Orchard Therapeutics (ORTX) 10-Q 13 May 21 2021 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2021	May 04, 2021
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1
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 Small Business	true
Entity Common Stock, Shares Outstanding		123,769,373
Entity Shell Company	false
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 Quarterly Report	true
Document Transition Report	false
Entity File Number	001-38722
Entity Emerging Growth Company	false
Entity Tax Identification Number	00-0000000
Entity Address, Country	GB
Title of 12(b) Security	American Depositary Shares, each representing one ordinary share, nominal value £0.10 per share
Trading Symbol	ORTX
Security Exchange Name	NASDAQ

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 78,883	$ 55,135
Marketable securities	219,543	136,813
Trade receivables		878
Prepaid expenses and other current assets	12,504	13,365
Research and development tax credit receivable, current	17,493	17,344
Total current assets	328,423	223,535
Non-current assets:
Operating lease right-of-use-assets	28,700	29,815
Property and equipment, net	4,591	4,781
Research and development tax credit receivable, net of current portion	3,552
Restricted cash	4,266	4,266
Other assets	19,581	18,540
Total assets	389,113	280,937
Current liabilities:
Accounts payable	7,905	8,823
Accrued expenses and other current liabilities	25,672	28,943
Operating lease liabilities	7,964	8,934
Notes payable current	6,944	4,861
Total current liabilities	48,485	51,561
Notes payable, long term	18,208	20,204
Operating lease liabilities, net of current portion	20,847	24,168
Other long-term liabilities	5,993	6,570
Total liabilities	93,533	102,503
Commitments and contingencies (see Note 12)
Shareholders’ equity:
Ordinary shares, £0.10 nominal value; 120,549,163 and 98,283,603 ordinary shares issued and outstanding at March 31, 2021 and December 31, 2020, respectively; 3,215,434 and nil non-voting ordinary shares issued and outstanding at March 31, 2021 and December 31, 2020, respectively	15,995	12,507
Additional paid-in capital	920,211	771,194
Accumulated other comprehensive income	196	373
Accumulated deficit	(640,822)	(605,640)
Total shareholders’ equity	295,580	178,434
Total liabilities and shareholders’ equity	$ 389,113	$ 280,937

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - £ / shares	Mar. 31, 2021	Dec. 31, 2020
Ordinary Shares, Nominal Value	£ 0.10	£ 0.10
Ordinary Shares, Issued	120,549,163	98,283,603
Ordinary Shares, Outstanding	120,549,163	98,283,603
Non-voting Ordinary Shares
Ordinary Shares, Issued	3,215,434
Ordinary Shares, Outstanding	3,215,434

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Income Statement [Abstract]
Type of Revenue [Extensible List]	us-gaap:ProductMember	us-gaap:ProductMember
Costs and operating expenses:
Research and development	$ 21,035	$ 24,836
Selling, general and administrative	14,051	20,145
Total costs and operating expenses	35,086	44,981
Loss from operations	(35,086)	(44,981)
Other income (expense):
Interest income	171	1,480
Interest expense	(538)	(613)
Other income (expense), net	1,358	(6,790)
Total other income (expense), net	991	(5,923)
Net loss before income tax	(34,095)	(50,904)
Income tax (expense) benefit	(1,087)	335
Net loss	(35,182)	(50,569)
Other comprehensive (loss) income:
Foreign currency translation adjustment	(64)	6,034
Unrealized loss on marketable securities	(113)	(1,021)
Total other comprehensive (loss) income:	(177)	5,013
Total comprehensive loss	$ (35,359)	$ (45,556)
Net loss per share, basic and diluted	$ (0.31)	$ (0.51)
Weighted average ordinary shares outstanding, basic and diluted	114,829,272	98,713,126

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Cash flows from operating activities:
Net loss	$ (35,182)	$ (50,569)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	495	482
Non-cash share-based compensation	6,268	9,479
Amortization of Strimvelis loss provision	(446)	(1,691)
Other non-cash adjustments	2,968	6,537
Changes in operating assets and liabilities:
Trade receivables	887	670
Research and development tax credit receivable	(3,554)	(3,385)
Prepaid expenses, other current assets and other assets	1,399	(2,500)
Operating leases, right-of-use assets	1,252	1,025
Accounts payable, accrued expenses and other current liabilities	(8,905)	(8,903)
Operating lease liabilities	(4,424)	(1,047)
Net cash used in operating activities	(39,242)	(49,902)
Cash flows from investing activities:
Proceeds from sales and maturities of marketable securities	47,200	68,143
Purchases of marketable securities	(130,387)
Payment of construction deposit		(10,000)
Purchases of property and equipment	(339)	(1,329)
Net cash (used in) provided by investing activities	(83,526)	56,814
Cash flows from financing activities:
Proceeds from equity plans	2,822	1,438
Taxes paid on net settlement of equity plan issuances	(296)
Proceeds from the issuance of ordinary shares in private placement	150,000
Payment of placement agent fees and offering costs	(6,092)
Net cash provided by financing activities	146,434	1,438
Effect of exchange rate changes on cash, cash equivalents, and restricted cash	82	(303)
Net increase in cash, cash equivalents and restricted cash	23,748	8,047
Cash, cash equivalents, and restricted cash, beginning of period	59,401	23,317
Cash, cash equivalents, and restricted cash, end of period	83,149	31,364
Supplemental disclosure of non-cash activities
Property and equipment and intangible assets included in accounts payable and accrued expenses	4,834	382
Offering costs included in accounts payable and accrued expenses	198
Lease assets obtained in exchange for new operating lease liabilities		3,752
Supplemental disclosure of cash flow information:
Cash paid for interest	$ 482	$ 485

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Shareholders' (Deficit) Equity (Unaudited) - USD ($) $ in Thousands	Total	Ordinary Shares	Additional Paid-in Capital	Accumulated Other Comprehensive Income	Accumulated Deficit
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	9,479		9,479
Exercise of share options	1,438	$ 30	1,408
Exercise of share options, Shares		230,836
Foreign currency translation adjustment	6,034			6,034
Unrealized loss on available for sale debt securities	(1,021)			(1,021)
Net loss	(50,569)				(50,569)
Balance at Mar. 31, 2020	264,554	$ 12,361	749,368	7,055	(504,230)
Balance, Shares at Mar. 31, 2020		97,154,565
Balance at Dec. 31, 2020	178,434	$ 12,507	771,194	373	(605,640)
Balance, Shares at Dec. 31, 2020		98,283,603
Share-based compensation expense	6,268		6,268
Exercise of share options	$ 2,822	$ 172	2,650
Exercise of share options, Shares	1,319,493	1,319,493
Vesting of restricted stock units, net of shares withheld for taxes	$ (296)	$ 6	(302)
Vesting of restricted stock units, net of shares withheld for taxes, shares		45,746
Sale of ordinary shares and non-voting ordinary shares, net of issuance costs	143,711	$ 3,310	140,401
Sale of ordinary shares and non-voting ordinary shares, net of issuance costs, Shares		24,115,755
Foreign currency translation adjustment	(64)			(64)
Unrealized loss on available for sale debt securities	(113)			(113)
Net loss	(35,182)				(35,182)
Balance at Mar. 31, 2021	$ 295,580	$ 15,995	$ 920,211	$ 196	$ (640,822)
Balance, Shares at Mar. 31, 2021		123,764,597

Condensed Consolidated Statem_4

Condensed Consolidated Statements Shareholders' (Deficit) Equity (Unaudited) (Parenthetical) $ in Thousands	3 Months Ended
	Mar. 31, 2021USD ($)
Statement Of Stockholders Equity [Abstract]
Sale of ordinary shares and non-voting ordinary shares, issuance costs	$ 6,290

Nature of the Business

Nature of the Business	3 Months Ended
	Mar. 31, 2021
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of the Business	1. Nature of the Business Orchard Therapeutics plc (the “Company”) is a global gene therapy company dedicated to transforming the lives of people affected by severe 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 standard marketing authorization from the European Commission for Libmeldy™ ( atidarsagene autotemcel arylsulfatase-A ARSA 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 (together (i) and (ii) the “Private Placement”). The Private Placement resulted in net proceeds to the Company of $143.7 million after deducting placement agent fees of $6.0 million and other issuance costs of $0.3 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. The Company’s business is subject to risks and uncertainties common to development-stage companies in the biotechnology industry. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s technology will be obtained, that any products developed will obtain necessary government regulatory approval or that any products, if approved, will be commercially viable. The Company operates in an environment of rapid technological innovation and substantial competition from pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees, consultants and service providers. Even if the Company’s product development efforts are successful in gaining regulatory approval, it is uncertain when, if ever, the Company will realize significant revenue from product sales. Through March 31, 2021, the Company funded its operations primarily with proceeds from the sale of convertible preferred shares, ADSs in the Company’s initial public offering (the “IPO”) and follow-on offering, ordinary shares in the Private Placement, proceeds from share issuances from employee equity plans, receipts from the United Kingdom (“UK”) research and development tax credit, and reimbursements from our research agreements with the University of California (“UCLA”) and the California Institute of Regenerative Medicine (“CIRM”). The Company has incurred recurring losses since its inception. As of March 31, 2021, the Company had an accumulated deficit of $640.8 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 March 31, 2021 of $298.4 million 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 shareholders.

