (341 ) Notes payable, long term $ 20,204

Share-based Compensation (Table

Share-based Compensation (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Assumptions Used in Black-Scholes Option Pricing Model	The relevant data used to determine the value of stock option awards are as follows: Year Ended December 31, 2020 2019 Risk-free interest rate 0.3 - 1.7% 1.4 - 2.6% Expected term (in years) 5.5 - 6.1 5.5 - 6.1 Expected volatility 70.7 - 75.2% 70.1 - 72.1% Expected dividend rate 0.00% 0.00%
Summary of Option Activity	The following table summarizes option activity under the plans for the year ended December 31, 2020: Shares Weighted Average Exercise Price per Share Weighted Average Remaining Contractual Life (Years) Aggregate Intrinsic Value Outstanding at December 31, 2019 12,216,140 $ 6.61 8.31 $ 91,133 Granted 5,846,152 11.37 Exercised (1,154,441 ) 2.99 Forfeited (3,012,208 ) 10.99 Outstanding and expected to vest at December 31, 2020 13,895,643 $ 7.96 7.16 $ 15,473 Exercisable, as of December 31, 2020 7,120,307 $ 5.98 5.73 $ 12,318
Summary of Award Activity	The following table summarizes restricted share unit award activity for the year-end December 31, 2020: Shares Weighted Average Fair Value per Share Unvested at December 31, 2019 556,422 $ 13.58 Granted 426,750 6.42 Vested — — Forfeited (339,172 ) 13.75 Unvested at December 31, 2020 644,000 $ 8.75
Share-based Compensation Expense	Share-based compensation expense related to share options, restricted share unit awards, and the employee stock purchase plan was classified in the consolidated statements of operations and comprehensive loss as follows: Year Ended December 31, 2020 2019 Research and development $ 11,679 $ 7,425 Selling, general and administrative 16,283 11,999 Total $ 27,962 $ 19,424

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Schedule of Income (Loss) before Income Tax	The components of income (loss) from operations before income taxes for the years ended December 31, 2020 and 2019 are as follows: December 31, 2020 2019 UK $ (155,614 ) $ (173,118 ) Non-UK 2,904 7,456 Loss before taxes $ (152,710 ) $ (165,662 )
Schedule of (Benefit from) Provision for Income Taxes	The (benefit from) provision for income taxes for the years ended December 31, 2020 and 2019 are as follows: December 31, 2020 2019 Current (benefit) provision Federal—United States $ 1,107 $ 888 State—United States 189 (275 ) Other foreign 230 89 Total current (benefit) provision 1,526 702 Deferred (benefit) provision Federal—United States (1,774 ) (2,820 ) State—United States (103 ) (122 ) Other foreign (380 ) — Total deferred (benefit) provision (2,257 ) (2,942 ) Total (benefit) provision for income taxes $ (731 ) $ (2,240 )
Schedule of Reconciliation of Income Tax (Benefit) Expense	The following table presents a reconciliation of income tax (benefit) expense computed at the UK statutory income tax rate to the effective income tax rate as reflected in the consolidated financial statements (in thousands): December 31, 2020 2019 Income taxes at United Kingdom statutory rate $ (29,015 ) $ (31,475 ) Change in valuation allowance 29,302 16,507 Reduction in research expense for credits granted 8,435 9,787 Change in tax rates (8,105 ) 8,109 Tax credits (1,369 ) (3,372 ) U.S. Deduction for foreign derived intangible income (1,254 ) (2,058 ) Permanent differences, including share-based compensation deduction shortfalls 1,265 344 U.S. state income taxes 68 (238 ) Foreign rate differential (58 ) 156 Total (benefit) provision for income taxes $ (731 ) $ (2,240 )
Schedule of Deferred Tax Assets and Liabilities	The following table presents the principal components of the Company’s deferred tax assets and liabilities as of December 31, 2020 and 2019: December 31, 2020 2019 Deferred tax assets Net operating loss carryforwards $ 75,502 $ 45,358 Amortization 22,599 21,741 Research and development credits 1,564 1,244 Share-based compensation 7,400 3,604 Accruals 1,001 1,286 Lease Liability 6,805 4,406 Other 3 1 Total deferred tax assets 114,874 77,640 Valuation allowance (103,890 ) (70,153 ) Fixed assets and right-of-use asset (5,765 ) (4,502 ) Other non-current assets (net deferred tax assets and liabilities) $ 5,219 $ 2,985
Schedule of Changes in Valuation Allowance for Deferred Tax Assets Related Primarily to Increase in U.K. Net Operating Loss Carryforwards and U.K. Amortization of Intangible Assets	Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2020 and 2019 related primarily to the increase in UK net operating loss carryforwards and UK amortization of intangible assets and were as follows: December 31, 2020 2019 Valuation allowance as of beginning of year $ (70,153 ) $ (51,281 ) Decreases recorded as benefit to income tax provision — — Increases recorded to income tax provision (29,302 ) (16,507 ) Effect of foreign currency translation (4,435 ) (2,365 ) Valuation allowance as of end of year $ (103,890 ) $ (70,153 )

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2020
Commitments And Contingencies Disclosure [Abstract]
Schedule of Annual Commitments Associated with the Contract	The following table outlines the annual commitments associated with the contract, as of December 31, 2020: Due in: Product manufacturing commitments (1) Dedicated manufacturing and development resources (2) Exclusive transduction suites (3) Total AGC Commitment 2021 $ 2,491 $ 8,524 $ 4,190 $ 15,205 2022 3,321 8,524 3,352 15,197 2023 3,321 8,524 3,352 15,197 2024 3,321 8,524 3,352 15,197 2025 1,661 4,262 838 6,761 Thereafter — — — — Total manufacturing commitments $ 14,115 $ 38,358 $ 15,084 $ 67,557 *Tabular disclosure above has been translated to U.S. Dollar, from Euro, using an exchange rate of €1.00 to $1.23. (1) The minimum product manufacturing commitments may be increased to the mid-seven figures per contract year upon achievement of certain milestones. (2) The Company may increase or decrease the usage of dedicated development services on a rolling basis with between six and 12-months’ prior written notice to AGC. The above table assumes continued usage of dedicated development services at current rates. (3) Refer to Note 10 for further information on the embedded operating lease agreement

