Orchard Therapeutics (ORTX) 10-Q 4 Nov 21 2021 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2021	Oct. 31, 2021
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
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		125,498,931
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
Entity Bankruptcy Proceedings, Reporting Current	true

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 74,324	$ 55,135
Marketable securities	179,819	136,813
Accounts receivable	1,073	878
Prepaid expenses and other current assets	17,962	13,365
Research and development tax credit receivable, current	18,284	17,344
Total current assets	291,462	223,535
Non-current assets:
Operating lease right-of-use-assets	26,267	29,815
Property and equipment, net	4,756	4,781
Research and development tax credit receivable	11,942
Restricted cash	4,266	4,266
Intangible assets, net	4,740	3,076
Other assets	12,224	15,464
Total assets	355,657	280,937
Current liabilities:
Accounts payable	6,096	8,823
Accrued expenses and other current liabilities	28,500	28,943
Deferred revenue	1,392
Operating lease liabilities	8,257	8,934
Notes payable, current		4,861
Total current liabilities	44,245	51,561
Notes payable, long-term	32,786	20,204
Deferred revenue, net of current portion	11,502
Operating lease liabilities, net of current portion	19,800	24,168
Other long-term liabilities	6,026	6,570
Total liabilities	114,359	102,503
Commitments and contingencies (see Note 13)
Shareholders’ equity:
Ordinary shares, £0.10 nominal value; 124,770,519 and 98,283,603 ordinary shares issued and outstanding at September 30, 2021 and December 31, 2020, respectively; 715,434 and nil non-voting ordinary shares issued and outstanding at September 30, 2021 and December 31, 2020, respectively	16,229	12,507
Additional paid-in capital	935,211	771,194
Accumulated other comprehensive income	3,734	373
Accumulated deficit	(713,876)	(605,640)
Total shareholders’ equity	241,298	178,434
Total liabilities and shareholders’ equity	$ 355,657	$ 280,937

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - £ / shares	Sep. 30, 2021	Dec. 31, 2020
Ordinary Shares, Nominal Value	£ 0.10	£ 0.10
Ordinary Shares, Issued	124,770,519	98,283,603
Ordinary Shares, Outstanding	124,770,519	98,283,603
Non-voting Ordinary Shares
Ordinary Shares, Issued	715,434
Ordinary Shares, Outstanding	715,434

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Total revenues	$ 1,192,000	$ 1,998,000	$ 1,192,000	$ 2,595,000
Costs and operating expenses:
Cost of product sales	$ 226,000	$ 667,000	$ 226,000	$ 858,000
Cost, Product and Service [Extensible List]	Product Sales, Net [Member]	Product Sales, Net [Member]	Product Sales, Net [Member]	Product Sales, Net [Member]
Research and development	$ 20,846,000	$ 14,678,000	$ 63,631,000	$ 71,082,000
Selling, general and administrative	13,039,000	12,956,000	41,353,000	48,760,000
Total costs and operating expenses	34,111,000	28,301,000	105,210,000	120,700,000
Loss from operations	(32,919,000)	(26,303,000)	(104,018,000)	(118,105,000)
Other (expense) income:
Interest income	65,000	534,000	349,000	2,906,000
Interest expense	(683,000)	(572,000)	(1,814,000)	(1,753,000)
Other (expense) income, net	(4,041,000)	5,510,000	(2,049,000)	(2,223,000)
Total other (expense) income, net	(4,659,000)	5,472,000	(3,514,000)	(1,070,000)
Net loss before income tax	(37,578,000)	(20,831,000)	(107,532,000)	(119,175,000)
Income tax benefit (expense)	1,133,000	541,000	(704,000)	816,000
Net loss	(36,445,000)	(20,290,000)	(108,236,000)	(118,359,000)
Other comprehensive income (loss)
Foreign currency translation adjustment	3,826,000	(5,310,000)	3,388,000	1,334,000
Unrealized gain (loss) on marketable securities	94,000	(308,000)	(27,000)	56,000
Total other comprehensive income (loss)	3,920,000	(5,618,000)	3,361,000	1,390,000
Total comprehensive loss	$ (32,525,000)	$ (25,908,000)	$ (104,875,000)	$ (116,969,000)
Net loss per share, basic and diluted	$ (290)	$ (200)	$ (880)	$ (1,190)
Weighted average number of ordinary shares outstanding, basic and diluted	127,376,562	99,664,616	122,765,516	99,255,370
Product Sales, Net [Member]
Total revenues	$ 700,000	$ 1,998,000	$ 700,000	$ 2,595,000
Collaboration Revenue [Member]
Total revenues	$ 492,000		$ 492,000

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Cash flows from operating activities:
Net loss	$ (108,236)	$ (118,359)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	1,651	1,558
Non-cash share-based compensation	17,198	21,221
Impairment of long-lived assets	5,650	5,650
Non-cash interest expense	297	378
Non-cash consideration for licenses		791
Amortization of Strimvelis loss provision	(898)	(2,018)
Amortization of premium on marketable securities	1,162	465
Deferred income taxes	758	(1,458)
Unrealized foreign currency	10,277	650
Changes in operating assets and liabilities:
Accounts receivable	(213)	(1,750)
Research and development tax credit receivable	(13,498)	(2,423)
Prepaid expenses, other current assets and other assets	(2,452)	(1,222)
Operating leases, right-of-use assets	3,725	4,253
Accounts payable, accrued expenses and other current liabilities	(9,473)	(16,166)
Deferred revenue	13,725
Operating lease liabilities	(5,724)	(6,010)
Net cash used in operating activities	(91,701)	(114,440)
Cash flows from investing activities:
Proceeds from sales and maturities of marketable securities	198,013	221,538
Purchases of marketable securities	(242,315)	(76,203)
Payment of construction deposit		(10,000)
Receipt of funds from construction deposit	216	1,876
Payments on intangible assets	887
Purchases of property and equipment	(1,955)	(2,551)
Net cash (used in) provided by investing activities	(46,928)	134,660
Cash flows from financing activities:
Proceeds from modification of credit facility, net of debt issuance costs paid	7,375
Proceeds from employee equity plans	2,761	2,831
Proceeds from issuance of shares as part of collaboration agreement	4,135
Proceeds from the issuance of ordinary shares in private placement	150,000
Payment of placement agent fees and offering costs	(6,355)
Net cash provided by financing activities	157,916	2,831
Effect of exchange rate changes on cash, cash equivalents, and restricted cash	(98)	(941)
Net increase in cash, cash equivalents and restricted cash	19,189	22,110
Cash, cash equivalents, and restricted cash, beginning of period	59,401	23,317
Cash, cash equivalents, and restricted cash, end of period	78,590	45,427
Supplemental disclosure of non-cash operating, investing and financing activities
Property, equipment and intangibles in accounts payable and accrued expenses	2,870	15
Lease assets obtained in exchange for new operating lease liabilities	350	17,486
Shares issued as part of license agreement		791
Supplemental disclosure of cash flow information:
Cash paid for taxes	1,651	2,375
Cash paid for interest	$ 1,517	$ 1,374