Basis of Presentation and Summa

Basis of Presentation and Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2021
Accounting Policies [Abstract]
Basis of Presentation and Summary of Significant Accounting Policies	2. Basis of Presentation and Summary of Significant Accounting Policies Basis of presentation The condensed consolidated interim financial statements of the Company are unaudited and have been prepared in accordance with generally accepted accounting principles in the United States of America (“U.S. GAAP”) for interim financial reporting and in accordance with Regulation S-X, Rule 10-01. Accordingly, they do not include all of the information and footnotes required by U.S. GAAP for complete financial statements. Any reference in these notes to applicable guidance is meant to refer to the authoritative U.S. GAAP as found in the Accounting Standards Codification (“ASC”), and Accounting Standards Update (“ASU”), of the Financial Accounting Standards Board (“FASB”). All intercompany accounts and transactions between the Company and its subsidiaries have been eliminated upon consolidation. The accompanying unaudited condensed consolidated interim financial statements should be read in conjunction with the audited consolidated financial statements and accompanying notes included in the Company’s Annual Report on Form 10-K filed with the SEC on March 2, 2021 (the “Annual Report”). The condensed consolidated balance sheet as of December 31, 2020 was derived from audited consolidated financial statements included in the Company’s Annual Report but does not include all disclosures required by U.S. GAAP. Certain information and footnote disclosures normally included in financial statements prepared in accordance with U.S. GAAP have been condensed or omitted from these interim financial statements. However, these interim financial statements include all adjustments, consisting only of normal recurring adjustments, which are, in the opinion of the Company’s management, necessary to fairly state the results of the interim period. The interim results are not necessarily indicative of results to be expected for the full year. 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. Use of estimates The preparation of condensed consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the condensed consolidated financial statements and the reported amounts of revenue and expenses during the reporting periods. Significant estimates and assumptions reflected in these condensed consolidated financial statements include, but are not limited to, the accrual for research and development expenses, the research and development tax credit receivable, 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 may also directly or indirectly impact the Company’s business, including impacts due to 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. 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 shareholders’ 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 $1.4 million and losses of $6.8 million for the three months ended March 31, 2021 and 2020, respectively, which is included in other income (expense), net in the condensed consolidated statements of operations and comprehensive loss. Cash and cash equivalents The Company considers all highly liquid investments purchased with original maturities of 90 days or less at acquisition to be cash equivalents. Marketable debt 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 the statement of operations; 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 condensed consolidated statements of operation. United Kingdom research and development tax credit 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 undertaken 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, 2021. The Company has qualified under the more favorable SME regime for the year ended December 31, 2020 and expects to qualify under the SME regime for the year ending December 31, 2021. 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 in the condensed consolidated statement of operations and comprehensive loss. The following table below outlines the changes to the research and development tax credit receivable, including amounts recognized as an offset to research and development expense during the period: Three Months Ended March 31, 2021 2020 Balance at beginning of period $ 17,344 $ 28,644 Recognition of credit claims as offset to research and development expense 3,554 3,417 (Receipt) of credit claims — — Foreign currency translation 147 (1,850 ) Balance at end of period $ 21,045 $ 30,211 As of March 31, 2021, the Company’s tax incentive receivable from the UK government was $21.1 million, of which $17.5 million was classified as current and $3.6 million was classified as long-term. As of December 31, 2020, the Company’s tax incentive receivable from the UK government was $17.3 million, all of which was classified as current. Restricted cash and construction deposits Cash and cash equivalents that are restricted as to withdrawal or use under the terms of certain contractual agreements are recorded as restricted cash on the Company’s condensed consolidated balance sheet. The Company has an outstanding letter of credit for $3.0 million associated with a lease and is required to hold this amount in a standalone bank account, as of March 31, 2021 and December 31, 2020. The Company is also contractually required to maintain cash collateral accounts associated with corporate credit cards and other leases in the amount of $1.3 million at March 31, 2021 and December 31, 2020. The Company includes the restricted cash balance in cash and cash equivalents when reconciling beginning-of-period and end-of-period total amounts shown on the condensed consolidated statements of cash flows. The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported in the condensed consolidated balance sheet that sum to the total of the amounts reported in the unaudited condensed consolidated statement of cash flows: March 31, December 31, 2021 2020 Cash and cash equivalents $ 78,883 $ 55,135 Restricted cash 4,266 4,266 Total cash, cash equivalents and restricted cash shown in the statement of cash flows $ 83,149 $ 59,401 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 and subleased to a third-party. Subject to the terms of the 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 condensed 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. 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 agreement costs and accruals The Company has entered into various research and development contracts. These agreements are cancelable, and related costs are recorded as research and development expenses as incurred. When billing terms under these contracts do not coincide with the timing of when the work is performed, the Company is required to make estimates of outstanding obligations as of period end to those third parties. Any accrual estimates are based on a number of factors, including the Company’s knowledge of the progress towards completion of the research and development activities, invoicing to date under the contracts, communication from the research institution or other companies of any actual costs incurred during the period that have not yet been invoiced, and the costs included in the contracts. Significant judgments and estimates may be made in determining the accrued balances at the end of any reporting period. Actual results could differ from the estimates made by the Company. The historical accrual estimates made by the Company have not been materially different from the actual costs. Share-based compensation The Company measures share-based awards granted to employees , consultants, and directors based on the fair value of the shares and options on the date of the grant and recognizes compensation expense for those awards over the requisite service period, which is the vesting period of the respective award. Forfeitures are accounted for as they occur. Comprehensive loss Comprehensive loss is composed of net loss and other comprehensive (loss) income. Other comprehensive (loss) income 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 (as detailed in Note 10), the Company is required to maintain commercial availability of Strimvelis in the European Union until such time that an alternative gene therapy is available. Strimvelis is not currently expected to generate sufficient cash flows to overcome the costs of maintaining the product and certain regulatory commitments; therefore, the Company initially recorded a liability associated with the loss contract of $18.4 million in 2018. The Company recognizes the amortization of the loss provision on a diminishing balance basis based on the actual net loss incurred associated with Strimvelis and the expected future net losses to be generated until such time as Strimvelis is no longer commercially available. The amortization of the provision is recorded as a reduction in research and development expense. The Company has 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 will continue to evaluate its future estimates for amortization of the Strimvelis loss provision Three Months Ended March 31, 2021 2020 Balance at beginning of period $ 4,482 $ 6,790 Amortization of loss provision (446 ) (1,691 ) Foreign currency translation 40 (348 ) Balance at end of period $ 4,076 $ 4,751 Of the balance as of March 31, 2021 noted in the table above, $1.1 million is classified as current, and $3.0 million is classified as non-current. Net loss per share Basic net loss per share is computed by dividing the net loss by the weighted average number of voting and non-voting ordinary shares outstanding for the period. Diluted net loss is computed by adjusting net loss based on the potential impact of dilutive securities. Diluted net loss per share is computed by dividing the diluted net 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: Three Months Ended March 31, 2021 2020 Share options 12,935,554 12,688,361 Unvested performance-based restricted share units 717,167 511,324 13,652,721 13,199,685 Recent Accounting Pronouncements Recently adopted accounting pronouncements In December 2019, the FASB issued ASU No. 2019-12, Simplifying the Accounting for Income Taxes (Topic 740) Income Taxes