Selected Quarterly Financial _2

Selected Quarterly Financial Information (unaudited) (Tables)	12 Months Ended
	Dec. 31, 2020
Quarterly Financial Information Disclosure [Abstract]
Summary of Unaudited Quarterly Financial Data	The following tables summarizes the unaudited quarterly financial data for the last two fiscal years: 2020 First Quarter Second Quarter Third Quarter Fourth Quarter Full year Total revenues $ — $ 597 $ 1,998 $ — $ 2,595 Total costs and operating expenses 44,981 47,418 28,301 38,873 159,573 Loss from operations (44,981 ) (46,821 ) (26,303 ) (38,873 ) (156,978 ) Net loss attributable to ordinary shareholders (50,569 ) (47,500 ) (20,290 ) (33,620 ) (151,979 ) Weighted average ordinary shares outstanding - basic and diluted 98,713,126 99,251,314 99,664,616 100,013,246 99,445,874 Earnings per share $ (0.51 ) $ (0.48 ) $ (0.20 ) $ (0.34 ) $ (1.53 ) 2019 First Quarter Second Quarter (2) Third Quarter Fourth Quarter Full year Total revenues $ — $ — $ 1,918 $ 595 $ 2,513 Total costs and operating expenses 28,283 54,152 43,330 49,621 175,386 Loss from operations (28,283 ) (54,152 ) (41,412 ) (49,026 ) (172,873 ) Net loss attributable to ordinary shareholders (30,739 ) (50,530 ) (36,737 ) (45,416 ) (163,422 ) Weighted average ordinary shares outstanding - basic and diluted 87,010,596 89,712,916 97,817,847 98,243,915 93,240,355 Earnings per share $ (0.35 ) $ (0.56 ) $ (0.38 ) $ (0.46 ) $ (1.75 )

Nature of Business and Basis _2

Nature of Business and Basis of Presentation - Additional Information (Details) - USD ($) $ in Thousands	Feb. 09, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Net loss		$ (33,620)	$ (20,290)	$ (47,500)	$ (50,569)	$ (45,416)	$ (36,737)	$ (50,530)	$ (30,739)	$ (151,979)	$ (163,422)
Accumulated deficit		605,640				$ 453,661				605,640	$ 453,661
Cash, cash equivalents and marketable securities		$ 191,900								$ 191,900
Subsequent Event | Securities Purchase Agreement | Non-voting Ordinary Shares
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Gross proceeds from private placement	$ 150,000

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Thousands	Feb. 28, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Summary Of Significant Accounting Policies [Line Items]
Description of significant off-balance sheet risk		The Company has no significant off-balance sheet risk, such as foreign currency contracts, options contracts, or other foreign hedging arrangements.
Foreign currency transaction gains		$ 3,400	$ 1,400
Property and equipment		4,781	7,596
Right-of-use assets		29,815	19,415
Proceeds from lines of credit		3,000	3,000
Cash collateral associated with corporate credit cards and other leases		1,300	1,300
Liability associated with the loss contract				$ 18,400
Strimvelis loss provision current		900	3,000
Strimvelis loss provision non current		3,600	3,800
Tax incentive receivable, current		$ 17,344	14,934
Tax incentive receivable, noncurrent			13,710
Change in accounting principle, accounting standards update, adopted		true
Change in accounting principle, accounting standards update, adoption date		Jan. 1, 2019
Accounting Standards Update [Extensible List]		us-gaap:AccountingStandardsUpdate201602Member
Subsequent Event
Summary Of Significant Accounting Policies [Line Items]
Marketing authorization renewal period	5 years
United Kingdom
Summary Of Significant Accounting Policies [Line Items]
Property and equipment		$ 3,700	2,600
Tax incentive receivable			28,600
Tax incentive receivable, current		$ 17,300	14,900
Tax incentive receivable, noncurrent			13,700
Incremental percentage of taxable profits		50.00%
United Kingdom | Research and Development Expense
Summary Of Significant Accounting Policies [Line Items]
Foreign currency transaction gains		$ 1,341	647
Additional tax credits		4,800
US
Summary Of Significant Accounting Policies [Line Items]
Property and equipment		1,100	5,000
Right-of-use assets		14,200	15,700
United Kingdom and European Union
Summary Of Significant Accounting Policies [Line Items]
Right-of-use assets		$ 15,600	$ 3,700

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Summary Of Significant Accounting Policies [Abstract]
Cash and cash equivalents	$ 55,135	$ 19,053
Restricted cash	4,266	4,264
Total cash, cash equivalents and restricted cash shown in the statement of cash flows	$ 59,401	$ 23,317	$ 339,681

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Schedule of Estimated Useful Lives of Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2020
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment useful life, term	Shorter of lease term or estimated useful life
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment useful life	4 years
Minimum | Lab Equipment
Property Plant And Equipment [Line Items]
Property and equipment useful life	5 years
Minimum | Office and Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment useful life	3 years
Maximum | Lab Equipment
Property Plant And Equipment [Line Items]
Property and equipment useful life	10 years
Maximum | Office and Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment useful life	5 years

Summary of Significant Accoun_7

Summary of Significant Accounting Policies - Schedule of Changes in Loss Provision (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Summary Of Significant Accounting Policies [Abstract]
Balance at beginning of period	$ 6,790	$ 10,339
Amortization of loss provision	(2,413)	(3,855)
Foreign currency translation	105	306
Balance at end of period	$ 4,482	$ 6,790

Summary of Significant Accoun_8

Summary of Significant Accounting Policies - Summary of Amounts recognized to Offset Research and Development Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Summary Of Significant Accounting Policies [Line Items]
Receipt of credit claims	$ (1,369)	$ (3,372)
Foreign currency translation	3,400	1,400
United Kingdom | Research and Development Expense
Summary Of Significant Accounting Policies [Line Items]
Balance at beginning of period	28,644	10,585
Recognition of credit claims as offset to research and development expense	21,130	17,564
Receipt of credit claims	(33,771)	(152)
Foreign currency translation	1,341	647
Balance at end of period	$ 17,344	$ 28,644