Condensed Consolidated Statem_3

Condensed Consolidated Statements Shareholders' 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	6,034			6,034
Unrealized gain (loss) on marketable 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, 2019	299,193	$ 12,331	738,481	2,042	(453,661)
Balance, Shares at Dec. 31, 2019		96,923,729
Foreign currency translation	1,334
Net loss	(118,359)
Balance at Sep. 30, 2020	207,067	$ 12,426	763,229	3,432	(572,020)
Balance, Shares at Sep. 30, 2020		97,659,771
Balance at Mar. 31, 2020	264,554	$ 12,361	749,368	7,055	(504,230)
Balance, Shares at Mar. 31, 2020		97,154,565
Share-based compensation expense	6,079		6,079
Exercise of share options and issuance of ESPP shares	1,102	$ 34	1,068
Exercise of share options and issuance of ESPP shares, shares		267,761
Ordinary shares issued as part of license agreements	791	$ 10	781
Ordinary shares issued as part of license agreements, Shares		75,413
Foreign currency translation	610			610
Unrealized gain (loss) on marketable securities	1,385			1,385
Net loss	(47,500)				(47,500)
Balance at Jun. 30, 2020	227,021	$ 12,405	757,296	9,050	(551,730)
Balance, Shares at Jun. 30, 2020		97,497,739
Share-based compensation expense	5,663		5,663
Exercise of share options	291	$ 21	270
Exercise of share options, Shares		162,032
Foreign currency translation	(5,310)			(5,310)
Unrealized gain (loss) on marketable securities	(308)			(308)
Net loss	(20,290)				(20,290)
Balance at Sep. 30, 2020	207,067	$ 12,426	763,229	3,432	(572,020)
Balance, Shares at Sep. 30, 2020		97,659,771
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
Vesting of restricted share units, net of shares withheld for taxes	(296)	$ 6	(302)
Vesting of restricted share units, net of shares withheld for taxes, shares		45,746
Sale of voting and non-voting ordinary shares, net of issuance costs	143,711	$ 3,310	140,401
Sale of voting and non-voting ordinary shares, net of issuance costs, Shares		24,115,755
Issuance costs associated with sale of voting and non-voting ordinary shares	(6,289)
Foreign currency translation	(64)			(64)
Unrealized gain (loss) on marketable 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
Balance at Dec. 31, 2020	$ 178,434	$ 12,507	771,194	373	(605,640)
Balance, Shares at Dec. 31, 2020		98,283,603
Exercise of share options, Shares	1,691,435
Foreign currency translation	$ 3,388
Net loss	(108,236)
Balance at Sep. 30, 2021	241,298	$ 16,229	935,211	3,734	(713,876)
Balance, Shares at Sep. 30, 2021		125,485,953
Balance at Mar. 31, 2021	295,580	$ 15,995	920,211	196	(640,822)
Balance, Shares at Mar. 31, 2021		123,764,597
Share-based compensation expense	5,541		5,541
Exercise of share options and issuance of ESPP shares	310	$ 15	295
Exercise of share options and issuance of ESPP shares, shares		118,500
Issuance costs associated with sale of voting and non-voting ordinary shares	(66)		(66)
Foreign currency translation	(374)			(374)
Unrealized gain (loss) on marketable securities	(8)			(8)
Net loss	(36,609)				(36,609)
Balance at Jun. 30, 2021	264,374	$ 16,010	925,981	(186)	(677,431)
Balance, Shares at Jun. 30, 2021		123,883,097
Share-based compensation expense	5,389		5,389
Exercise of share options	4	$ 47	(43)
Exercise of share options, Shares		356,217
Vesting of restricted share units, net of shares withheld for taxes	(79)	$ 2	(81)
Vesting of restricted share units, net of shares withheld for taxes, shares		18,901
Issuance of shares as part of collaboration agreement	4,135	$ 170	3,965
Issuance of shares as part of collaboration agreement, shares		1,227,738
Foreign currency translation	3,826			3,826
Unrealized gain (loss) on marketable securities	94			94
Net loss	(36,445)				(36,445)
Balance at Sep. 30, 2021	$ 241,298	$ 16,229	$ 935,211	$ 3,734	$ (713,876)
Balance, Shares at Sep. 30, 2021		125,485,953

Condensed Consolidated Statem_4

Condensed Consolidated Statements Shareholders' Equity (Unaudited) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2021	Mar. 31, 2021
Statement Of Stockholders Equity [Abstract]
Sale of voting and non-voting ordinary shares, issuance costs	$ 66	$ 6,289

Nature of the Business

Nature of the Business	9 Months Ended
	Sep. 30, 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 $143.6 million after deducting placement agent fees of $6.0 million and other issuance costs of $0.4 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 September 30, 2021, the Company has financed its operations primarily with proceeds from the sale of equity securities, including American Depositary Shares (“ADSs”) in the Company’s initial public offering (“IPO”) and follow-on offering, ordinary shares in the private placement, and convertible preferred shares. The Company has also financed operations through proceeds from the Company’s senior term facilities agreement (the “Amended Credit Facility”) with MidCap Financial (Ireland) Limited (“MidCap Financial”), research grants from the California Institute of Regenerative Medicine (“CIRM”), upfront payments from the Company’s collaboration agreement and share purchase agreement with Pharming Group N.V., and proceeds associated 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 (“RDEC”) program. 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. unable to continue operations. Although management continues to pursue these plans, there is no assurance that the Company will be successful in obtaining sufficient funding on terms acceptable to the Company to fund continuing operations, if at all.