Fair Value Measurements and Mar

Fair Value Measurements and Marketable Debt Securities	3 Months Ended
	Mar. 31, 2021
Fair Value Disclosures [Abstract]
Fair Value Measurements and Marketable Debt Securities	3. Fair Value Measurements and Marketable Debt Securities The following tables present information about the Company’s financial assets that have been measured at fair value as of March 31, 2021 and indicate the fair value of the hierarchy of the valuation inputs utilized to determine such fair value. In general, fair values determined by Level 1 inputs utilize quoted prices (unadjusted) in active markets for identical assets or liabilities. Fair value determined by Level 2 inputs utilize observable inputs other than Level 1 prices, such as quoted prices for similar assets or liabilities, quoted market prices in markets that are not active or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the related assets or liabilities. Fair values determined by Level 3 inputs are unobservable data points for the asset or liability, and include situations where there is little, if any, market activity for the asset or liability. During the three months ended March 31, 2021, there were no transfers between Level 1 and Level 2 financial assets. The following table summarizes the Company’s cash equivalents and marketable debt securities as of March 31, 2021: Fair Value Measurements at March 31, 2021 Using: Level 1 Level 2 Level 3 Total Cash equivalents Money market funds $ 26,091 $ — $ — $ 26,091 Corporate bonds — $ 5,174 — 5,174 Commercial paper — 23,854 — 23,854 Total cash equivalents $ 26,091 $ 29,028 $ — $ 55,119 Marketable securities Corporate bonds $ — $ 96,879 $ — $ 96,879 Commercial paper — 122,664 — $ 122,664 Total marketable securities $ — $ 219,543 $ — $ 219,543 Total $ 26,091 $ 248,571 $ — $ 274,662 The following table summarizes the Company’s cash equivalents and marketable debt securities as of December 31, 2020: Fair Value Measurements at 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 U.S. 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 carrying amount reflected in the condensed consolidated balance sheets for research and development tax incentive receivable, trade 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 Debt Securities The following table summarizes the Company’s marketable debt securities as of March 31, 2021: At March 31, 2021 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Credit Losses Fair Value Corporate bonds $ 102,030 $ 68 $ (45 ) $ — $ 102,053 Commercial paper 146,551 2 (35 ) — 146,518 Total $ 248,581 $ 70 $ (80 ) $ — $ 248,571 The following table summarizes the Company’s marketable securities as of December 31, 2020: At December 31, 2020 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 available-for-sale debt securities by contractual maturity, as of March 31, 2021 and December 31, 2020: At March 31, 2021 At December 31, 2020 Due in one year $ 242,579 $ 132,056 Due after one year through three years 5,992 10,757 Total $ 248,571 $ 142,813

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets	3 Months Ended
	Mar. 31, 2021
Prepaid Expense And Other Assets Current [Abstract]
Prepaid Expenses and Other Current Assets	4. Prepaid Expenses and Other Current Assets Prepaid expenses and other current assets consist of the following: March 31, December 31, 2021 2020 Prepaid external research and development expenses $ 2,132 $ 1,421 Inventories 1,180 665 Other prepayments 4,634 4,930 VAT receivable 894 2,780 Construction deposit - current 1,634 1,552 Non-trade receivables 2,030 2,017 Total prepaid expenses and other current assets $ 12,504 $ 13,365

Property and Equipment, Net

Property and Equipment, Net	3 Months Ended
	Mar. 31, 2021
Property Plant And Equipment [Abstract]
Property and Equipment, Net	5. Property and Equipment, net Property and equipment, net consisted of the following: March 31, December 31, 2021 2020 Property and equipment: Lab equipment $ 5,159 $ 5,114 Leasehold improvements 2,486 2,522 Furniture and fixtures 304 304 Office and computer equipment 765 763 Construction-in-process 601 302 Property and equipment $ 9,315 $ 9,005 Less: accumulated depreciation (4,724 ) (4,224 ) Property and equipment, net $ 4,591 $ 4,781 Depreciation expense was $0.5 million and $0.5 million for the three months ended March 31, 2021 and 2020, respectively.

Other Assets

Other Assets	3 Months Ended
	Mar. 31, 2021
Other Assets Noncurrent Disclosure [Abstract]
Other Assets	6. Other Assets Other assets consist of the following: March 31, December 31, 2021 2020 Intangible assets - license milestones $ 4,914 $ 3,076 Deferred tax assets 4,449 5,219 Deposits 1,019 1,144 Deferred financing costs 647 975 Other non-current assets 2,061 1,554 Construction deposits - long-term 6,491 6,572 Total other assets $ 19,581 $ 18,540

Accrued Expenses and Other Liab

Accrued Expenses and Other Liabilities	3 Months Ended
	Mar. 31, 2021
Accrued Liabilities And Other Liabilities [Abstract]
Accrued Expenses and Other Liabilities	7. Accrued Expenses and Other Liabilities Accrued expenses and other liabilities consisted of the following: March 31, December 31, 2021 2020 Accrued external research and development expenses $ 8,210 $ 8,878 Accrued payroll and related expenses 7,084 11,881 Accrued professional fees 758 791 Accrued other 3,616 3,401 Accrued milestone payments 4,914 3,076 Strimvelis loss provision - current portion 1,090 916 Total accrued expenses and other liabilities $ 25,672 $ 28,943

Notes Payable

Notes Payable	3 Months Ended
	Mar. 31, 2021
Debt Disclosure [Abstract]
Notes Payable	8. Notes Payable In May 2019 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 million in cash and cash equivalent investments; and the third term loan being a $25.0 million term loan available no earlier than July 1, 2020 and no later than September 30, 2021 upon certain regulatory approvals and evidence of the Company having $125 million in cash and cash equivalent investments. Each term loan under the Credit Facility bears interest at an annual rate equal to 6% plus LIBOR. The 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 for all payment dates prior to 36 months following the date of the Credit Facility. The term loans under the Credit Facility will begin amortizing on either the 24-month or the 36-month anniversary of the Credit Facility (as applicable), with equal monthly payments of principal plus interest to be made by the Borrower to the Lenders in consecutive monthly installments until the Loan Maturity Date. In addition, a final payment of 4.5% is due on the Loan Maturity Date. The Company accrues the final payment amount of $1.1 million associated with the first term loan, to outstanding debt by charges to interest expense using the effective-interest method from the date of issuance through the maturity date. The Credit Facility includes affirmative and negative covenants. The affirmative covenants include, among others, covenants requiring the Company to maintain their legal existence and governmental approvals, deliver certain financial reports, maintain insurance coverage, maintain property, pay taxes, satisfy certain requirements regarding accounts and comply with laws and regulations. The negative covenants include, among others, restrictions on the Company transferring collateral, incurring additional indebtedness, engaging in mergers or acquisitions, paying dividends or making other distributions, making investments, creating liens, amending material agreements and organizational documents, selling assets, changing the nature of the business and undergoing a change in control, in some cases subject to certain exceptions. The Company is also subject to an ongoing minimum cash financial covenant in which the Company must maintain unrestricted cash in an amount not less than $ 20.0 million following the utilization of the second term loan and not less than $ 35.0 million following the utilization of the third term loan . Notes payable consist of the following: At March 31, At December 31, 2021 2020 Notes payable, net of issuance costs $ 24,687 $ 24,659 Less current portion (6,944 ) (4,861 ) Notes payable, net of current portion 17,743 19,798 Accretion related to final payment 465 406 Notes payable, long term $ 18,208 $ 20,204 As of March 31, 2021, the estimated future principal payments due are as follows: Aggregate Minimum Payments 2021 (April - December) 4,861 2022 8,333 2023 8,334 2024 4,597 2025 — Thereafter — Total 26,125 Less: current portion (6,944 ) Less: unamortized portion of final payment (660 ) Less: unamortized debt issuance costs (313 ) Notes payable, long term $ 18,208 During the three months ended March 31, 2021 and 2020, the Company recognized $0.5 million and $0.6 million of interest expense, respectively, related to the initial term loan. The effective annual interest rate for the three months ended March 31, 2021 on the outstanding debt under the term loan was approximately 8.6%.