Summary of Significant Accoun_9

Summary of Significant Accounting Policies - Securities Excluded in the Computation of Diluted Net Loss Per Ordinary Share (Details) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common shares attributable to anti-dilutive shares	11,887,871	10,808,360
Share options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common shares attributable to anti-dilutive shares	11,071,555	10,056,864
Unvested Shares from Share Plan and Consulting Agreement
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common shares attributable to anti-dilutive shares	816,316	751,496

Fair Value Measurements and M_3

Fair Value Measurements and Marketable Securities - Additional Information (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Fair value of financial assets transfers between level 1 to level 2	$ 0	$ 0
Fair value of financial assets transfers between level 2 to level 1	$ 0	$ 0

Fair Value Measurements and M_4

Fair Value Measurements and Marketable Securities - Schedule of Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Cash equivalents
Total cash equivalents	$ 12,650	$ 13,153
Marketable securities
Total marketable securities	136,813	305,937
Total	149,463	319,090
Level 1
Cash equivalents
Total cash equivalents	6,650	202
Marketable securities
Total	6,650	202
Level 2
Cash equivalents
Total cash equivalents	6,000	12,951
Marketable securities
Total marketable securities	136,813	305,937
Total	142,813	318,888
Money Market Funds
Cash equivalents
Total cash equivalents	6,650	202
Money Market Funds | Level 1
Cash equivalents
Total cash equivalents	6,650	202
Corporate Bonds
Cash equivalents
Total cash equivalents	3,001
Marketable securities
Total marketable securities	93,358	259,900
Corporate Bonds | Level 2
Cash equivalents
Total cash equivalents	3,001
Marketable securities
Total marketable securities	93,358	259,900
U.S. Government Securities
Cash equivalents
Total cash equivalents		3,159
Marketable securities
Total marketable securities	2,997
U.S. Government Securities | Level 2
Cash equivalents
Total cash equivalents		3,159
Marketable securities
Total marketable securities	2,997
Commercial Paper
Cash equivalents
Total cash equivalents	2,999	9,792
Marketable securities
Total marketable securities	40,458	46,037
Commercial Paper | Level 2
Cash equivalents
Total cash equivalents	2,999	9,792
Marketable securities
Total marketable securities	$ 40,458	$ 46,037

Fair Value Measurements and M_5

Fair Value Measurements and Marketable Securities - Schedule of Level 2 Cash Equivalents and Marketable Securities (Details) - Level 2 - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 142,728	$ 318,622
Gross Unrealized Gains	134	327
Gross Unrealized Losses	(49)	(61)
Fair Value	142,813	318,888
U.S. Government Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	3,000	3,159
Gross Unrealized Losses	(4)
Fair Value	2,996	3,159
Corporate Bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	96,259	259,669
Gross Unrealized Gains	133	285
Gross Unrealized Losses	(32)	(54)
Fair Value	96,360	259,900
Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	43,469	55,794
Gross Unrealized Gains	1	42
Gross Unrealized Losses	(13)	(7)
Fair Value	$ 43,457	$ 55,829

Fair Value Measurements and M_6

Fair Value Measurements and Marketable Securities - Schedule of Available-for-Sale Marketable Debt Securities by contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Maturities in one year or less	$ 132,056	$ 250,490
Maturities between one and three years	10,757	68,398
Total	$ 142,813	$ 318,888

Revenue Recognition - Additiona

Revenue Recognition - Additional Information (Details)	12 Months Ended
	Dec. 31, 2020
Revenue From Contract With Customer [Abstract]
General payment terms, maximum	60 days

Prepaid Expenses and Other Cu_3

Prepaid Expenses and Other Current Assets - Schedule of Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Prepaid Expense And Other Assets Current [Abstract]
Prepaid external research and development expenses	$ 1,421	$ 1,121
Inventories	665
Other prepayments	4,930	2,800
VAT receivable	2,780	1,091
Construction deposit - current	1,552
Non-trade receivables	2,017	3,518
Total prepaid expenses and other current assets	$ 13,365	$ 8,530

Property and Equipment - Schedu

Property and Equipment - Schedule of Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property Plant And Equipment [Line Items]
Property and equipment	$ 9,005	$ 10,756
Less: accumulated depreciation	(4,224)	(3,160)
Property and equipment, net	4,781	7,596
Lab Equipment
Property Plant And Equipment [Line Items]
Property and equipment	5,114	6,377
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment	2,522	1,839
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment	304	508
Office and IT Equipment
Property Plant And Equipment [Line Items]
Property and equipment	763	184
Construction-In-Progress
Property Plant And Equipment [Line Items]
Property and equipment	$ 302	$ 1,848

Property and Equipment - Additi

Property and Equipment - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Property Plant And Equipment [Abstract]
Depreciation expense	$ 2,004	$ 1,675

Other Assets - Schedule of Othe

Other Assets - Schedule of Other Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Other Assets Noncurrent Disclosure [Abstract]
Intangible assets - license milestones	$ 3,076
Deferred tax assets	5,219	$ 2,985
Deposits	1,144	1,108
Deferred financing costs	975	307
Other non-current assets	1,554
Construction deposits - long-term	6,572
Total other assets	$ 18,540	$ 4,400

Accrued Expenses and Other Li_3

Accrued Expenses and Other Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Accrued Liabilities And Other Liabilities [Abstract]
Accrued external research and development expenses	$ 8,878	$ 16,215
Accrued payroll and related expenses	11,881	12,381
Accrued professional fees	791	1,321
Accrued other	6,477	5,069
Strimvelis liability - current portion	916	2,994
Total accrued expenses and other current liabilities	$ 28,943	$ 37,980