Basis of Presentation and Summa

Basis of Presentation and Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 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 based in 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, collaboration agreement milestones, 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 losses of $4.0 million and gains of $5.9 million for the three months ended September 30, 2021 and 2020, respectively. The Company recorded realized and unrealized foreign currency transaction losses of $2.0 million and $1.5 million for the nine months ended 30, 2021 and 2020, respectively. These amounts are included in other income (expense) 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 securities Marketable securities consist of investments with original maturities greater than ninety days. The Company has classified its investments with maturities beyond one year as short term, based on their highly liquid nature and because such marketable securities represent the investment of cash that is available for current operations. The Company considers its investment portfolio of investments as available-for-sale. Accordingly, these investments are recorded at fair value, which is based on quoted market prices or other observable inputs. Unrealized gains and losses are recorded as a component of other comprehensive income (loss). Realized gains and losses are determined on a specific identification basis and are included in other income (loss). Amortization and accretion of discounts and premiums is also recorded in other income (loss). When the fair value is below the amortized cost of the asset, an estimate of expected credit losses is made and is limited to the amount by which fair value is less than amortized cost. The credit-related impairment amount is recognized in net income; the remaining impairment amount and unrealized gains are reported as a component of accumulated other comprehensive income (loss) in shareholders’ equity. Credit losses are recognized through the use of an allowance for credit losses account and subsequent improvements in expected credit losses are recognized as a reversal of the allowance account. If the Company has the intent to sell the security or it is more likely than not that the Company will be required to sell the security prior to recovery of its amortized cost basis the allowance for credit loss is written off and the excess of the amortized cost basis of the asset over its fair value is recorded in the statement of operations. 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 SME program and the 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, for the periods ended September 30, 202 1 and 20 20 : Three Months Ended September 30, Nine Months Ended September 30, 2021 2020 2021 2020 Balance at beginning of period $ 25,371 $ 32,729 $ 17,343 $ 28,644 Recognition of credit claims as offset to research and development expense 5,698 10,104 13,498 17,169 Receipt of credit claims — (13,558 ) — (14,573 ) Foreign currency translation (843 ) 1,212 (615 ) (753 ) Balance at end of period $ 30,226 $ 30,487 $ 30,226 $ 30,487 As of September 30, 2021, the Company’s tax incentive receivable from the U.K. government was $30.2 million, of which $18.3 million was classified as current and $11.9 million was classified as noncurrent. As of December 31, 2020, the Company’s tax incentive receivable from the U.K. 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 September 30, 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 September 30, 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: September 30, December 31, 2021 2020 (in thousands) Cash and cash equivalents $ 74,324 $ 55,135 Restricted cash 4,266 4,266 Total cash, cash equivalents and restricted cash shown in the statement of cash flows $ 78,590 $ 59,401 The Company also has $7.9 million in an escrow account associated with the construction of the Company’s leased facility in Fremont, California, which the Company has ceased construction and build-out, and has subleased the facility to a third-party who intends to perform construction and build-out of the facility. Intangible assets, net Intangible assets, net consist of accruals and payments of milestones associated with the Company’s approved products, net of accumulated amortization. The Company amortizes its intangible assets using the straight-line method over their estimated economic lives and periodically reviews for impairment. The Company has not recognized any impairment charges related to intangible assets to date. Product sales The Company’s product sales of Strimvelis are currently distributed exclusively at the San Raffaele Hospital in Milan, Italy. San Raffaele Hospital will purchase and pay for Strimvelis and submit a claim to the payer. The Company’s contracted sales with San Raffaele Hospital contain a single performance obligation and the Company recognizes revenue from product sales when the Company has satisfied its performance obligation by transferring control of Strimvelis to San Raffaele Hospital. Control of the product generally transfers upon the completion of the scheduled Strimvelis treatment. The Company’s product sales represent total net product sales of Strimvelis. The Company evaluated the variable consideration under Accounting Standards Codification (ASC) 606, Revenue from Contracts with Customers , and there is currently no variable consideration included in the transaction price for Strimvelis. Costs to manufacture and deliver the product and those associated with administering the therapy are included in cost of product sales. As the product is sold in direct relation to a scheduled treatment, the Company estimates that there is limited risk of product return, including the risk of product expiration. During the three months and nine months ended September 30, 2021, the Company had $0.7 million in sales of Strimvelis. 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 preclinical and clinical development activities and clinical trials, as well as costs to develop a manufacturing process, perform analytical testing and manufacture clinical trial materials. The Company also includes the costs related to the Company’s collaboration arrangement in research and development expense. 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 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 income (loss). Other comprehensive income (loss) consists of unrealized gains and losses on marketable 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 an amount equal to the lease payments in a similar economic environment. The Company estimated the incremental borrowing rate based on the Company’s currently outstanding credit facility as inputs to the analysis to calculate a spread, adjusted for factors that reflect the profile of secured borrowing over the expected term of the lease. The components of a lease should be split into three categories: lease components (e.g., land, building, etc.), non-lease components (e.g., common area maintenance, utilities, performance of manufacturing services, purchase of inventory, etc.), and non-components (e.g., property taxes, insurance, etc.). Then the fixed contract consideration (including any related to non-components) must be allocated based on fair values to the lease components and non-lease components. Although separation of lease and non-lease components is required, certain practical expedients are available to entities. Entities electing the practical expedient would not separate lease and non-lease components. Rather, they would account for each lease component and the related non-lease component together as a single component. The Company has elected not to apply the practical expedient and with respect to its lease of manufacturing space at a contract manufacturing organization, the Company has instead allocated the consideration between the lease and non-lease components of the contract. The Company calculated the fair value of the lease component using financial information readily available as part of its master services arrangement. The remainder of the consideration was allocated to the non-lease components. 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. Impairment of long-lived assets Long-lived assets consist of property and equipment and operating lease right-of-use assets. Long-lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset group for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset group to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset group are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset group over its fair value, as determined in accordance with the related accounting literature. Strimvelis loss provision As part of the GSK transaction, the Company is required to maintain commercial availability of Strimvelis in the European Union until such time that an alternative gene therapy is available (see Note 12). 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 the Strimvelis program and the expected future net losses to be generated until such time as Strimvelis is no longer commercially available. The amortization of the provision is recorded as a credit to research and development expense. The Company has made an estimate of the expected future losses associated with Strimvelis and will adjust this estimate as facts and circumstances change regarding the commercial availability and costs of maintaining and selling Strimvelis. The Company does not update the accrued loss provision for any subsequent adjustment of the future losses, however, the timing of recognizing the amortization of what was originally recorded is adjusted for the updated future losses. The following table below outlines the changes to the Strimvelis loss provision for the periods ended September 30, 2021 and 2020: Three Months Ended September 30, Nine Months Ended September 30, 2021 2020 2021 2020 Balance at beginning of period $ 3,735 $ 4,392 $ 4,482 $ 6,790 Amortization of loss provision (84 ) — (898 ) (2,018 ) Foreign currency translation (105 ) 191 (38 ) (189 ) Balance at end of period $ 3,546 $ 4,583 $ 3,546 $ 4,583 As of September 30, 2021, $0.6 million is classified as current, and $2.9 million is classified as non-current. As of December 31, 2020, $0.9 million was classified as current, and $3.6 million was 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: As of September 30, 2021 2020 Share options 14,659,035 11,980,433 Unvested performance-based restricted share units 663,500 867,574 15,322,535 12,848,007 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 Securities	9 Months Ended
	Sep. 30, 2021
Fair Value Disclosures [Abstract]
Fair Value Measurements and Marketable Securities	3. Fair value measurements and marketable securities The following tables present information about the Company’s financial assets that have been measured at fair value as of September 30, 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 and nine months ended September 30, 2021, there were no transfers between Level 1 and Level 2 financial assets. The following table summarizes the Company’s cash equivalents and marketable securities as of September 30, 2021: Fair Value Measurements at September 30, 2021 Using: Level 1 Level 2 Level 3 Total Cash equivalents Money market funds $ 24,014 $ — $ — $ 24,014 Corporate bonds 1,002 1,002 Commercial paper — 23,098 — 23,098 Total cash equivalents $ 24,014 $ 24,100 $ — $ 48,114 Marketable securities Corporate bonds $ — $ 102,877 $ — $ 102,877 Commercial paper — 76,942 — 76,942 Total marketable securities $ — $ 179,819 $ — $ 179,819 Total $ 24,014 $ 203,919 $ — 227,933 The following table summarizes the Company’s cash equivalents and marketable 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 securities The following table summarizes the Company’s marketable securities as of September 30, 2021: At September 30, 2021 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Credit Losses Fair Value Corporate bonds $ 103,915 $ 4 $ (41 ) $ — $ 103,878 Commercial paper 100,052 3 (14 ) — 100,041 Total $ 203,967 $ 7 $ (55 ) $ — $ 203,919 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 September 30, 2021 and December 31, 2020 At September 30, 2021 At December 31, 2020 Due in one year $ 192,325 $ 132,056 Due after one year through three years 11,594 10,757 Total $ 203,919 $ 142,813

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets	9 Months Ended
	Sep. 30, 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: September 30, December 31, 2021 2020 Prepaid external research and development expenses $ 3,165 $ 1,421 Inventories 1,712 665 Other prepayments 2,453 4,930 VAT receivable 1,189 2,780 Construction deposit - current 4,968 1,552 Non-trade receivables 4,475 2,017 Total prepaid expenses and other current assets $ 17,962 $ 13,365

Property and Equipment, Net

Property and Equipment, Net	9 Months Ended
	Sep. 30, 2021
Property Plant And Equipment [Abstract]
Property and Equipment, Net	5. Property and equipment, net Property and equipment, net consisted of the following: September 30, December 31, 2021 2020 Property and equipment: Lab equipment $ 5,492 $ 5,114 Leasehold improvements 2,445 2,522 Furniture and fixtures 303 304 Office and computer equipment 1,172 763 Construction-in-process 906 302 Property and equipment $ 10,318 $ 9,005 Less: accumulated depreciation (5,562 ) (4,224 ) Property and equipment, net $ 4,756 $ 4,781 Depreciation expense was $0.5 million and $0.5 million for the three months ended September 30, 2021 and 2020, respectively. Depreciation expense was $1.5 million and $1.6 million for the nine months ended September 30, 2021 and 2020, respectively. During the second quarter of 2020, the Company took impairment charges of $2.3 million related to construction-in-process and $0.8 million related to laboratory equipment (see Note 11).

Intangible Assets, Net

Intangible Assets, Net	9 Months Ended
	Sep. 30, 2021
Goodwill And Intangible Assets Disclosure [Abstract]
Intangible Assets, Net	6. Intangible assets, net Intangible assets, net of accumulated amortization, consisted of the following: At September 30, 2021 At December 31, 2020 Cost Accumulated amortization Net Cost Accumulated amortization Net License milestones $ 4,841 $ (101 ) $ 4,740 $ 3,076 $ — $ 3,076 Total $ 4,841 $ (101 ) $ 4,740 $ 3,076 $ — $ 3,076 License milestones consist of capitalized milestone payments or accruals of payments the Company has deemed probable upon receiving regulatory approval of Libmeldy in the EU. The license milestones are being amortized on a straight-line basis over the remaining useful life of the related patents of approximately twelve years. For the three months and nine months ended September 30, 2021, amortization of intangible assets totaled $0.1 million. For the three months and nine months ended September 30, 2020, amortization of intangible assets was nil. The effect of foreign currency translation on the net carrying value of intangible assets during 2021 was not material. The following table summarizes the estimated future amortization for intangible assets for the next five years and thereafter: As of September 30, 2021 (October - December) $ 103 2022 410 2023 410 2024 410 2025 410 Thereafter 2,997 Total $ 4,740

Other Assets

Other Assets	9 Months Ended
	Sep. 30, 2021
Other Assets Noncurrent Disclosure [Abstract]
Other Assets	7. Other assets Other assets consist of the following: September 30, December 31, 2021 2020 Deferred tax assets 4,468 5,219 Deposits 996 1,144 Deferring financing costs 708 975 Other non-current assets 3,077 1,554 Construction deposits - long-term 2,975 6,572 Total other assets $ 12,224 $ 15,464