Share-Based Compensation

Share-Based Compensation	3 Months Ended
	Mar. 31, 2021
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Share-Based Compensation	9. 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 board of directors has determined not to make any further awards under the 2016 plan following the Company’s IPO. As of March 31, 2021, 3,542,397 shares remained available for issuance under the 2018 Plan, 1,000,000 shares remained available for issuance under the Inducement Plan, and 1,470,104 shares remained available for issuance under the ESPP. Share option activity The following table summarizes option activity under the plans for three months ended March 31, 2021: Number of Options Weighted Average Exercise Price Outstanding at December 31, 2020 13,895,643 $ 7.96 Granted 4,506,256 5.97 Exercised (1,319,493 ) 2.06 Forfeited (1,192,274 ) 11.16 Outstanding at March 31, 2021 15,890,132 $ 7.65 Vested and expected to vest, as of March 31, 2021 15,890,132 $ 7.65 Exercisable, March 31, 2021 6,116,939 $ 6.73 The weighted-average grant date fair value of share options granted during the three months ended March 31, 2021 was $3.87 per share. Restricted Share Units Performance-based restricted share units The Company has issued performance-based restricted share units (“RSUs”) to certain executives and members of its senior management, with vesting linked to the achievement of three specific regulatory and research and development milestones and one market condition based upon the volume weighted-average price (“VWAP”) of the Company’s ADSs for a certain period. Upon achievement of any of the aforementioned milestones, one third of the RSU’s will vest, and the award will become fully vested upon achievement of three of the four performance conditions. In April 2020, 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. 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 this award were deemed probable and none vested during the three months ended March 31, 2021. The maximum aggregate total fair value of the outstanding performance-based RSUs is $3.5 million. The fair value associated with the shares that could vest based on the market-based condition was recognized as expense over an average derived service period of 1.3 years. The fair value associated with the performance-based conditions will be recognized when achievement of the milestones becomes probable, if at all. 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. Time-based restricted share units Time-based restricted share units generally vest in equal annual installments over a three-year The following table summarizes award activity for the three months ended March 31, 2021: Performance-based RSUs Time-based RSUs Total RSUs Weighted Average Grant Date Fair Value Unvested and outstanding at December 31, 2020 464,000 180,000 644,000 $ 8.75 Granted — 20,000 20,000 5.98 Vested (89,667 ) — (89,667 ) 9.40 Forfeited (34,500 ) (40,000 ) (74,500 ) 11.80 Unvested and outstanding at March 31, 2021 339,833 160,000 499,833 $ 7.99 The amount of compensation cost recognized for the three months ended March 31, 2021 and 2020 for the market condition associated with the performance-based RSUs was not material. Share-based compensation expense Share-based compensation expense recorded as research and development and general and administrative expenses is as follows: March 31, March 31, 2021 2020 Research and development $ 2,876 $ 3,110 General and administrative 3,392 6,369 Total share-based compensation $ 6,268 $ 9,479 During the three months ended March 31, 2020, the Company recognized $2.7 million of share-based compensation expense to selling, general and administrative expense related to the modification of share option awards associated with the separation of the Company’s former Chief Executive Officer. As of March 31, 2021, total unrecognized compensation cost related to unvested share options and time-based RSU’s was approximately $53.5 million. This amount is expected to be recognized over a weighted average period of approximately 2.9 years. As of March 31, 2021, the total unrecognized compensation cost related to performance-based RSUs is a maximum of $3.5 million, dependent upon achievement of the aforementioned milestones.

License Agreements

License Agreements	3 Months Ended
	Mar. 31, 2021
License Agreements [Abstract]
License Agreements	10. License Agreements GSK asset purchase and license agreement In April 2018, the Company completed an asset purchase and license agreement (the “GSK Agreement”) with subsidiaries of GSK to acquire a portfolio of autologous ex vivo 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. As part of the GSK Agreement the Company is also 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. The Company will pay GSK non-refundable royalties and milestone payments in relation to the gene therapy programs acquired and the Company-developed product candidate, 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 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. Telethon-OSR research and development collaboration and license agreement s In connection with the Company’s entering into the GSK Agreement in April 2018, 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, up to an aggregate of approximately €31.0 million ($36.2 million at March 31, 2021). 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. 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 Oxford BioMedica license, development and supply agreement In November 2016, and amended in June 2017, May 2018, July 2018, September 2018, May 2019 and April 2020, the Company entered into an arrangement with Oxford BioMedica plc whereby Oxford BioMedica granted an exclusive intellectual property license to the Company for the purposes of research, development, and commercialization of collaboration products, and whereby Oxford BioMedica will provide process development services (“Oxford BioMedica Development Agreement”). As part of the consideration to rights and licenses granted under the Oxford BioMedica Development 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. No milestones were met during the three months ended March 31, 2020. 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 another 75,413 ordinary shares to Oxford BioMedica with a total value of $0.8 million which was recorded to research and development expense. T he Company may also pay low single-digit percentage royalties on net sales of collaborated product generated under the Oxford BioMedica Agreement.

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2021
Income Tax Disclosure [Abstract]
Income Taxes	11. Income Taxes The Company recorded income tax expense 1.1 benefit

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2021
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	12. Commitments and Contingencies Legal proceedings The Company is not a party to any material litigation and does not have contingency reserves established for any litigation liabilities. Manufacturing and technology development master agreement with AGC Biologics The Company is party to an Agreement with AGC Biologic S.p.A (“AGC”) pursuant to which 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 current commitments associated with the agreement, as of March 31, 2021: Due in: Product manufacturing commitments Dedicated manufacturing and development resources Exclusive transduction suites Total remaining AGC commitment 2021 (April - December) $ 2,379 $ 7,796 $ — $ 10,175 2022 3,173 8,143 3,202 14,518 2023 3,173 8,143 3,202 14,518 2024 3,173 8,143 3,202 14,518 2025 1,586 4,072 1,601 7,259 Total manufacturing commitments 13,484 36,297 11,207 60,988 *Tabular disclosure above has been translated to U.S. Dollar, from Euro, using an exchange rate of €1.00 to $1.175. Lease commitments The Company leases office and laboratory space and has an embedded lease at AGC. There have been no material changes to the Company’s lease commitments as reported in the Company’s Annual Report on Form 10-K.

Employee Benefit Plans

Employee Benefit Plans	3 Months Ended
	Mar. 31, 2021
Compensation And Retirement Disclosure [Abstract]
Employee Benefit Plans	13. Employee Benefit Plans The Company makes contributions to private defined contribution employee benefit plans on behalf of its employees. The Company provides employee contributions of up to six percent of each employee’s annual salary based on the jurisdiction the employees are located. The Company paid $0.5 million and $0.6 million in matching contributions for the three months ended March 31, 2021 and 2020, respectively.

Related Party Transactions

Related Party Transactions	3 Months Ended
	Mar. 31, 2021
Related Party Transactions [Abstract]
Related Party Transactions	14. Related Party Transactions GSK In April 2018, the Company completed the GSK Agreement with subsidiaries of GSK to acquire a portfolio of autologous ex vivo As of March 31, 2021, and December 31, 2020, the Company had accounts payable and accrued expenses due to GSK of nil and $0.1 million, respectively. During the three months ended March 31, 2021 and 2020 the Company made payments of $0.1 million and nil, respectively, to settle accounts payable due to GSK. During the three months ended March 31, 2021 and 2020, there were no sales of Strimvelis and the Company incurred no royalties due to GSK.