Restructuring Charges - Additio

Restructuring Charges - Additional Information (Details) - USD ($)	1 Months Ended	3 Months Ended	12 Months Ended
	May 31, 2020	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Restructuring Cost And Reserve [Line Items]
Restructuring costs			$ 1,854,000	$ 0
Operating lease right-of-use-assets			29,815,000	$ 19,415,000
Undiscounted cash flow estimate			17,752,000
Right of use asset, impairment charge			$ 2,781,000
Office Workforce Menlo Park, California
Restructuring Cost And Reserve [Line Items]
Number of positions eliminated, percent	25.00%
Construction-in-process, impairment charges	$ 2,300,000	$ 2,285,000
Operating lease right-of-use-assets	$ 13,800,000
Discount rate	4.60%
Fair value of ROU asset	$ 11,200,000
Right of use asset, impairment charge	2,600,000	$ 2,605,000
Office Workforce Menlo Park, California | Minimum
Restructuring Cost And Reserve [Line Items]
Undiscounted cash flow estimate	11,700,000
Office Workforce Menlo Park, California | Maximum
Restructuring Cost And Reserve [Line Items]
Undiscounted cash flow estimate	$ 19,100,000

Restructuring Charges - Summary

Restructuring Charges - Summary of Cash Restructuring Charges (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Restructuring And Related Activities [Abstract]
Charged to expense	$ 1,854,000	$ 0
Payments made	(1,848,000)
Balance at end of period	$ 6,000

Restructuring Charges - Summa_2

Restructuring Charges - Summary of Impairment of Long Lived Assets (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended	12 Months Ended
	May 31, 2020	Jun. 30, 2020	Dec. 31, 2020
Restructuring Cost And Reserve [Line Items]
Operating lease right-of-use asset			$ 2,781
Charge included in research and development expense			$ 5,650
Office Workforce Menlo Park, California
Restructuring Cost And Reserve [Line Items]
Operating lease right-of-use asset	$ 2,600	$ 2,605
Construction-in-progress	$ 2,300	2,285
Laboratory equipment, Asset write-downs		760
Charge included in research and development expense		$ 5,650

Leases - Additional Information

Leases - Additional Information (Details) $ in Thousands, € in Millions	1 Months Ended						12 Months Ended
	Jul. 31, 2019	Jan. 31, 2019	Dec. 31, 2018USD ($)	Mar. 31, 2018	Jan. 31, 2018	Nov. 30, 2017	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2020EUR (€)	Mar. 31, 2020USD ($)
Operating Leases [Line Items]
Operating leases expiration term							Jul. 2, 2025
Operating lease right-of-use-assets							$ 29,815	$ 19,415
Operating lease liabilities							33,102
Proceeds from lines of credit							3,000	3,000
Operating lease, liability, noncurrent							24,168	$ 15,320
Operating lease, right-of-use asset obtained in exchange for lease liabilities							17,500
Office Space Menlo Park Lease Agreement [Member]
Operating Leases [Line Items]
Operating leases expiration term						Dec. 31, 2020
Operating lease duration for free rent						1 month
Laboratory Space Menlo Park Lease Agreement [Member]
Operating Leases [Line Items]
Operating leases expiration term		Dec. 31, 2020
Office and Laboratory Space Menlo Park Lease Agreement [Member]
Operating Leases [Line Items]
Annual rental payment							1,900
Five Years Lease Term Terminates In January 2023 [Member] | Office Space London United Kingdom Lease Agreement [Member]
Operating Leases [Line Items]
Operating leases expiration term			Jan. 31, 2023		Jan. 31, 2023
Annual rental payment							1,700
Lease Term Terminates in September 2022 [Member] | Office Space Boston Lease Agreement [Member]
Operating Leases [Line Items]
Operating leases expiration term				Sep. 30, 2022
Annual rental payment							400
Lease Term Terminates in September 2026 [Member] | Office Space Boston Lease Agreement [Member]
Operating Leases [Line Items]
Operating leases expiration term	Sep. 30, 2026
Annual rental payment							900
Operating leases Commenced term	Jan. 31, 2020
Boston and London Office Space Lease Agreements [Member]
Operating Leases [Line Items]
Operating lease right-of-use-assets							5,400
Operating lease liabilities							5,700
Fremont Lease Agreement [Member]
Operating Leases [Line Items]
Operating leases expiration term			May 31, 2030
Annual rental payment							3,100
Operating lease duration for free rent			8 months
Operating lease right-of-use-assets							10,500
Reduction of standby letter of credit amount percentage			25.00%
Proceeds from lines of credit			$ 3,000
Operating lease, liability, noncurrent							14,400
Escrow deposit							8,100			$ 10,000
Increase decrease in escrow deposit							1,900
Fremont Lease Agreement [Member] | Prepaid Expenses and Other Current Assets [Member]
Operating Leases [Line Items]
Escrow deposit							1,600
Fremont Lease Agreement [Member] | Other Assets [Member]
Operating Leases [Line Items]
Escrow deposit							6,500
Fremont Lease Agreement [Member] | Maximum [Member]
Operating Leases [Line Items]
Tenant improvements allowance							5,300
Fremont Sublease Agreement
Operating Leases [Line Items]
Annual rental payment							$ 2,100
Operating lease duration for free rent							12 months
Fremont Sublease Agreement | Other Long-Term Liabilities [Member]
Operating Leases [Line Items]
Security deposit							$ 2,600
Fremont Sublease Agreement | Maximum [Member]
Operating Leases [Line Items]
Tenant improvements allowance							$ 5,300
G S K Agreement
Operating Leases [Line Items]
Operating leases expiration term							Jul. 2, 2025
Operating lease right-of-use-assets							$ 13,900
Operating lease, liability, noncurrent							$ 13,100
Operating lease term							5 years		5 years
Lessee, operating lease, option to extend							The agreement may be extended for an additional two years by mutual agreement of the Company and AGC
Lessee, operating Lease, existence of option to extend [true false]							true
Operating lease extended term							2 years
Lease component manufacturing suites commitment amount per contract year | €									€ 2.7
Dedicated manufacturing and development resources commitment amount per contract year | €									€ 10.2

Leases - Summary of Lease Cost

Leases - Summary of Lease Cost Under Operating Leases (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Fixed lease cost	$ 7,593	$ 5,589
Impairment of right-of-use assets	2,781
Variable lease cost	2,131	1,436
Sublease income	(181)
Total lease cost	12,324	7,025
Operating cash flows used for operating leases	$ 8,447	$ 5,738
Weighted-average remaining lease term (years)	6 years 7 months 6 days	8 years 2 months 12 days
Weighted-average discount rate	8.60%	9.30%