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	9 Months Ended
	Sep. 30, 2021
Accrued Liabilities And Other Liabilities [Abstract]
Accrued Expenses and Other Current Liabilities	8. Accrued expenses and other current liabilities Accrued expenses and other liabilities consisted of the following: September 30, December 31, 2021 2020 Accrued external research and development expenses $ 11,599 $ 8,878 Accrued payroll and related expenses, including severance 10,100 11,881 Accrued professional fees 993 791 Accrued other 2,671 3,401 Accrued milestone payments 2,585 3,076 Strimvelis liability - current portion 552 916 Total accrued expenses and other liabilities $ 28,500 $ 28,943

Notes Payable

Notes Payable	9 Months Ended
	Sep. 30, 2021
Debt Disclosure [Abstract]
Notes Payable	9. Notes payable In May 2019 In May 2021, the Company amended and restated the Original Credit Facility (the “Amended Credit Facility”). Under the Amended Credit Facility, the Lenders agreed to make term loans available to the Company in the aggregate amount of $100.0 million, including increasing the principal on the initial term loan to $33.0 million, from $25.0 million. To date, the Company has borrowed $33.0 million under the amended initial term loan. The remaining $67.0 million under the Amended Credit Facility may be drawn down in the form of a second and third term loan, the second term loan being a $33.0 million term loan available no earlier than July 1, 2022 and no later than July 1, 2023 upon certain regulatory approvals and evidence of the Company having $100 million in cash and cash equivalent investments; and the third term loan being a $34.0 million term loan available no earlier than July 1, 2023 and no later than July 1, 2024 upon evidence of the Company having $100 million in cash and cash equivalent investments and attaining a pre-specified trailing 12-month revenue target. Prior to execution of the Amended Credit Facility, each term loan under the Original Credit Facility bore interest at an annual rate equal to 6.0% plus LIBOR. The Company was required to make interest-only payments on the term loan for all payment dates prior to 24 months following the date of the Original Credit Facility, unless the third tranche was drawn, in which case for all payment dates prior to 36 months following the date of the Original Credit Facility. The term loans prior to the Amended Credit Facility were to begin amortizing on either the 24-month or the 36-month anniversary of the Original 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% was due on the loan maturity date. The Company accrued the final payment amount of $1.1 million associated with the first term loan of the Original Credit Facility, to outstanding debt by charges to interest expense using the effective-interest method from the date of issuance through the date of the Amended Credit Facility. Upon execution of the Amended Credit Facility, the Company was required to make a payment of $0.5 million for the accrued final payment associated with the Original Credit Facility, which was netted against proceeds from the additional initial term loan. Each term loan under the Amended Credit Facility bears interest at an annual rate equal to 5.95% plus LIBOR. The Company is required to make interest-only payments on the term loan for 18 months following the date of the Amended Credit Facility, unless the Company is eligible for the second tranche, in which case the Company may elect to make interest-only payments for 30 months The Amended Credit Facility includes affirmative and negative covenants. The affirmative covenants include, among others, covenants requiring the Company to maintain their legal existence and governmental approvals, deliver certain financial reports, maintain insurance coverage, maintain property, pay taxes, satisfy certain requirements regarding accounts and comply with laws and regulations. The negative covenants include, among others, restrictions on the Company transferring collateral, incurring additional indebtedness, engaging in mergers or acquisitions, paying dividends or making other distributions, making investments, creating liens, amending material agreements and organizational documents, selling assets, changing the nature of the business and undergoing a change in control, in some cases subject to certain exceptions. The Company is also subject to an ongoing minimum cash financial covenant in which the Company must maintain unrestricted cash in an amount not less than $20.0 million following the utilization of the second term loan and not less than $35.0 million following the utilization of the third term loan. As of September 30, 2021, and December 31, 2020, notes payable consist of the following: September 30, December 31, 2021 2020 Notes payable, net of issuance costs $ 32,647 $ 24,659 Less: current portion $ — (4,861 ) Notes payable, net of current portion $ 32,647 $ 19,798 Accretion related to final payment 139 406 Notes payable, long term $ 32,786 $ 20,204 As of September 30, 2021, the estimated future principal payments due are as follows: Aggregate Minimum Payments 2021 (October - December) — 2022 785 2023 9,429 2024 9,429 2025 9,429 Thereafter 5,083 Total 34,155 Less current portion — Less unamortized portion of final payment (1,016 ) Less unamortized debt issuance costs (353 ) Notes payable, long term $ 32,786 During the three months and nine months ended September 30, 2021, the Company recognized $0.7 million and $1.8 million of interest expense, respectively, related to the term loan. During the three months and nine months ended September 30, 2020 the Company recognized $0.6 million and $1.8 million of interest expense, respectively, related to the term loan. The effective annual interest rate as of September 30, 2021 on the outstanding debt under the term loan was approximately 8.5%.

Share-Based Compensation

Share-Based Compensation	9 Months Ended
	Sep. 30, 2021
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Share-Based Compensation	10. 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 determined not to make any further awards under the 2016 plan following the Company’s IPO. As of September 30, 2021, 6,429,862 shares remained available for issuance under the 2018 Plan, 721,500 shares remained available for issuance under the 2020 Inducement Plan, and 1,341,016 shares remained available for issuance under the ESPP. Share option activity The following table summarizes option activity under the plans for nine months ended September 30, 2021: Number of Options Weighted Average Exercise Price Outstanding at December 31, 2020 13,895,643 $ 7.96 Granted 8,104,206 4.89 Exercised (1,691,435 ) 1.62 Forfeited (2,467,243 ) 10.92 Outstanding at September 30, 2021 17,841,171 $ 6.76 Vested and expected to vest, as of September 30, 2021 17,841,171 $ 6.76 Exercisable, September 30, 2021 7,334,390 $ 6.95 The weighted-average grant date fair value of share options granted during the nine months ended September 30, 2021 was $3.19. 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 RSUs will vest, and the award will become fully vested upon achievement of three of the four performance conditions. CEO award The Company granted 195,000 performance-based RSUs to its Chief Executive Officer in April 2020. The award vests on January 2, 2024 as to 1/3 of the award for each of the first three to occur of four milestones, if each such milestone is achieved by the Company on or before December 31, 2023 and Dr. Gaspar remains continuously employed with the Company through January 2, 2024. The milestones relate to achievement of specific clinical and regulatory milestones. The following table summarizes award activity for the nine months ended September 30, 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 — 47,500 47,500 4.94 Vested (89,667 ) (41,667 ) (131,334 ) 9.15 Forfeited (51,500 ) (62,500 ) (114,000 ) 9.94 Unvested and outstanding at September 30, 2021 322,833 123,333 446,166 $ 7.89 The maximum aggregate total fair value of all outstanding RSUs is $3.3 million, which relates to performance-based conditins. The fair value associated with the performance-based conditions will be recognized when achievement of the milestones becomes probable, if at all. The Company determined that, as of June 30, 2021, none of the regulatory and research and development milestones associated with any outstanding RSUs were deemed probable. The amount of compensation cost recognized for the nine months ended September 30, 2021 and 2020 for the market condition associated with the performance-based RSUs was nil million and $0.3 million, respectively. The amount of compensation cost recognized for the three months ended September 30, 2021 and 2020 for the market condition associated with the performance-based RSUs was nil and nil, respectively. Share-based compensation expense Share-based compensation expense recorded as research and development and general and administrative expenses is as follows: Three months ended September 30, Nine months ended September 30, 2021 2020 2021 2020 Research and development $ 2,096 $ 2,617 $ 7,106 $ 8,398 General and administrative 3,293 3,046 10,092 12,823 Total share-based compensation $ 5,389 $ 5,663 $ 17,198 $ 21,221 During the nine months ended September 30, 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 September 30, 2021, total unrecognized compensation cost related to unvested share option grants was approximately $45.1 million. This amount is expected to be recognized over a weighted average period of approximately 2.9 years. As of September 30, 2021, the total unrecognized compensation cost related to performance-based RSUs is a maximum of $3.3 million, dependent upon achievement of the aforementioned milestones.