Basis of Presentation and Sum_2

Basis of Presentation and Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2021
Accounting Policies [Abstract]
Basis of presentation	Basis of presentation The condensed consolidated interim financial statements of the Company are unaudited and have been prepared in accordance with generally accepted accounting principles in the United States of America (“U.S. GAAP”) for interim financial reporting and in accordance with Regulation S-X, Rule 10-01. Accordingly, they do not include all of the information and footnotes required by U.S. GAAP for complete financial statements. Any reference in these notes to applicable guidance is meant to refer to the authoritative U.S. GAAP as found in the Accounting Standards Codification (“ASC”), and Accounting Standards Update (“ASU”), of the Financial Accounting Standards Board (“FASB”). All intercompany accounts and transactions between the Company and its subsidiaries have been eliminated upon consolidation. The accompanying unaudited condensed consolidated interim financial statements should be read in conjunction with the audited consolidated financial statements and accompanying notes included in the Company’s Annual Report on Form 10-K filed with the SEC on March 2, 2021 (the “Annual Report”). The condensed consolidated balance sheet as of December 31, 2020 was derived from audited consolidated financial statements included in the Company’s Annual Report but does not include all disclosures required by U.S. GAAP. Certain information and footnote disclosures normally included in financial statements prepared in accordance with U.S. GAAP have been condensed or omitted from these interim financial statements. However, these interim financial statements include all adjustments, consisting only of normal recurring adjustments, which are, in the opinion of the Company’s management, necessary to fairly state the results of the interim period. The interim results are not necessarily indicative of results to be expected for the full year. 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.
Use of estimates	Use of estimates The preparation of condensed consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the condensed consolidated financial statements and the reported amounts of revenue and expenses during the reporting periods. Significant estimates and assumptions reflected in these condensed consolidated financial statements include, but are not limited to, the accrual for research and development expenses, the research and development tax credit receivable, 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 may also directly or indirectly impact the Company’s business, including impacts due to 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.
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 shareholders’ 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 $1.4 million and losses of $6.8 million for the three months ended March 31, 2021 and 2020, respectively, which is included in other income (expense), net in the condensed consolidated statements of operations and comprehensive loss.
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 acquisition to be cash equivalents.
Marketable debt securities	Marketable debt 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 the statement of operations; 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 condensed consolidated statements of operation.
United Kingdom Research and development tax credits	United Kingdom research and development tax credit 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 undertaken 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, 2021. The Company has qualified under the more favorable SME regime for the year ended December 31, 2020 and expects to qualify under the SME regime for the year ending December 31, 2021. 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 in the condensed consolidated statement of operations and comprehensive loss. The following table below outlines the changes to the research and development tax credit receivable, including amounts recognized as an offset to research and development expense during the period: Three Months Ended March 31, 2021 2020 Balance at beginning of period $ 17,344 $ 28,644 Recognition of credit claims as offset to research and development expense 3,554 3,417 (Receipt) of credit claims — — Foreign currency translation 147 (1,850 ) Balance at end of period $ 21,045 $ 30,211 As of March 31, 2021, the Company’s tax incentive receivable from the UK government was $21.1 million, of which $17.5 million was classified as current and $3.6 million was classified as long-term. As of December 31, 2020, the Company’s tax incentive receivable from the UK government was $17.3 million, all of which was classified as current.
Restricted cash and construction deposits	Restricted cash and construction deposits Cash and cash equivalents that are restricted as to withdrawal or use under the terms of certain contractual agreements are recorded as restricted cash on the Company’s condensed consolidated balance sheet. The Company has an outstanding letter of credit for $3.0 million associated with a lease and is required to hold this amount in a standalone bank account, as of March 31, 2021 and December 31, 2020. The Company is also contractually required to maintain cash collateral accounts associated with corporate credit cards and other leases in the amount of $1.3 million at March 31, 2021 and December 31, 2020. The Company includes the restricted cash balance in cash and cash equivalents when reconciling beginning-of-period and end-of-period total amounts shown on the condensed consolidated statements of cash flows. The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported in the condensed consolidated balance sheet that sum to the total of the amounts reported in the unaudited condensed consolidated statement of cash flows: March 31, December 31, 2021 2020 Cash and cash equivalents $ 78,883 $ 55,135 Restricted cash 4,266 4,266 Total cash, cash equivalents and restricted cash shown in the statement of cash flows $ 83,149 $ 59,401 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 and subleased to a third-party. Subject to the terms of the 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 condensed 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.
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 agreement costs and accruals	Research agreement 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 share-based awards granted to employees , consultants, and directors based on the fair value of the shares and options on the date of the grant and recognizes compensation expense for those awards over the requisite service period, which is the vesting period of the respective award. Forfeitures are accounted for as they occur.
Comprehensive loss	Comprehensive loss Comprehensive loss is composed of net loss and other comprehensive (loss) income. Other comprehensive (loss) income 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 (as detailed in Note 10), the Company is required to maintain commercial availability of Strimvelis in the European Union until such time that an alternative gene therapy is available. Strimvelis is not currently expected to generate sufficient cash flows to overcome the costs of maintaining the product and certain regulatory commitments; therefore, the Company initially recorded a liability associated with the loss contract of $18.4 million in 2018. The Company recognizes the amortization of the loss provision on a diminishing balance basis based on the actual net loss incurred associated with Strimvelis and the expected future net losses to be generated until such time as Strimvelis is no longer commercially available. The amortization of the provision is recorded as a reduction in research and development expense. The Company has 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 will continue to evaluate its future estimates for amortization of the Strimvelis loss provision Three Months Ended March 31, 2021 2020 Balance at beginning of period $ 4,482 $ 6,790 Amortization of loss provision (446 ) (1,691 ) Foreign currency translation 40 (348 ) Balance at end of period $ 4,076 $ 4,751 Of the balance as of March 31, 2021 noted in the table above, $1.1 million is classified as current, and $3.0 million is classified as non-current.
Net loss per share	Net loss per share Basic net loss per share is computed by dividing the net loss by the weighted average number of voting and non-voting ordinary shares outstanding for the period. Diluted net loss is computed by adjusting net loss based on the potential impact of dilutive securities. Diluted net loss per share is computed by dividing the diluted net 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: Three Months Ended March 31, 2021 2020 Share options 12,935,554 12,688,361 Unvested performance-based restricted share units 717,167 511,324 13,652,721 13,199,685
Recently adopted accounting pronouncements	Recently adopted accounting pronouncements In December 2019, the FASB issued ASU No. 2019-12, Simplifying the Accounting for Income Taxes (Topic 740) Income Taxes

Basis of Presentation and Sum_3

Basis of Presentation and Summary of Significant Accounting Policies (Tables)	3 Months Ended
	Mar. 31, 2021
Accounting Policies [Abstract]
Summary of Amounts Recognized to Offset Research and Development Expense	The following table below outlines the changes to the research and development tax credit receivable, including amounts recognized as an offset to research and development expense during the period: Three Months Ended March 31, 2021 2020 Balance at beginning of period $ 17,344 $ 28,644 Recognition of credit claims as offset to research and development expense 3,554 3,417 (Receipt) of credit claims — — Foreign currency translation 147 (1,850 ) Balance at end of period $ 21,045 $ 30,211
Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash	The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported in the condensed consolidated balance sheet that sum to the total of the amounts reported in the unaudited condensed consolidated statement of cash flows: March 31, December 31, 2021 2020 Cash and cash equivalents $ 78,883 $ 55,135 Restricted cash 4,266 4,266 Total cash, cash equivalents and restricted cash shown in the statement of cash flows $ 83,149 $ 59,401
Schedule of Changes in Loss Provision	The following table below outlines the changes to the Strimvelis loss provision for the periods ended March 31, 2021 and 2020: Three Months Ended March 31, 2021 2020 Balance at beginning of period $ 4,482 $ 6,790 Amortization of loss provision (446 ) (1,691 ) Foreign currency translation 40 (348 ) Balance at end of period $ 4,076 $ 4,751
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: Three Months Ended March 31, 2021 2020 Share options 12,935,554 12,688,361 Unvested performance-based restricted share units 717,167 511,324 13,652,721 13,199,685

Fair Value Measurements and M_2

Fair Value Measurements and Marketable Debt Securities (Tables)	3 Months Ended
	Mar. 31, 2021
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract]
Schedule of Cash Equivalents and Marketable Debt Securities	The following table summarizes the Company’s cash equivalents and marketable debt securities as of March 31, 2021: Fair Value Measurements at March 31, 2021 Using: Level 1 Level 2 Level 3 Total Cash equivalents Money market funds $ 26,091 $ — $ — $ 26,091 Corporate bonds — $ 5,174 — 5,174 Commercial paper — 23,854 — 23,854 Total cash equivalents $ 26,091 $ 29,028 $ — $ 55,119 Marketable securities Corporate bonds $ — $ 96,879 $ — $ 96,879 Commercial paper — 122,664 — $ 122,664 Total marketable securities $ — $ 219,543 $ — $ 219,543 Total $ 26,091 $ 248,571 $ — $ 274,662 The following table summarizes the Company’s cash equivalents and marketable debt securities as of December 31, 2020: Fair Value Measurements at 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 U.S. 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
Schedule of Marketable Debt Securities	The following table summarizes the Company’s marketable debt securities as of March 31, 2021: At March 31, 2021 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Credit Losses Fair Value Corporate bonds $ 102,030 $ 68 $ (45 ) $ — $ 102,053 Commercial paper 146,551 2 (35 ) — 146,518 Total $ 248,581 $ 70 $ (80 ) $ — $ 248,571 The following table summarizes the Company’s marketable securities as of December 31, 2020: At December 31, 2020 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
Schedule of Available-for-Sale Debt Securities by Contractual Maturity	The following table summarizes the Company’s available-for-sale debt securities by contractual maturity, as of March 31, 2021 and December 31, 2020: At March 31, 2021 At December 31, 2020 Due in one year $ 242,579 $ 132,056 Due after one year through three years 5,992 10,757 Total $ 248,571 $ 142,813