Leases - Schedule of Future Min

Leases - Schedule of Future Minimum Base Rent Commitments Under Operating Leases (Details) $ in Thousands	Dec. 31, 2020USD ($)
Gross lease payments
2021	$ 8,941
2022	8,093
2023	7,043
2024	7,067
2025	4,657
Thereafter	15,053
Total future minimum lease payments	50,854
Less: imputed interest	(17,752)
Total operating lease payments	33,102
Gross sublease receipts
2021	(181)
2022	(2,180)
2023	(2,245)
2024	(2,312)
2025	(2,382)
Thereafter	(11,413)
Total future minimum lease payments	(20,713)
Net lease payments
2021	8,760
2022	5,913
2023	4,798
2024	4,755
2025	2,275
Thereafter	3,640
Total future minimum lease payments	$ 30,141

Leases - Schedule of Future M_2

Leases - Schedule of Future Minimum Base Rent Commitments Under Operating Leases (Parenthetical) (Details)	Dec. 31, 2020
GBP
Operating Leases [Line Items]
Lease translation rate	1.36
Euro
Operating Leases [Line Items]
Lease translation rate	1.23

Notes Payable - Additional Info

Notes Payable - Additional Information (Details) - USD ($)	1 Months Ended	12 Months Ended
	May 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Line of credit facility agreement date	May 31, 2019
Credit facility maximum borrowings	$ 75,000,000
Proceeds from lines of credit		$ 3,000,000	$ 3,000,000
Credit facility, remaining borrowings	$ 50,000,000
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	$ 1,100,000
Line of credit facility interest rate description		Each term loan under the Credit Facility bears interest at an annual rate equal to 6% plus LIBOR. The Company is required to make interest-only payments on the term loan for all payment dates prior to 24 months following the date of the Credit Facility, unless the third tranche is drawn, in which case the Company is required to make interest-only payments for all payment dates prior to 36 months following the date of the Credit Facility. The term loans under the Credit Facility will begin amortizing on either the 24-month or the 36-month anniversary of the Credit Facility (as applicable), with equal monthly payments of principal plus interest to be made by the Company to the Lenders in consecutive monthly installments until the Loan Maturity Date. In addition, a final payment of 4.5% is due on the Loan Maturity Date.
Line of credit facility covenants description		The Company is also subject to an ongoing minimum cash financial covenant in which the Company must maintain unrestricted cash in an amount not less than $20.0 million following the utilization of the second term loan and not less than $35.0 million following the utilization of the third term loan
Term Loan
Debt Instrument [Line Items]
Debt instrument, interest expense		$ 2,300,000	$ 1,500,000
Debt instrument, effective annual interest rate		9.30%
LIBOR
Debt Instrument [Line Items]
Debt instrument, annual interest rate	6.00%
Minimum | Second Tranche (Term Loan Available from July 1, 2020 to March 31, 2021)
Debt Instrument [Line Items]
Line of credit facility minimum cash financial covenant	$ 20,000,000
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	35,000,000
Initial Term Loan
Debt Instrument [Line Items]
Proceeds from lines of credit	25,000,000
Second Tranche (Term Loan Available from July 1, 2020 to March 31, 2021)
Debt Instrument [Line Items]
Credit facility maximum borrowings	25,000,000
Line of Credit Facility, Covenant Terms		upon submission of certain regulatory filings and evidence of the Company having $100.0 million in cash and cash equivalent investments
Second Tranche (Term Loan Available from July 1, 2020 to March 31, 2021) | Minimum
Debt Instrument [Line Items]
Cash and cash equivalent investments	100,000,000
Third Tranche (Term Loan Available from July 1, 2020 to September 30, 2021)
Debt Instrument [Line Items]
Credit facility maximum borrowings	25,000,000
Line of Credit Facility, Covenant Terms		upon certain regulatory approvals and evidence of the Company having $125.0 million in cash and cash equivalent investments
Third Tranche (Term Loan Available from July 1, 2020 to September 30, 2021) | Minimum
Debt Instrument [Line Items]
Cash and cash equivalent investments	$ 125,000,000
Second and Third Tranche (Term Loan)
Debt Instrument [Line Items]
Credit facility maximum borrowings		$ 50,000,000

Notes Payable - Summary of Note

Notes Payable - Summary of Notes Payable (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Long Term Notes And Loans [Abstract]
Notes payable, net of issuance costs	$ 24,659	$ 24,541
Less: current portion	(4,861)
Notes payable, net of current portion	19,798	24,541
Accretion related to final payment	406	158
Notes payable, long term	$ 20,204	$ 24,699

Notes Payable - Summary of Futu

Notes Payable - Summary of Future Principal Payments Due (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Long Term Debt By Maturity [Abstract]
2021	$ 4,861
2022	8,333
2023	8,334
2024	4,597
Total	26,125
Less current portion	(4,861)
Less unamortized portion of final payment	(719)
Less unamortized debt issuance costs	(341)
Notes payable, long term	$ 20,204	$ 24,699

Shareholders' Equity and Conv_2

Shareholders' Equity and Convertible Preferred Shares - Additional Information (Details)	1 Months Ended	12 Months Ended
	Jun. 30, 2019USD ($)$ / sharesshares	Dec. 31, 2020USD ($)VotingRight	Dec. 31, 2019USD ($)VotingRight	Dec. 31, 2020GBP (£)£ / sharesshares	Apr. 30, 2020shares	Dec. 31, 2019GBP (£)£ / sharesshares	Nov. 30, 2018shares	Sep. 30, 2018shares
Class Of Stock [Line Items]
Ordinary shares voting rights description		each holder of ordinary shares and ADSs is entitled to one vote per ordinary share
Ordinary shares dividends declared		$ 0	$ 0
Number of ordinary voting rights | VotingRight		1	1
Ordinary Shares, Par Value | £ / shares				£ 0.10		£ 0.10
Issuance of ADRs in initial public offering, net of issuance costs			$ 130,270,000
Payment of offering expenses			$ 605,000
Ordinary Shares, Issued | shares				98,283,603		96,923,729
Consulting Agreement | Advisor
Class Of Stock [Line Items]
Ordinary Shares, Issued | shares				22,758
Oxford BioMedica License, Development and Supply Agreement
Class Of Stock [Line Items]
Ordinary Shares, Issued | shares				588,220	75,413		150,826	150,826
American Depositary Shares | Follow-On Public Offering
Class Of Stock [Line Items]
Issuance of ordinary shares in follow-on public offering net of issuance, Shares | shares	9,725,268
Issue price per share | $ / shares	$ 14.25
Issuance of ADRs in initial public offering, net of issuance costs	$ 129,700,000
Underwriting discounts and commissions	8,300,000
American Depositary Shares | Underwriter Discounts and Commissions
Class Of Stock [Line Items]
Payment of offering expenses	$ 600,000
Maximum
Class Of Stock [Line Items]
Ordinary shares maximum nominal value | £				£ 13,023,851.50		£ 13,023,851.50