Restructuring Charges

Restructuring Charges	9 Months Ended
	Sep. 30, 2021
Restructuring And Related Activities [Abstract]
Restructuring Charges	11. Restructuring charges On May 4, 2020, the Company committed to a new strategic plan and restructuring intended to enable the Company to advance its corporate strategy while reducing overall operating expenses, including ceasing construction and build-out of its Fremont, California manufacturing facility, closing its office in Menlo Park, California, reducing its workforce by approximately 25% across the Company, eliminating a number of future positions expected to be recruited in 2020 and 2021, reducing its investment in the future development for certain programs, and other cost-saving measures (collectively, the “Restructuring”). The workforce reductions took place during the second and third quarters of 2020. Cash restructuring charges Accrued restructuring and severance costs are included in accrued expenses and other current liabilities in the condensed consolidated balance sheet. Activity for the 2020 fiscal year is summarized as follows: Three months ended September 30, 2020 Nine months ended September 30, 2020 Balance at beginning of period $ 841 $ — Charged to expense (9 ) 1,575 Payments made (511 ) (1,254 ) Balance at end of period $ 321 $ 321 During the nine months ended September 30, 2021, the Company had no restructuring charges. During the nine months ended September 30, 2021 and 2020, nil and $1.1 million, respectively, of the employee termination benefits has been classified on the condensed consolidated statement of operations as research and development expense and nil and $0.5 million, respectively, has been classified as selling, general, and administrative expense. Impairment of long-lived assets During the three months and nine months ended September 30, 2021, the Company had no non-cash restructuring charges. During the three months and nine months ended September 30, 2020, the Company took the following non-cash charges to research and development expense associated with the impairment of construction-in-process associated with the Fremont manufacturing facility, partial impairment of the right-of-use asset for the Fremont manufacturing facility lease (the “Fremont ROU asset”), and a write-down of laboratory equipment from the Company’s Menlo Park, CA facility: Asset write-downs Operating lease right-of-use asset $ 2,605 Construction-in-process 2,285 Laboratory equipment 760 Charge taken to research and development expense 5,650 The Company assessed the Fremont construction-in-process for impairment in May 2020 upon the Restructuring. The construction-in-process was related to design costs, and was determined to have no potential future value, and an impairment charge of $2.3 million was taken for the full value of the construction-in-process asset. The Company assessed the Fremont ROU asset for impairment in May 2020 upon the Restructuring when the carrying value of the asset was $13.8 million. The Fremont ROU asset represented the asset group for the impairment assessment. Upon failing the first step of the long-lived asset impairment model where the undiscounted cash flows were less than the carrying value of the Fremont ROU asset, the Company performed the second step by comparing the fair value of the Fremont ROU asset to its carrying value. The fair value of the Fremont ROU asset is a non-recurring fair value measurement that was measured using a probability-weighted discounted cash flow approach, which estimated the present value of potential sublease income to be generated by the facility, less costs incurred to sublease the facility. The significant assumptions inherent in estimating the various probability weighted scenarios included the undiscounted forecasted sublease income less costs incurred, which included assumptions of the expected income and timing of entering into a future sublease, and a market-participant discount rate that reflects a potential discount rate. The Company selected the assumptions used in the fair value estimate using current market data associated with the potential sublease income and market participant discount rates. The undiscounted cash flows utilized in the fair value estimate ranged from $11.7 million to $19.1 million to be generated over the remainder of the lease term. The market-participant discount rate utilized in the fair value estimate was 4.6%. These assumptions represent level 3 inputs of the fair value hierarchy. As of the assessment date in May 2020, the fair value of the Fremont ROU asset was $11.2 million, and the Company recorded a $2.6 million impairment charge related to the asset. The remaining carrying value of the Fremont ROU asset will be amortized over the remaining lease term on a straight-line basis. In December 2020, the Company executed a sublease for the Fremont manufacturing facility with an unrelated third-party for the remaining lease term. No further impairment was necessary as a result of the sublease. The occurrence of a triggering event for the Fremont ROU asset in future periods could result in additional impairment charges if the estimated fair value of the asset is determined to be lower than the carrying value.

License Agreements

License Agreements	9 Months Ended
	Sep. 30, 2021
License Agreements [Abstract]
License Agreements	12. 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 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. The Company will pay a flat mid-single digit percentage royalty on the annual net sales of Strimvelis. 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 agreements 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. 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. 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. The Company may also pay low single-digit percentage royalties on net sales of collaborated product generated under the Oxford BioMedica Agreement.

Collaboration Agreement with Ph

Collaboration Agreement with Pharming Group N.V.	9 Months Ended
	Sep. 30, 2021
Collaborative Arrangement [Abstract]
Collaboration Agreement with Pharming Group N.V.	13. Collaboration agreement with Pharming Group N.V. Overview On July 1, 2021, the Company entered into a strategic collaboration with Pharming Group N.V. (“Pharming”) to research, develop, manufacture and commercialize OTL-105, an investigational ex vivo Under the terms of the Collaboration Agreement, Pharming was granted worldwide rights to OTL-105 and will be responsible for clinical development, regulatory filings and commercialization of the investigational gene therapy, including associated costs. The Company will lead the completion of IND-enabling activities and oversee manufacturing of OTL-105 during preclinical and clinical development, which will be funded by Pharming. In addition, both the Company and Pharming will explore the application of non-toxic conditioning regimen for use with OTL-105 administration. The Company received an upfront payment of $10.0 million in cash from Pharming. The Company is also eligible to receive up to $189.5 million in development, regulatory and sales milestones as well as mid-single to low double-digit royalty payments on future worldwide sales. Share Purchase Agreement The Company also entered into a Share Purchase Agreement with Pharming on July 1, 2021 (the “SPA”), pursuant to which the Company issued 1,227,738 ordinary shares to Pharming for total consideration of $7.5 million. The consideration is payment for the fair value of ordinary shares with a fair value of $4.1 million plus a $3.4 million premium on the fair value of the Company’s ordinary shares. The “Collaboration Agreement” and the “SPA” are referred to together as the “Pharming Agreements.” Accounting Analysis At the commencement of the arrangement, two units of accounting were identified, which are the issuance of 1,227,738 of the Company’s ordinary shares as part of the SPA, and the license and collaboration agreement, which conveys the license and provides for the Company to provide research, development, manufacturing services for OTL-105. The Pharming Agreements were entered into concurrently as part of a single commercial objective, and the Company considers them a single arrangement for accounting purposes. The total upfront payments of $17.5 million comprises $4.1 million attributed to the equity sold to Pharming and $13.4 million attributed to the Collaboration Agreement. In determining the fair value of the common stock issued to Pharming as part of the SPA, the Company used an option pricing valuation model to take into consideration certain holding period restrictions on the shares. The fair value of the Company’s common shares was considered a level 2 fair value measurement within the fair value hierarchy. Based on the fair value adjustments made by management, the fair value of the shares issued was determined to be $4.1 million with the excess proceeds of $3.4 million being allocation to the Collaboration Agreement. The Company recognizes revenue under Accounting Standards Codification (“ASC”) 606, Revenue from Contracts with Customer (“ASC 606”). the Company determined that it could not assert that it was probable that a significant reversal in the amount of cumulative revenue recognized will not occur. The Company re-evaluates the transaction price as of the end of each reporting period. The Company also considered the existence of any significant financing component within the Pharming Agreements given their upfront payment structure. Based upon this assessment, the Company concluded that the up-front payments were provided for valid business reasons and not for the purpose of providing financing. Accordingly, the Company has concluded that the upfront payment structure of the Pharming Agreements does not result in the existence of a significant financing component. The Company recognizes revenue associated with the performance obligation as the research, development and manufacturing services are provided using an input method, based on the cumulative costs incurred compared to the total estimated costs expected to be incurred to satisfy the performance obligation. The transfer of control to the customer occurs over the time period that the research, development and manufacturing services are to be provided by the Company, and this cost-to-cost method is, in management’s judgment, the best measure of progress towards satisfying the performance obligation. Reimbursement for research, development, and manufacturing services are recognized as the costs are incurred consistent with the cost-to-cost method. The Company's continuing obligations to provide research, development, and manufacturing services is based on the results of such efforts, and the estimated costs associated with the remaining efforts required to complete the performance obligations may change, which may materially impact revenue recognition. The Company regularly evaluates and, when necessary, updates the costs associated with the remaining effort under the Collaboration Agreement. Accordingly, revenue may fluctuate from period to period due to revisions to estimated costs, resulting in a change in the measure of progress for the performance obligation, or if the transaction price changes due to inclusion of any milestone payments that become unconstrained. The following table summarizes research and development costs incurred and collaboration revenue recognized in connection with the Company’s performance under the Collaboration Agreement: Three months ended September 30, Nine months ended September 30, 2021 2020 2021 2020 Reimbursement revenue $ 428 $ — $ 428 $ — Upfront and milestone payment revenue 64 — 64 — Total collaboration revenue recognized $ 492 $ — $ 492 $ — The Company had $0.4 million and nil due from Pharming included in accounts receivable as of September 30, 2021 and December 31, 2020, respectively. As of September 30, 2021, the Company had contract liabilities of $12.9 million, which is classified as either current or long-term deferred revenue in the condensed consolidated balance sheet based on the period over which this is expected to be recognized. The deferred revenue balance represents the portion of the upfront payments received that are partially unsatisfied as of September 30, 2021.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2021
Income Tax Disclosure [Abstract]
Income Taxes	14. Income taxes The Company recorded income tax expense 0.7