Prepaid Expenses and Other Cu_2

Prepaid Expenses and Other Current Assets (Tables)	3 Months Ended
	Mar. 31, 2021
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: March 31, December 31, 2021 2020 Prepaid external research and development expenses $ 2,132 $ 1,421 Inventories 1,180 665 Other prepayments 4,634 4,930 VAT receivable 894 2,780 Construction deposit - current 1,634 1,552 Non-trade receivables 2,030 2,017 Total prepaid expenses and other current assets $ 12,504 $ 13,365

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	3 Months Ended
	Mar. 31, 2021
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment, Net	Property and equipment, net consisted of the following: March 31, December 31, 2021 2020 Property and equipment: Lab equipment $ 5,159 $ 5,114 Leasehold improvements 2,486 2,522 Furniture and fixtures 304 304 Office and computer equipment 765 763 Construction-in-process 601 302 Property and equipment $ 9,315 $ 9,005 Less: accumulated depreciation (4,724 ) (4,224 ) Property and equipment, net $ 4,591 $ 4,781

Other Assets (Tables)

Other Assets (Tables)	3 Months Ended
	Mar. 31, 2021
Other Assets Noncurrent Disclosure [Abstract]
Schedule of Other Assets	Other assets consist of the following: March 31, December 31, 2021 2020 Intangible assets - license milestones $ 4,914 $ 3,076 Deferred tax assets 4,449 5,219 Deposits 1,019 1,144 Deferred financing costs 647 975 Other non-current assets 2,061 1,554 Construction deposits - long-term 6,491 6,572 Total other assets $ 19,581 $ 18,540

Accrued Expenses and Other Li_2

Accrued Expenses and Other Liabilities (Tables)	3 Months Ended
	Mar. 31, 2021
Accrued Liabilities And Other Liabilities [Abstract]
Schedule of Accrued Expenses and Other Liabilities	Accrued expenses and other liabilities consisted of the following: March 31, December 31, 2021 2020 Accrued external research and development expenses $ 8,210 $ 8,878 Accrued payroll and related expenses 7,084 11,881 Accrued professional fees 758 791 Accrued other 3,616 3,401 Accrued milestone payments 4,914 3,076 Strimvelis loss provision - current portion 1,090 916 Total accrued expenses and other liabilities $ 25,672 $ 28,943

Notes Payable (Tables)

Notes Payable (Tables)	3 Months Ended
	Mar. 31, 2021
Debt Disclosure [Abstract]
Summary of Notes Payable	Notes payable consist of the following: At March 31, At December 31, 2021 2020 Notes payable, net of issuance costs $ 24,687 $ 24,659 Less current portion (6,944 ) (4,861 ) Notes payable, net of current portion 17,743 19,798 Accretion related to final payment 465 406 Notes payable, long term $ 18,208 $ 20,204
Summary of Estimated Future Principal Payments Due	As of March 31, 2021, the estimated future principal payments due are as follows: Aggregate Minimum Payments 2021 (April - December) 4,861 2022 8,333 2023 8,334 2024 4,597 2025 — Thereafter — Total 26,125 Less: current portion (6,944 ) Less: unamortized portion of final payment (660 ) Less: unamortized debt issuance costs (313 ) Notes payable, long term $ 18,208

Share-Based Compensation (Table

Share-Based Compensation (Tables)	3 Months Ended
	Mar. 31, 2021
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Option Activity	The following table summarizes option activity under the plans for three months ended March 31, 2021: Number of Options Weighted Average Exercise Price Outstanding at December 31, 2020 13,895,643 $ 7.96 Granted 4,506,256 5.97 Exercised (1,319,493 ) 2.06 Forfeited (1,192,274 ) 11.16 Outstanding at March 31, 2021 15,890,132 $ 7.65 Vested and expected to vest, as of March 31, 2021 15,890,132 $ 7.65 Exercisable, March 31, 2021 6,116,939 $ 6.73
Summary of Award Activity	The following table summarizes award activity for the three months ended March 31, 2021: Performance-based RSUs Time-based RSUs Total RSUs Weighted Average Grant Date Fair Value Unvested and outstanding at December 31, 2020 464,000 180,000 644,000 $ 8.75 Granted — 20,000 20,000 5.98 Vested (89,667 ) — (89,667 ) 9.40 Forfeited (34,500 ) (40,000 ) (74,500 ) 11.80 Unvested and outstanding at March 31, 2021 339,833 160,000 499,833 $ 7.99
Share-based Compensation Expense	Share-based compensation expense recorded as research and development and general and administrative expenses is as follows: March 31, March 31, 2021 2020 Research and development $ 2,876 $ 3,110 General and administrative 3,392 6,369 Total share-based compensation $ 6,268 $ 9,479

Commitments and Contingencies (

Commitments and Contingencies (Tables)	3 Months Ended
	Mar. 31, 2021
Commitments And Contingencies Disclosure [Abstract]
Schedule of Annual Commitments Associated with the Contract	The following table outlines the current commitments associated with the agreement, as of March 31, 2021: Due in: Product manufacturing commitments Dedicated manufacturing and development resources Exclusive transduction suites Total remaining AGC commitment 2021 (April - December) $ 2,379 $ 7,796 $ — $ 10,175 2022 3,173 8,143 3,202 14,518 2023 3,173 8,143 3,202 14,518 2024 3,173 8,143 3,202 14,518 2025 1,586 4,072 1,601 7,259 Total manufacturing commitments 13,484 36,297 11,207 60,988 *Tabular disclosure above has been translated to U.S. Dollar, from Euro, using an exchange rate of €1.00 to $1.175.

Nature of the Business - Additi

Nature of the Business - Additional Information (Details) $ / shares in Units, $ in Thousands	Feb. 09, 2021USD ($)$ / sharesshares	Mar. 31, 2021USD ($)	Mar. 31, 2021£ / shares	Dec. 31, 2020USD ($)	Dec. 31, 2020£ / shares
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Nominal value of shares issued and sold | £ / shares			£ 0.10		£ 0.10
Placement agent fees		$ 6,092
Accumulated deficit		640,822		$ 605,640
Cash, cash equivalents and marketable securities		$ 298,400
Securities Purchase Agreement
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Net proceeds from private placement	$ 143,700
Securities Purchase Agreement | Private Placement
Organization Consolidation And Presentation Of Financial Statements [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
Placement agent fees	$ 6,000
Other issuance costs	$ 300
Private placement, transaction date	Feb. 4, 2021
Securities Purchase Agreement | Private Placement | Non-voting Ordinary Shares
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of ordinary shares issued and sold | shares	3,215,434
Nominal value of shares issued and sold | $ / shares	$ 0.10

Basis of Presentation and Sum_4

Basis of Presentation and Summary of Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2018
Summary Of Significant Accounting Policies [Line Items]
Foreign currency transaction gain (loss)	$ 1,400	$ (6,800)
Tax incentive receivable, current	17,493		$ 17,344
Tax incentive receivable, long term	3,552
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	1,090		916
Strimvelis loss provision non-current	$ 3,000
Change in accounting principle, accounting standards update, adopted	true
Change in accounting principle, accounting standards update, adoption date	Jan. 1, 2021
Change in accounting principle, accounting standards update, immaterial effect	true
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201912Member
Fremont Lease Agreement [Member]
Summary Of Significant Accounting Policies [Line Items]
Escrow deposit	$ 8,100	$ 10,000
Increase decrease in escrow deposit	1,900
Fremont Lease Agreement [Member] | Prepaid Expenses and Other Current Assets [Member]
Summary Of Significant Accounting Policies [Line Items]
Escrow deposit	1,600
Fremont Lease Agreement [Member] | Other Assets [Member]
Summary Of Significant Accounting Policies [Line Items]
Escrow deposit	6,500
United Kingdom
Summary Of Significant Accounting Policies [Line Items]
Tax incentive receivable	21,100
Tax incentive receivable, current	17,500		$ 17,300
Tax incentive receivable, long term	$ 3,600

Basis of Presentation and Sum_5

Basis of Presentation and Summary of Significant Accounting Policies - Summary of Amounts recognized to Offset Research and Development Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Summary Of Significant Accounting Policies [Line Items]
Foreign currency transaction gain (loss)	$ 1,400	$ (6,800)
United Kingdom | Research and Development Expense
Summary Of Significant Accounting Policies [Line Items]
Balance at beginning of period	17,344	28,644
Recognition of credit claims as offset to research and development expense	3,554	3,417
Foreign currency transaction gain (loss)	147	(1,850)
Balance at end of period	$ 21,045	$ 30,211