Share-based Compensation - Addi

Share-based Compensation - Additional Information (Details) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended	12 Months Ended
	Apr. 30, 2020USD ($)shares	Dec. 31, 2020USD ($)shares	Dec. 31, 2020USD ($)$ / sharesshares	Dec. 31, 2019USD ($)$ / sharesshares	Dec. 31, 2020£ / shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Exercise price of option | £ / shares					£ 0.00001
Award vesting period			4 years
Maximum term of option			10 years
Total intrinsic value of share options exercised			$ 5,000	$ 17,200
Total proceeds from share options exercised			3,900	2,000
Employee equity plan proceeds received after year end		$ 200	$ 200
Weighted average grant date fair value of share options granted | $ / shares			$ 7.22	$ 8.67
Compensation cost recognized			$ 27,962	$ 19,424
Number of shares granted | shares			426,750
Unvested options outstanding | shares		6,775,336	6,775,336
Total unrecognized compensation cost of stock option and time-based RSUs		$ 46,400	$ 46,400
Compensation cost expected to be recognized weighted average period			2 years 6 months 7 days
Total unrecognized compensation cost of performance-based RSUs		4,000	$ 4,000
Performance-based Restricted Share Units ("RSUs")
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares vested | shares			0	0
Compensation cost recognized			$ 300	$ 1,200
Performance-based Restricted Share Units ("RSUs") | Chief Executive Officer
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares granted | shares	195,000
Total grant date fair value	$ 1,400
Performance-based Restricted Share Units ("RSUs") | Chief Executive Officer | Achievement of At Least Three of Four Milestones
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting rights			The award vests on January 2, 2024 as to 1/3 of the award for each of the first three to occur of four milestones, if each such milestone is achieved by the Company on or before December 31, 2023
Performance-based Restricted Share Units ("RSUs") | Libmeldy
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Compensation cost recognized		$ 1,200
Time-based Restricted Share Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting period			3 years
2018 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Remaining shares available for issuance | shares		6,611,693	6,611,693
2018 ESPP
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Remaining shares available for issuance | shares		1,470,104	1,470,104
Inducement Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Remaining shares available for issuance | shares		1,000,000	1,000,000

Share-based Compensation - Assu

Share-based Compensation - Assumptions Used in Black-Scholes Option Pricing Model (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate, minimum	0.30%	1.40%
Risk-free interest rate, maximum	1.70%	2.60%
Expected volatility, minimum	70.70%	70.10%
Expected volatility, maximum	75.20%	72.10%
Expected dividend rate	0.00%	0.00%
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	5 years 6 months	5 years 6 months
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	6 years 1 month 6 days	6 years 1 month 6 days

Share-based Compensation - Summ

Share-based Compensation - Summary of Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Options, Outstanding at December 31, 2019	12,216,140
Number of Options, Granted	5,846,152
Number of Options, Exercised	(1,154,441)
Number of Options, Forfeited	(3,012,208)
Number of Options, Outstanding and expected to vest at December 31, 2020	13,895,643	12,216,140
Number of Options, Exercisable as of December 31, 2020	7,120,307
Weighted Average Exercise Price per Share, Outstanding at December 31, 2019	$ 6.61
Weighted Average Exercise Price per Share, Granted	11.37
Weighted Average Exercise Price per Share, Exercised	2.99
Weighted Average Exercise Price per Share, Forfeited	10.99
Weighted Average Exercise Price per Share, Outstanding and expected to vest at December 31,2020	7.96	$ 6.61
Weighted Average Exercise Price per Share, Exercisable as of December 31,2020	$ 5.98
Weighted Average Remaining Contractual Life, Outstanding at December 31, 2019		8 years 3 months 21 days
Weighted Average Remaining Contractual Life, Outstanding and expected to vest at December 31, 2020	7 years 1 month 28 days
Weighted Average Remaining Contractual Life, Exercisable as of December 31, 2020	5 years 8 months 23 days
Aggregate Intrinsic Value, Outstanding	$ 15,473	$ 91,133
Aggregate Intrinsic Value, Exercisable as of December 31, 2020	$ 12,318

Share-based Compensation - Su_2

Share-based Compensation - Summary of Award Activity (Details)	12 Months Ended
	Dec. 31, 2020$ / sharesshares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Shares, Unvested at December 31, 2019 | shares	556,422
Number of shares granted | shares	426,750
Shares, Forfeited | shares	(339,172)
Shares, Unvested at December 31, 2020 | shares	644,000
Weighted Average Fair Value per share, Unvested at December 31, 2019 | $ / shares	$ 13.58
Weighted Average Fair Value per Share, Granted | $ / shares	6.42
Weighted Average Fair Value per share, Forfeited | $ / shares	13.75
Weighted Average Fair Value per Share, Unvested at December 31, 2020 | $ / shares	$ 8.75

Share-based Compensation - Shar

Share-based Compensation - Share-based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation	$ 27,962	$ 19,424
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation	11,679	7,425
Selling General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation	$ 16,283	$ 11,999