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2021
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	15. 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 entered into an agreement with AGC Biologic S.p.A (“AGC”) in July 2020 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 September 30, 2021: Due in: Product manufacturing commitments Dedicated manufacturing and development resources Exclusive manufacturing suites Total remaining AGC commitment 2021 (October - December) $ 1,416 $ 5,234 $ — $ 6,650 2022 2,744 8,750 3,216 14,710 2023 3,186 8,177 3,216 14,579 2024 3,186 8,177 3,216 14,579 2025 1,593 4,089 1,608 7,290 Total commitment $ 12,125 $ 34,427 $ 11,256 $ 57,808 *Tabular disclosure above has been translated to U.S. Dollar, from Euro, using an exchange rate of €1.00 to $1.18. Lease commitments The Company leases office and laboratory space and has an embedded lease at AGC for use of exclusive manufacturing suites. 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	9 Months Ended
	Sep. 30, 2021
Compensation And Retirement Disclosure [Abstract]
Employee Benefit Plans	16. 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 $1.4 million in matching contributions for the three months and nine months ended September 30, 2021, respectively. The Company paid $0.3 million and $1.3 million in matching contributions for the three months and nine months ended September 30, 2020, respectively.

Related Party Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2021
Related Party Transactions [Abstract]
Related Party Transactions	17. 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 September 30, 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 nine months ended September 30, 2021, the Company recorded no expense associated with transactions with GSK. During the nine months ended September 30, 2020 the Company entered into a global license agreement with GSK for use of their lentiviral stable cell line technology whereby the Company recorded $1.3 million of in-process research and development expense associated with upfront payments made to GSK. During the nine months ended September 30, 2021, the Company made $0.1 million in payment on accounts payable due to GSK. During the nine months ended September 30, 2020, the Company made $5.8 million in payments on accounts payable due to GSK.

Basis of Presentation and Sum_2

Basis of Presentation and Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 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 based in 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, collaboration agreement milestones, 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 losses of $4.0 million and gains of $5.9 million for the three months ended September 30, 2021 and 2020, respectively. The Company recorded realized and unrealized foreign currency transaction losses of $2.0 million and $1.5 million for the nine months ended 30, 2021 and 2020, respectively. These amounts are included in other income (expense) 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 securities	Marketable securities Marketable securities consist of investments with original maturities greater than ninety days. The Company has classified its investments with maturities beyond one year as short term, based on their highly liquid nature and because such marketable securities represent the investment of cash that is available for current operations. The Company considers its investment portfolio of investments as available-for-sale. Accordingly, these investments are recorded at fair value, which is based on quoted market prices or other observable inputs. Unrealized gains and losses are recorded as a component of other comprehensive income (loss). Realized gains and losses are determined on a specific identification basis and are included in other income (loss). Amortization and accretion of discounts and premiums is also recorded in other income (loss). When the fair value is below the amortized cost of the asset, an estimate of expected credit losses is made and is limited to the amount by which fair value is less than amortized cost. The credit-related impairment amount is recognized in net income; the remaining impairment amount and unrealized gains are reported as a component of accumulated other comprehensive income (loss) in shareholders’ equity. Credit losses are recognized through the use of an allowance for credit losses account and subsequent improvements in expected credit losses are recognized as a reversal of the allowance account. If the Company has the intent to sell the security or it is more likely than not that the Company will be required to sell the security prior to recovery of its amortized cost basis the allowance for credit loss is written off and the excess of the amortized cost basis of the asset over its fair value is recorded in the statement of operations.
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 SME program and the 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, for the periods ended September 30, 202 1 and 20 20 : Three Months Ended September 30, Nine Months Ended September 30, 2021 2020 2021 2020 Balance at beginning of period $ 25,371 $ 32,729 $ 17,343 $ 28,644 Recognition of credit claims as offset to research and development expense 5,698 10,104 13,498 17,169 Receipt of credit claims — (13,558 ) — (14,573 ) Foreign currency translation (843 ) 1,212 (615 ) (753 ) Balance at end of period $ 30,226 $ 30,487 $ 30,226 $ 30,487 As of September 30, 2021, the Company’s tax incentive receivable from the U.K. government was $30.2 million, of which $18.3 million was classified as current and $11.9 million was classified as noncurrent. As of December 31, 2020, the Company’s tax incentive receivable from the U.K. 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 September 30, 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 September 30, 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: September 30, December 31, 2021 2020 (in thousands) Cash and cash equivalents $ 74,324 $ 55,135 Restricted cash 4,266 4,266 Total cash, cash equivalents and restricted cash shown in the statement of cash flows $ 78,590 $ 59,401 The Company also has $7.9 million in an escrow account associated with the construction of the Company’s leased facility in Fremont, California, which the Company has ceased construction and build-out, and has subleased the facility to a third-party who intends to perform construction and build-out of the facility.
Intangible assets, net	Intangible assets, net Intangible assets, net consist of accruals and payments of milestones associated with the Company’s approved products, net of accumulated amortization. The Company amortizes its intangible assets using the straight-line method over their estimated economic lives and periodically reviews for impairment. The Company has not recognized any impairment charges related to intangible assets to date.
Product sales	Product sales The Company’s product sales of Strimvelis are currently distributed exclusively at the San Raffaele Hospital in Milan, Italy. San Raffaele Hospital will purchase and pay for Strimvelis and submit a claim to the payer. The Company’s contracted sales with San Raffaele Hospital contain a single performance obligation and the Company recognizes revenue from product sales when the Company has satisfied its performance obligation by transferring control of Strimvelis to San Raffaele Hospital. Control of the product generally transfers upon the completion of the scheduled Strimvelis treatment. The Company’s product sales represent total net product sales of Strimvelis. The Company evaluated the variable consideration under Accounting Standards Codification (ASC) 606, Revenue from Contracts with Customers , and there is currently no variable consideration included in the transaction price for Strimvelis. Costs to manufacture and deliver the product and those associated with administering the therapy are included in cost of product sales. As the product is sold in direct relation to a scheduled treatment, the Company estimates that there is limited risk of product return, including the risk of product expiration. During the three months and nine months ended September 30, 2021, the Company had $0.7 million in sales of Strimvelis.
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 preclinical and clinical development activities and clinical trials, as well as costs to develop a manufacturing process, perform analytical testing and manufacture clinical trial materials. The Company also includes the costs related to the Company’s collaboration arrangement in research and development expense. 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 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 income (loss). Other comprehensive income (loss) consists of unrealized gains and losses on marketable 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 an amount equal to the lease payments in a similar economic environment. The Company estimated the incremental borrowing rate based on the Company’s currently outstanding credit facility as inputs to the analysis to calculate a spread, adjusted for factors that reflect the profile of secured borrowing over the expected term of the lease. The components of a lease should be split into three categories: lease components (e.g., land, building, etc.), non-lease components (e.g., common area maintenance, utilities, performance of manufacturing services, purchase of inventory, etc.), and non-components (e.g., property taxes, insurance, etc.). Then the fixed contract consideration (including any related to non-components) must be allocated based on fair values to the lease components and non-lease components. Although separation of lease and non-lease components is required, certain practical expedients are available to entities. Entities electing the practical expedient would not separate lease and non-lease components. Rather, they would account for each lease component and the related non-lease component together as a single component. The Company has elected not to apply the practical expedient and with respect to its lease of manufacturing space at a contract manufacturing organization, the Company has instead allocated the consideration between the lease and non-lease components of the contract. The Company calculated the fair value of the lease component using financial information readily available as part of its master services arrangement. The remainder of the consideration was allocated to the non-lease components. 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.
Impairment of long-lived assets	Impairment of long-lived assets Long-lived assets consist of property and equipment and operating lease right-of-use assets. Long-lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset group for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset group to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset group are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset group over its fair value, as determined in accordance with the related accounting literature.
Strimvelis loss provision	Strimvelis loss provision As part of the GSK transaction, the Company is required to maintain commercial availability of Strimvelis in the European Union until such time that an alternative gene therapy is available (see Note 12). 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 the Strimvelis program and the expected future net losses to be generated until such time as Strimvelis is no longer commercially available. The amortization of the provision is recorded as a credit to research and development expense. The Company has made an estimate of the expected future losses associated with Strimvelis and will adjust this estimate as facts and circumstances change regarding the commercial availability and costs of maintaining and selling Strimvelis. The Company does not update the accrued loss provision for any subsequent adjustment of the future losses, however, the timing of recognizing the amortization of what was originally recorded is adjusted for the updated future losses. The following table below outlines the changes to the Strimvelis loss provision for the periods ended September 30, 2021 and 2020: Three Months Ended September 30, Nine Months Ended September 30, 2021 2020 2021 2020 Balance at beginning of period $ 3,735 $ 4,392 $ 4,482 $ 6,790 Amortization of loss provision (84 ) — (898 ) (2,018 ) Foreign currency translation (105 ) 191 (38 ) (189 ) Balance at end of period $ 3,546 $ 4,583 $ 3,546 $ 4,583 As of September 30, 2021, $0.6 million is classified as current, and $2.9 million is classified as non-current. As of December 31, 2020, $0.9 million was classified as current, and $3.6 million was 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: As of September 30, 2021 2020 Share options 14,659,035 11,980,433 Unvested performance-based restricted share units 663,500 867,574 15,322,535 12,848,007
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)	9 Months Ended
	Sep. 30, 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, for the periods ended September 30, 202 1 and 20 20 : Three Months Ended September 30, Nine Months Ended September 30, 2021 2020 2021 2020 Balance at beginning of period $ 25,371 $ 32,729 $ 17,343 $ 28,644 Recognition of credit claims as offset to research and development expense 5,698 10,104 13,498 17,169 Receipt of credit claims — (13,558 ) — (14,573 ) Foreign currency translation (843 ) 1,212 (615 ) (753 ) Balance at end of period $ 30,226 $ 30,487 $ 30,226 $ 30,487
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: September 30, December 31, 2021 2020 (in thousands) Cash and cash equivalents $ 74,324 $ 55,135 Restricted cash 4,266 4,266 Total cash, cash equivalents and restricted cash shown in the statement of cash flows $ 78,590 $ 59,401
Schedule of Changes in Loss Provision	The following table below outlines the changes to the Strimvelis loss provision for the periods ended September 30, 2021 and 2020: Three Months Ended September 30, Nine Months Ended September 30, 2021 2020 2021 2020 Balance at beginning of period $ 3,735 $ 4,392 $ 4,482 $ 6,790 Amortization of loss provision (84 ) — (898 ) (2,018 ) Foreign currency translation (105 ) 191 (38 ) (189 ) Balance at end of period $ 3,546 $ 4,583 $ 3,546 $ 4,583
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: As of September 30, 2021 2020 Share options 14,659,035 11,980,433 Unvested performance-based restricted share units 663,500 867,574 15,322,535 12,848,007