Basis of Presentation and Sum_6

Basis of Presentation and Summary of Significant Accounting Policies - Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Summary Of Significant Accounting Policies [Abstract]
Cash and cash equivalents	$ 78,883	$ 55,135
Restricted cash	4,266	4,266
Total cash, cash equivalents and restricted cash shown in the statement of cash flows	$ 83,149	$ 59,401	$ 31,364	$ 23,317

Basis of Presentation and Sum_7

Basis of Presentation and Summary of Significant Accounting Policies - Schedule of Changes in Loss Provision (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Summary Of Significant Accounting Policies [Abstract]
Balance at beginning of period	$ 4,482	$ 6,790
Amortization of loss provision	(446)	(1,691)
Foreign currency translation	40	(348)
Balance at end of period	$ 4,076	$ 4,751

Basis of Presentation and Sum_8

Basis of Presentation and Summary of Significant Accounting Policies - Securities Excluded in the Computation of Diluted Net Loss Per Ordinary Share (Details) - shares	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common shares attributable to anti-dilutive shares	13,652,721	13,199,685
Share options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common shares attributable to anti-dilutive shares	12,935,554	12,688,361
Unvested Performance-based Restricted Share Units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common shares attributable to anti-dilutive shares	717,167	511,324

Fair Value Measurements and M_3

Fair Value Measurements and Marketable Debt Securities - Additional Information (Details) $ in Thousands	Mar. 31, 2021USD ($)
Fair Value Disclosures [Abstract]
Fair value of financial assets transfers between level 1 to level 2	$ 0
Fair value of financial assets transfers between level 2 to level 1	$ 0

Fair Value Measurements and M_4

Fair Value Measurements and Marketable Debt Securities - Schedule of Cash Equivalents and Marketable Debt Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Cash equivalents
Total cash equivalents	$ 55,119	$ 12,650
Total cash equivalents	55,119	12,650
Marketable securities
Total marketable securities	219,543	136,813
Total	274,662	149,463
Level 1
Cash equivalents
Total cash equivalents	26,091	6,650
Total cash equivalents	26,091	6,650
Marketable securities
Total	26,091	6,650
Level 2
Cash equivalents
Total cash equivalents	29,028	6,000
Total cash equivalents	29,028	6,000
Marketable securities
Total marketable securities	219,543	136,813
Total	248,571	142,813
Money Market Funds
Cash equivalents
Total cash equivalents	26,091	6,650
Total cash equivalents	26,091	6,650
Money Market Funds | Level 1
Cash equivalents
Total cash equivalents	26,091	6,650
Total cash equivalents	26,091	6,650
Corporate Bonds
Cash equivalents
Total cash equivalents	5,174	3,001
Total cash equivalents	5,174	3,001
Marketable securities
Total marketable securities	96,879	93,358
Corporate Bonds | Level 2
Cash equivalents
Total cash equivalents	5,174	3,001
Total cash equivalents	5,174	3,001
Marketable securities
Total marketable securities	96,879	93,358
Commercial Paper
Cash equivalents
Total cash equivalents	23,854	2,999
Total cash equivalents	23,854	2,999
Marketable securities
Total marketable securities	122,664	40,458
Commercial Paper | Level 2
Cash equivalents
Total cash equivalents	23,854	2,999
Total cash equivalents	23,854	2,999
Marketable securities
Total marketable securities	$ 122,664	40,458
U.S. Government Securities
Marketable securities
Total marketable securities		2,997
U.S. Government Securities | Level 2
Marketable securities
Total marketable securities		$ 2,997

Fair Value Measurements and M_5

Fair Value Measurements and Marketable Debt Securities - Schedule of Marketable Debt Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 248,581	$ 142,728
Gross Unrealized Gains	70	134
Gross Unrealized Losses	(80)	(49)
Fair Value	248,571	142,813
U.S. Government Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost		3,000
Gross Unrealized Losses		(4)
Fair Value		2,996
Corporate Bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	102,030	96,259
Gross Unrealized Gains	68	133
Gross Unrealized Losses	(45)	(32)
Fair Value	102,053	96,360
Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	146,551	43,469
Gross Unrealized Gains	2	1
Gross Unrealized Losses	(35)	(13)
Fair Value	$ 146,518	$ 43,457

Fair Value Measurements and M_6

Fair Value Measurements and Marketable Debt Securities - Schedule of Available-for-Sale Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Due in one year	$ 242,579	$ 132,056
Due after one year through three years	5,992	10,757
Total	$ 248,571	$ 142,813

Prepaid Expenses and Other Cu_3

Prepaid Expenses and Other Current Assets - Schedule of Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Prepaid Expense And Other Assets Current [Abstract]
Prepaid external research and development expenses	$ 2,132	$ 1,421
Inventories	1,180	665
Other prepayments	4,634	4,930
VAT receivable	894	2,780
Construction deposit - current	1,634	1,552
Non-trade receivables	2,030	2,017
Total prepaid expenses and other current assets	$ 12,504	$ 13,365

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Property Plant And Equipment [Line Items]
Property and equipment	$ 9,315	$ 9,005
Less: accumulated depreciation	(4,724)	(4,224)
Property and equipment, net	4,591	4,781
Lab Equipment
Property Plant And Equipment [Line Items]
Property and equipment	5,159	5,114
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment	2,486	2,522
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment	304	304
Office and Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment	765	763
Construction-In-Process
Property Plant And Equipment [Line Items]
Property and equipment	$ 601	$ 302

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Property Plant And Equipment [Abstract]
Depreciation expense	$ 495	$ 482

Other Assets - Schedule of Othe

Other Assets - Schedule of Other Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Other Assets Noncurrent Disclosure [Abstract]
Intangible assets - license milestones	$ 4,914	$ 3,076
Deferred tax assets	4,449	5,219
Deposits	1,019	1,144
Deferred financing costs	647	975
Other non-current assets	2,061	1,554
Construction deposits - long-term	6,491	6,572
Total other assets	$ 19,581	$ 18,540

Accrued Expenses and Other Li_3

Accrued Expenses and Other Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Accrued Liabilities And Other Liabilities [Abstract]
Accrued external research and development expenses	$ 8,210	$ 8,878
Accrued payroll and related expenses	7,084	11,881
Accrued professional fees	758	791
Accrued other	3,616	3,401
Accrued milestone payments	4,914	3,076
Strimvelis loss provision - current portion	1,090	916
Total accrued expenses and other liabilities	$ 25,672	$ 28,943

Accrued Expenses and Other Li_4

Accrued Expenses and Other Liabilities - Additional Information (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Accrued Liabilities And Other Liabilities [Line Items]
Accrued milestone payments	$ 4,914	$ 3,076
license Intangibles
Accrued Liabilities And Other Liabilities [Line Items]
Accrued milestone payments	$ 1,800		$ 0

Notes Payable - Additional Info

Notes Payable - Additional Information (Details) - USD ($)	1 Months Ended	3 Months Ended		12 Months Ended
	May 31, 2019	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
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 for all payment dates prior to 36 months following the date of the Credit Facility. The term loans under the Credit Facility will begin amortizing on either the 24-month or the 36-month anniversary of the Credit Facility (as applicable), with equal monthly payments of principal plus interest to be made by the Borrower to the Lenders in consecutive monthly installments until the Loan Maturity Date. In addition, a final payment of 4.5% is due on the Loan Maturity Date.
Line of credit facility covenants description		The Company is also subject to an ongoing minimum cash financial covenant in which the Company must maintain unrestricted cash in an amount not less than $20.0 million following the utilization of the second term loan and not less than $35.0 million following the utilization of the third term loan
Term Loan
Debt Instrument [Line Items]
Debt instrument, interest expense		$ 500,000	$ 600,000
Debt instrument, effective annual interest rate		8.60%
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 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 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

Notes Payable - Summary of Note

Notes Payable - Summary of Notes Payable (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Long Term Notes And Loans [Abstract]
Notes payable, net of issuance costs	$ 24,687	$ 24,659
Less current portion	(6,944)	(4,861)
Notes payable, net of current portion	17,743	19,798
Accretion related to final payment	465	406
Notes payable, long term	$ 18,208	$ 20,204