License and Research Agreemen_2

License and Research Agreements - Additional Information (Details) $ in Thousands, € in Millions	Apr. 30, 2020USD ($)shares	Apr. 30, 2018USD ($)shares	Apr. 30, 2018GBP (£)shares	Dec. 31, 2020USD ($)shares	Dec. 31, 2020GBP (£)	Dec. 31, 2019USD ($)Milestoneshares	Dec. 31, 2017USD ($)shares	Dec. 31, 2016USD ($)shares	Dec. 31, 2020USD ($)shares	Dec. 31, 2020EUR (€)	Dec. 31, 2020GBP (£)shares	Dec. 31, 2020EUR (€)shares	Dec. 31, 2018USD ($)shares	Nov. 30, 2018shares	Sep. 30, 2018shares	Apr. 30, 2018GBP (£)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Remaining liability				$ 4,482		$ 6,790			$ 4,482				$ 10,339
Ordinary shares issued | shares				98,283,603		96,923,729			98,283,603		98,283,603	98,283,603
Research and development				$ 93,730		$ 117,363
GSK Asset Purchase and License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total consideration		$ 133,600	£ 94,200,000
Upfront payment		14,200	10,000,000
Remaining liability		18,400														£ 12,900,000
Inventory purchase liability		6,900														4,900,000
Transaction costs		800	600,000
In-process research and development expense		$ 133,600	£ 94,200,000
Payment of tiered royalty, maximum percentage				20.00%					20.00%		20.00%	20.00%
Milestone payments payable upon achievement of certain sales milestones				$ 38,100					$ 38,100		£ 90,000,000	€ 31
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. To date, Telethon-OSR received €17.0 million in upfront and milestone payments from the Company upon entering into the agreement and shortly thereafter, resulting in $19.4 million in in-process research and development expense.	In May 2019, the Company entered into a license agreement with Telethon-OSR, under which Telethon-OSR granted to the Company an exclusive worldwide license for the research, development, manufacture and commercialization of Telethon-OSR’s ex vivo autologous HSC lentiviral based gene therapy for the treatment of mucopolysaccharidosis type I (“MPS-I”), including the Hurler variant. To date, Telethon-OSR received €17.0 million in upfront and milestone payments from the Company upon entering into the agreement and shortly thereafter, resulting in $19.4 million in in-process research and development expense.
GSK Asset Purchase and License Agreement | Convertible Preferred Shares
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Shares issued for asset acquisition | shares		12,455,252	12,455,252
Aggregate value of shares issued for asset acquisition		$ 93,400														£ 65,800,000
Telethon O S R License Agreements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront and milestone payments | €										€ 17
In-process research and development expense for upfront and milestone payments									19,400
Milestone payments payable upon achievement of certain development regulatory and commercial milestones				$ 34,400					$ 34,400			€ 28
UCLB/UCLA Technology
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payments								$ 800
UCLB Technology and Manufacturing Technology
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payments								$ 1,100
UCLB License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Ordinary shares issued | shares							1,224,094	3,441,290					4,665,384
UCLB/UCLA License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Payments to acquire clinical data							$ 800
Annual administration fee				100
Payments upon achievement of specified regulatory milestones | £					£ 19,900,000
Research and development				$ 100		$ 100
Oxford BioMedica License, Development and Supply Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Ordinary shares issued | shares	75,413			588,220					588,220		588,220	588,220		150,826	150,826
Number of milestones met | Milestone						0
Oxford BioMedica License, Development and Supply Agreement | Research and Development Expense
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payment	$ 800

Income Taxes - Schedule of Comp

Income Taxes - Schedule of Components of Income (Loss) from Operations Before Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
UK	$ (155,614)	$ (173,118)
Non-UK	2,904	7,456
Net loss before income tax	$ (152,710)	$ (165,662)

Income Taxes - Schedule of (Ben

Income Taxes - Schedule of (Benefit from) Provision for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Current (benefit) provision
Federal—United States	$ 1,107	$ 888
State—United States	189	(275)
Other foreign	230	89
Total current (benefit) provision	1,526	702
Deferred (benefit) provision
Federal—United States	(1,774)	(2,820)
State—United States	(103)	(122)
Other foreign	(380)
Total deferred (benefit) provision	(2,257)	(2,942)
Total (benefit) provision for income taxes	$ (731)	$ (2,240)

Income Taxes - Schedule of Reco

Income Taxes - Schedule of Reconciliation of Income Tax (Benefit) Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income taxes at United Kingdom statutory rate	$ (29,015)	$ (31,475)
Change in valuation allowance	29,302	16,507
Reduction in research expense for credits granted	8,435	9,787
Change in tax rates	(8,105)	8,109
Tax credits	(1,369)	(3,372)
U.S. Deduction for foreign derived intangible income	(1,254)	(2,058)
Permanent differences, including share-based compensation deduction shortfalls	1,265	344
U.S. state income taxes	68	(238)
Foreign rate differential	(58)	156
Total (benefit) provision for income taxes	$ (731)	$ (2,240)

Income Taxes - Schedule of Defe

Income Taxes - Schedule of Deferred Tax Assets and Liabilities Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets
Net operating loss carryforwards	$ 75,502	$ 45,358
Amortization	22,599	21,741
Research and development credits	1,564	1,244
Share-based compensation	7,400	3,604
Accruals	1,001	1,286
Lease Liability	6,805	4,406
Other	3	1
Total deferred tax assets	114,874	77,640
Valuation allowance	(103,890)	(70,153)	$ (51,281)
Fixed assets and right-of-use asset	(5,765)	(4,502)
Other non-current assets (net deferred tax assets and liabilities)	$ 5,219	$ 2,985

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Operating Loss Carryforwards [Line Items]
Accrued interest or penalties related to uncertain tax positions	$ 0	$ 0
Earliest Tax Year
Operating Loss Carryforwards [Line Items]
Income tax year open to examination	2017
Latest Tax Year
Operating Loss Carryforwards [Line Items]
Income tax year open to examination	2020
U.S. Federal General Business and U.S. State Research and Development
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	$ 2,000,000
United Kingdom
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	$ 390,100,000	$ 266,800,000

Income Taxes - Schedule of Chan

Income Taxes - Schedule of Changes in Valuation Allowance for Deferred Tax Assets Related Primarily to Increase in U.K. Net Operating Loss Carryforwards and U.K. Amortization of Intangible Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Valuation allowance as of beginning of year	$ (70,153)	$ (51,281)
Increases recorded to income tax provision	(29,302)	(16,507)
Effect of foreign currency translation	(4,435)	(2,365)
Valuation allowance as of end of year	$ (103,890)	$ (70,153)