Fair Value Measurements and M_2

Fair Value Measurements and Marketable Securities (Tables)	9 Months Ended
	Sep. 30, 2021
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract]
Schedule of Cash Equivalents and Marketable Securities	The following table summarizes the Company’s cash equivalents and marketable securities as of September 30, 2021: Fair Value Measurements at September 30, 2021 Using: Level 1 Level 2 Level 3 Total Cash equivalents Money market funds $ 24,014 $ — $ — $ 24,014 Corporate bonds 1,002 1,002 Commercial paper — 23,098 — 23,098 Total cash equivalents $ 24,014 $ 24,100 $ — $ 48,114 Marketable securities Corporate bonds $ — $ 102,877 $ — $ 102,877 Commercial paper — 76,942 — 76,942 Total marketable securities $ — $ 179,819 $ — $ 179,819 Total $ 24,014 $ 203,919 $ — 227,933 The following table summarizes the Company’s cash equivalents and marketable 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 Securities	The following table summarizes the Company’s marketable securities as of September 30, 2021: At September 30, 2021 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Credit Losses Fair Value Corporate bonds $ 103,915 $ 4 $ (41 ) $ — $ 103,878 Commercial paper 100,052 3 (14 ) — 100,041 Total $ 203,967 $ 7 $ (55 ) $ — $ 203,919 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 September 30, 2021 and December 31, 2020 At September 30, 2021 At December 31, 2020 Due in one year $ 192,325 $ 132,056 Due after one year through three years 11,594 10,757 Total $ 203,919 $ 142,813

Prepaid Expenses and Other Cu_2

Prepaid Expenses and Other Current Assets (Tables)	9 Months Ended
	Sep. 30, 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: September 30, December 31, 2021 2020 Prepaid external research and development expenses $ 3,165 $ 1,421 Inventories 1,712 665 Other prepayments 2,453 4,930 VAT receivable 1,189 2,780 Construction deposit - current 4,968 1,552 Non-trade receivables 4,475 2,017 Total prepaid expenses and other current assets $ 17,962 $ 13,365

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	9 Months Ended
	Sep. 30, 2021
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment, Net	Property and equipment, net consisted of the following: September 30, December 31, 2021 2020 Property and equipment: Lab equipment $ 5,492 $ 5,114 Leasehold improvements 2,445 2,522 Furniture and fixtures 303 304 Office and computer equipment 1,172 763 Construction-in-process 906 302 Property and equipment $ 10,318 $ 9,005 Less: accumulated depreciation (5,562 ) (4,224 ) Property and equipment, net $ 4,756 $ 4,781

Intangible Assets, Net (Tables)

Intangible Assets, Net (Tables)	9 Months Ended
	Sep. 30, 2021
Goodwill And Intangible Assets Disclosure [Abstract]
Schedule of Intangible Assets Net of Accumulated Amortization	Intangible assets, net of accumulated amortization, consisted of the following: At September 30, 2021 At December 31, 2020 Cost Accumulated amortization Net Cost Accumulated amortization Net License milestones $ 4,841 $ (101 ) $ 4,740 $ 3,076 $ — $ 3,076 Total $ 4,841 $ (101 ) $ 4,740 $ 3,076 $ — $ 3,076
Schedule of Estimated Future Amortization For Intangible Assets	The following table summarizes the estimated future amortization for intangible assets for the next five years and thereafter: As of September 30, 2021 (October - December) $ 103 2022 410 2023 410 2024 410 2025 410 Thereafter 2,997 Total $ 4,740

Other Assets (Tables)

Other Assets (Tables)	9 Months Ended
	Sep. 30, 2021
Other Assets Noncurrent Disclosure [Abstract]
Schedule of Other Assets	Other assets consist of the following: September 30, December 31, 2021 2020 Deferred tax assets 4,468 5,219 Deposits 996 1,144 Deferring financing costs 708 975 Other non-current assets 3,077 1,554 Construction deposits - long-term 2,975 6,572 Total other assets $ 12,224 $ 15,464

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities (Tables)	9 Months Ended
	Sep. 30, 2021
Accrued Liabilities And Other Liabilities [Abstract]
Schedule of Accrued Expenses and Other Current Liabilities	Accrued expenses and other liabilities consisted of the following: September 30, December 31, 2021 2020 Accrued external research and development expenses $ 11,599 $ 8,878 Accrued payroll and related expenses, including severance 10,100 11,881 Accrued professional fees 993 791 Accrued other 2,671 3,401 Accrued milestone payments 2,585 3,076 Strimvelis liability - current portion 552 916 Total accrued expenses and other liabilities $ 28,500 $ 28,943