Notes Payable - Summary of Esti

Notes Payable - Summary of Estimated Future Principal Payments Due (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Long Term Debt By Maturity [Abstract]
2021 (April - December)	$ 4,861
2022	8,333
2023	8,334
2024	4,597
Total	26,125
Less: current portion	(6,944)
Less: unamortized portion of final payment	(660)
Less: unamortized debt issuance costs	(313)
Notes payable, long term	$ 18,208	$ 20,204

Share-Based Compensation - Addi

Share-Based Compensation - Additional Information (Details) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended
	Apr. 30, 2020USD ($)shares	Mar. 31, 2021USD ($)Regulatory$ / sharesshares	Dec. 31, 2020USD ($)	Mar. 31, 2020USD ($)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted-average grant date fair value of share options granted | $ / shares		$ 3.87
Number of shares granted		20,000
RSUs vested		89,667
Compensation cost recognized | $		$ 6,268		$ 9,479
Total unrecognized compensation cost of stock option and time-based RSUs | $		$ 53,500
Compensation cost expected to be recognized weighted average period		2 years 10 months 24 days
Total unrecognized compensation cost of performance-based RSUs | $		$ 3,500
Selling, General and Administrative Expenses
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Compensation cost recognized | $		$ 2,700
Performance-based Restricted Share Units ("RSUs")
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
RSUs vested		89,667
Award vesting period		1 year 3 months 18 days
Maximum aggregate total fair value of outstanding RSUs | $		$ 3,500
Number of Regulatory | Regulatory		2
Performance-based Restricted Share Units ("RSUs") | Chief Executive Officer
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares granted	195,000
Total grant date fair value | $	$ 1,400
RSUs vested		0
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]
Number of shares granted		20,000
Award vesting period		3 years
2018 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Remaining shares available for issuance		3,542,397
Inducement Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Remaining shares available for issuance		1,000,000
2018 ESPP
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Remaining shares available for issuance		1,470,104

Share-Based Compensation - Summ

Share-Based Compensation - Summary of Option Activity (Details)	3 Months Ended
	Mar. 31, 2021$ / sharesshares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Options, Outstanding at December 31, 2020 | shares	13,895,643
Number of Options, Granted | shares	4,506,256
Number of Options, Exercised | shares	(1,319,493)
Number of Options, Forfeited | shares	(1,192,274)
Number of Options, Outstanding at March 31, 2021 | shares	15,890,132
Number of Options, Vested and expected to vest, as of March 31, 2021 | shares	15,890,132
Number of Options, Exercisable, March 31, 2021 | shares	6,116,939
Weighted Average Exercise Price, Outstanding at December 31, 2020 | $ / shares	$ 7.96
Weighted Average Exercise Price, Granted | $ / shares	5.97
Weighted Average Exercise Price, Exercised | $ / shares	2.06
Weighted Average Exercise Price, Forfeited | $ / shares	11.16
Weighted Average Exercise Price, Outstanding at March 31, 2021 | $ / shares	7.65
Weighted Average Exercise Price, Vested and expected to vest, as of March 31, 2021 | $ / shares	7.65
Weighted Average Exercise Price, Exercisable, March 31, 2021 | $ / shares	$ 6.73

Share-Based Compensation - Su_2

Share-Based Compensation - Summary of Award Activity (Details)	3 Months Ended
	Mar. 31, 2021$ / sharesshares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
RSUs, Unvested and outstanding at December 31, 2020	644,000
RSUs,, Granted	20,000
RSUs, Vested	(89,667)
RSUs, Forfeited	(74,500)
RSUs, Unvested and outstanding at March 31, 2021	499,833
Weighted Average Grant Date Fair Value, Unvested and outstanding at December 31, 2020 | $ / shares	$ 8.75
Weighted Average Grant Date Fair Value, Granted | $ / shares	5.98
Weighted Average Grant Date Fair Value, Vested | $ / shares	9.40
Weighted Average Grant Date Fair Value, Forfeited | $ / shares	11.80
Weighted Average Grant Date Fair Value, Unvested and outstanding at March 31, 2021 | $ / shares	$ 7.99
Performance-based RSUs
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
RSUs, Unvested and outstanding at December 31, 2020	464,000
RSUs, Vested	(89,667)
RSUs, Forfeited	(34,500)
RSUs, Unvested and outstanding at March 31, 2021	339,833
Time-based RSUs
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
RSUs, Unvested and outstanding at December 31, 2020	180,000
RSUs,, Granted	20,000
RSUs, Forfeited	(40,000)
RSUs, Unvested and outstanding at March 31, 2021	160,000

Share-Based Compensation - Shar

Share-Based Compensation - Share-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation	$ 6,268	$ 9,479
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation	2,876	3,110
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation	$ 3,392	$ 6,369

License Agreements - Additional

License Agreements - Additional Information (Details) € in Millions, £ in Millions, $ in Millions

1 Months Ended

3 Months Ended

Apr. 30, 2020USD ($)shares

May 31, 2019USD ($)

May 31, 2019EUR (€)

Apr. 30, 2018Program

Mar. 31, 2021USD ($)shares

Mar. 31, 2020Milestone

Jun. 30, 2018USD ($)

Mar. 31, 2021GBP (£)shares

Mar. 31, 2021EUR (€)shares

Dec. 31, 2020shares

Nov. 30, 2018shares

Sep. 30, 2018shares

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Ordinary Shares, Issued | shares

120,549,163

120,549,163

120,549,163

98,283,603

GSK Asset Purchase and License Agreement

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Number of additional programs in preclinical development

3

Number of late-stage clinical gene therapy programs

2

Number of earlier stage clinical gene therapy program

1

Number of additional earlier-stage development programs

3

Total consideration | $

$ 133.6

Payment of tiered royalty, maximum percentage

20.00%

20.00%

20.00%

Milestone payments payable upon achievement of certain sales milestones | £

£ 90

Telethon-OSR License Agreements

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Milestone payments payable upon achievement of certain sales milestones

$ 36.2

€ 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 (“MPS-IH”). Under the terms of the agreement, Telethon-OSR received €15.0 million in upfront and milestone payments from the Company upon entering into the agreement, resulting in $17.2 million in in-process research and development expense.

Upfront and milestone payments | €

€ 15

In-process research and development expense for upfront and milestone payments | $

$ 17.2

Milestone payments payable upon achievement of certain development regulatory and commercial milestones

$ 32.8

€ 28

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

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 | $

$ 0.8

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Income Tax Disclosure [Abstract]
Income tax expense (benefit)	$ 1,087	$ (335)

Commitments and Contingencies -

Commitments and Contingencies - Schedule of Annual Commitments Associated with the Contract (Details) $ in Thousands	Mar. 31, 2021USD ($)
Other Commitments [Line Items]
2021 (April - December)	$ 10,175
2022	14,518
2023	14,518
2024	14,518
2025	7,259
Total manufacturing commitments	60,988
Product Manufacturing Commitments
Other Commitments [Line Items]
2021 (April - December)	2,379
2022	3,173
2023	3,173
2024	3,173
2025	1,586
Total manufacturing commitments	13,484
Dedicated Manufacturing And Development Resources
Other Commitments [Line Items]
2021 (April - December)	7,796
2022	8,143
2023	8,143
2024	8,143
2025	4,072
Total manufacturing commitments	36,297
Exclusive Transduction Suites
Other Commitments [Line Items]
2022	3,202
2023	3,202
2024	3,202
2025	1,601
Total manufacturing commitments	$ 11,207

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Annual Commitments Associated with the Contract (Parenthetical) (Details) - Mar. 31, 2021	USD ($)	EUR (€)
Commitments And Contingencies Disclosure [Abstract]
Annual Commitments Exchange Rate	$ 1.175	€ 1

Employee Benefit Plans - Additi

Employee Benefit Plans - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Compensation And Retirement Disclosure [Abstract]
Maximum annual contribution employer matches per employee, percent	6.00%
Contribution expenses	$ 0.5	$ 0.6

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Details) $ in Thousands	1 Months Ended	3 Months Ended
	Apr. 30, 2018Program	Mar. 31, 2021USD ($)	Mar. 31, 2020USD ($)	Dec. 31, 2020USD ($)
GSK Asset Purchase and License Agreement
Related Party Transaction [Line Items]
Number of additional programs in preclinical development | Program	3
GSK
Related Party Transaction [Line Items]
Due to related party transactions, accounts payable				$ 100
Due to related party transactions, accrued expenses				$ 100
Payments to settle accounts payable		$ 100
GSK | Strimvelis
Related Party Transaction [Line Items]
Sales of strimvelis		0	$ 0
Royalty expense		$ 0	$ 0