Commitments and Contingencies -

Commitments and Contingencies - Schedule of Annual Commitments Associated with the Contract (Details) $ in Thousands	Dec. 31, 2020USD ($)
Other Commitments [Line Items]
2021	$ 15,205
2022	15,197
2023	15,197
2024	15,197
2025	6,761
Total manufacturing commitments	67,557
Product Manufacturing Commitments
Other Commitments [Line Items]
2021	2,491
2022	3,321
2023	3,321
2024	3,321
2025	1,661
Total manufacturing commitments	14,115
Dedicated Manufacturing and Development Resources
Other Commitments [Line Items]
2021	8,524
2022	8,524
2023	8,524
2024	8,524
2025	4,262
Total manufacturing commitments	38,358
Exclusive Transduction Suites
Other Commitments [Line Items]
2021	4,190
2022	3,352
2023	3,352
2024	3,352
2025	838
Total manufacturing commitments	$ 15,084

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Annual Commitments Associated with the Contract (Parenthetical) (Details) - Dec. 31, 2020	USD ($)	EUR (€)
Commitments And Contingencies Disclosure [Abstract]
Annual commitments exchange rate	$ 1.23	€ 1

Commitments and Contingencies_3

Commitments and Contingencies - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Other Commitments [Line Items]
Shares, vest	4 years
Ordinary Shares, Issued	98,283,603	96,923,729
Research and development	$ 93,730,000	$ 117,363,000
Compensation cost recognized	$ 27,962,000	19,424,000
Consulting Agreement | Advisor
Other Commitments [Line Items]
Cash payments		$ 100,000
Shares, issued		91,034
Shares, vest		4 years
Ordinary Shares, Issued	22,758
Research and development	$ 300,000
Compensation cost recognized	$ 0	$ 0
Agreement
Other Commitments [Line Items]
Initial term of agreement	5 years
Agreement maturity date	Jul. 2, 2025
Agreement extension term	2 years
Clinical Development And Regulatory Milestone | Consulting Agreement | Advisor
Other Commitments [Line Items]
Shares, issued		92,035
Ordinary Shares, Issued	22,758

Benefit Plans - Additional Info

Benefit Plans - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Compensation And Retirement Disclosure [Abstract]
Contribution expenses	$ 1.6	$ 1.3
Maximum annual contribution employer matches per employee, percent	6.00%

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Apr. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Related Party Transaction [Line Items]
Loss provision associated with contract				$ 18,400
Research and development		$ 93,730	$ 117,363
Payments on accounts payable		1,553	6,413
Payment on inventory purchase liability			3,300
GSK
Related Party Transaction [Line Items]
License upfront fee	$ 14,200
Inventory purchased	6,900
Transaction costs	$ 800
Due to related party transactions, accounts payable		100
Due to related party transactions, accrued expenses		100
Research and development		1,200
Payments on accounts payable		$ 5,800
GSK | Transition Services Agreement
Related Party Transaction [Line Items]
Payments on accounts payable			7,200
Payment on inventory purchase liability			3,600
GSK | Strimvelis
Related Party Transaction [Line Items]
Royalty Expense			$ 100
GSK | Convertible Preferred Shares
Related Party Transaction [Line Items]
Ordinary shares Issuable Upon Conversion	12,455,252
Preferred stock, Value	$ 93,400
Loss provision associated with contract	$ 18,400

Selected Quarterly Financial _3

Selected Quarterly Financial Information (unaudited) - Summary of Unaudited Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Quarterly Financial Information Disclosure [Abstract]
Total revenues		$ 1,998	$ 597		$ 595	$ 1,918			$ 2,595	$ 2,513
Total costs and operating expenses	$ 38,873	28,301	47,418	$ 44,981	49,621	43,330	$ 54,152	$ 28,283	159,573	175,386
Loss from operations	(38,873)	(26,303)	(46,821)	(44,981)	(49,026)	(41,412)	(54,152)	(28,283)	(156,978)	(172,873)
Net loss attributable to ordinary shareholders	$ (33,620)	$ (20,290)	$ (47,500)	$ (50,569)	$ (45,416)	$ (36,737)	$ (50,530)	$ (30,739)	$ (151,979)	$ (163,422)
Weighted average ordinary shares outstanding - basic and diluted	100,013,246	99,664,616	99,251,314	98,713,126	98,243,915	97,817,847	89,712,916	87,010,596	99,445,874	93,240,355
Earnings per share	$ (0.34)	$ (0.20)	$ (0.48)	$ (0.51)	$ (0.46)	$ (0.38)	$ (0.56)	$ (0.35)	$ (1.53)	$ (1.75)

Subsequent Events - Additional

Subsequent Events - Additional Information (Details) $ / shares in Units, $ in Thousands	Feb. 09, 2021USD ($)$ / sharesshares	Dec. 31, 2019USD ($)	Dec. 31, 2020£ / shares	Dec. 31, 2019£ / shares
Subsequent Event [Line Items]
Nominal value of shares issued and sold | £ / shares			£ 0.10	£ 0.10
Issuance of ADSs in follow-on offering, net of issuance costs | $		$ 605
Subsequent Event | Securities Purchase Agreement
Subsequent Event [Line Items]
Number of ordinary shares issued and sold | shares	20,900,321
Nominal value of shares issued and sold | $ / shares	$ 0.10
Purchase price per share | $ / shares	$ 6.22
Closing price of share date	Feb. 4, 2021
Subsequent Event | Securities Purchase Agreement | Non-voting Ordinary Shares
Subsequent Event [Line Items]
Gross proceeds from private placement | $	$ 150,000
Subsequent Event | Securities Purchase Agreement | Private Placement | Non-voting Ordinary Shares
Subsequent Event [Line Items]
Number of ordinary shares issued and sold | shares	3,215,434
Nominal value of shares issued and sold | $ / shares	$ 0.10
Issuance of ADSs in follow-on offering, net of issuance costs | $	$ 6,000
Private placement, transaction date	Feb. 4, 2021