Notes Payable (Tables)

Notes Payable (Tables)	9 Months Ended
	Sep. 30, 2021
Debt Disclosure [Abstract]
Summary of Notes Payable	As of September 30, 2021, and December 31, 2020, notes payable consist of the following: September 30, December 31, 2021 2020 Notes payable, net of issuance costs $ 32,647 $ 24,659 Less: current portion $ — (4,861 ) Notes payable, net of current portion $ 32,647 $ 19,798 Accretion related to final payment 139 406 Notes payable, long term $ 32,786 $ 20,204
Summary of Estimated Future Principal Payments Due	As of September 30, 2021, the estimated future principal payments due are as follows: Aggregate Minimum Payments 2021 (October - December) — 2022 785 2023 9,429 2024 9,429 2025 9,429 Thereafter 5,083 Total 34,155 Less current portion — Less unamortized portion of final payment (1,016 ) Less unamortized debt issuance costs (353 ) Notes payable, long term $ 32,786

Share-Based Compensation (Table

Share-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2021
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Option Activity	The following table summarizes option activity under the plans for nine months ended September 30, 2021: Number of Options Weighted Average Exercise Price Outstanding at December 31, 2020 13,895,643 $ 7.96 Granted 8,104,206 4.89 Exercised (1,691,435 ) 1.62 Forfeited (2,467,243 ) 10.92 Outstanding at September 30, 2021 17,841,171 $ 6.76 Vested and expected to vest, as of September 30, 2021 17,841,171 $ 6.76 Exercisable, September 30, 2021 7,334,390 $ 6.95
Summary of Award Activity	The following table summarizes award activity for the nine months ended September 30, 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 — 47,500 47,500 4.94 Vested (89,667 ) (41,667 ) (131,334 ) 9.15 Forfeited (51,500 ) (62,500 ) (114,000 ) 9.94 Unvested and outstanding at September 30, 2021 322,833 123,333 446,166 $ 7.89
Share-based Compensation Expense	Share-based compensation expense recorded as research and development and general and administrative expenses is as follows: Three months ended September 30, Nine months ended September 30, 2021 2020 2021 2020 Research and development $ 2,096 $ 2,617 $ 7,106 $ 8,398 General and administrative 3,293 3,046 10,092 12,823 Total share-based compensation $ 5,389 $ 5,663 $ 17,198 $ 21,221

Restructuring Charges (Tables)

Restructuring Charges (Tables)	9 Months Ended
	Sep. 30, 2021
Restructuring And Related Activities [Abstract]
Summary of Cash Restructuring Charges	Accrued restructuring and severance costs are included in accrued expenses and other current liabilities in the condensed consolidated balance sheet. Activity for the 2020 fiscal year is summarized as follows: Three months ended September 30, 2020 Nine months ended September 30, 2020 Balance at beginning of period $ 841 $ — Charged to expense (9 ) 1,575 Payments made (511 ) (1,254 ) Balance at end of period $ 321 $ 321
Summary of Impairment of Long Lived Assets	During the three months and nine months ended September 30, 2021, the Company had no non-cash restructuring charges. During the three months and nine months ended September 30, 2020, the Company took the following non-cash charges to research and development expense associated with the impairment of construction-in-process associated with the Fremont manufacturing facility, partial impairment of the right-of-use asset for the Fremont manufacturing facility lease (the “Fremont ROU asset”), and a write-down of laboratory equipment from the Company’s Menlo Park, CA facility: Asset write-downs Operating lease right-of-use asset $ 2,605 Construction-in-process 2,285 Laboratory equipment 760 Charge taken to research and development expense 5,650

Collaboration Agreement with _2

Collaboration Agreement with Pharming Group N.V. (Tables)	9 Months Ended
	Sep. 30, 2021
Collaborative Arrangement [Abstract]
Summary of Research and Development Costs Incurred and Collaboration Revenue Recognized	The following table summarizes research and development costs incurred and collaboration revenue recognized in connection with the Company’s performance under the Collaboration Agreement: Three months ended September 30, Nine months ended September 30, 2021 2020 2021 2020 Reimbursement revenue $ 428 $ — $ 428 $ — Upfront and milestone payment revenue 64 — 64 — Total collaboration revenue recognized $ 492 $ — $ 492 $ —

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 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 September 30, 2021: Due in: Product manufacturing commitments Dedicated manufacturing and development resources Exclusive manufacturing suites Total remaining AGC commitment 2021 (October - December) $ 1,416 $ 5,234 $ — $ 6,650 2022 2,744 8,750 3,216 14,710 2023 3,186 8,177 3,216 14,579 2024 3,186 8,177 3,216 14,579 2025 1,593 4,089 1,608 7,290 Total commitment $ 12,125 $ 34,427 $ 11,256 $ 57,808 *Tabular disclosure above has been translated to U.S. Dollar, from Euro, using an exchange rate of €1.00 to $1.18.

Nature of the Business - Additi

Nature of the Business - Additional Information (Details) $ / shares in Units, $ in Thousands	Feb. 09, 2021USD ($)shares	Sep. 30, 2021USD ($)	Sep. 30, 2021£ / shares	Feb. 09, 2021£ / shares	Feb. 09, 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		$ 6,355
Accumulated deficit		713,876				$ 605,640
Cash, cash equivalents and marketable securities		$ 254,100
Securities Purchase Agreement
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Net proceeds from private placement	$ 143,600
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	$ 400
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 ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2018
Summary Of Significant Accounting Policies [Line Items]
Foreign currency transaction gain (loss)	$ 4,000,000	$ 5,900,000	$ 2,000,000	$ (1,500,000)
Tax incentive receivable, current	18,284,000		18,284,000		$ 17,344,000
Tax incentive receivable, long term	11,942,000		11,942,000
Proceeds from lines of credit			3,000,000		3,000,000
Cash collateral associated with corporate credit cards and other leases	1,300,000		1,300,000		1,300,000
Sales of strimvelis	1,192,000	1,998,000	1,192,000	2,595,000
Liability associated with the loss contract							$ 18,400,000
Strimvelis loss provision current	552,000		552,000		916,000
Strimvelis loss provision non-current	$ 2,900,000		$ 2,900,000		3,600,000
Change in accounting principle, accounting standards update, adopted	true		true
Change in accounting principle, accounting standards update, adoption date	Jan. 1, 2021		Jan. 1, 2021
Change in accounting principle, accounting standards update, immaterial effect	true		true
New accounting pronouncement or change in accounting principle, description			ASU No. 2019-12
Product Sales, Net [Member]
Summary Of Significant Accounting Policies [Line Items]
Sales of strimvelis	$ 700,000	$ 1,998,000	$ 700,000	$ 2,595,000
Fremont Lease Agreement [Member]
Summary Of Significant Accounting Policies [Line Items]
Escrow deposit	7,900,000		7,900,000			$ 10,000,000
Increase decrease in escrow deposit			2,100,000
Fremont Lease Agreement [Member] | Prepaid Expenses and Other Current Assets [Member]
Summary Of Significant Accounting Policies [Line Items]
Escrow deposit	5,000,000		5,000,000
Fremont Lease Agreement [Member] | Other Assets [Member]
Summary Of Significant Accounting Policies [Line Items]
Escrow deposit	2,900,000		2,900,000
United Kingdom
Summary Of Significant Accounting Policies [Line Items]
Tax incentive receivable	30,200,000		30,200,000
Tax incentive receivable, current	18,300,000		18,300,000		$ 17,300,000
Tax incentive receivable, long term	$ 11,900,000		$ 11,900,000

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		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Summary Of Significant Accounting Policies [Line Items]
Foreign currency transaction gain (loss)	$ 4,000	$ 5,900	$ 2,000	$ (1,500)
United Kingdom | Research and Development Expense
Summary Of Significant Accounting Policies [Line Items]
Balance at beginning of period	25,371	32,729	17,343	28,644
Recognition of credit claims as offset to research and development expense	5,698	10,104	13,498	17,169
Receipt of credit claims		(13,558)		(14,573)
Foreign currency transaction gain (loss)	(843)	1,212	(615)	(753)
Balance at end of period	$ 30,226	$ 30,487	$ 30,226	$ 30,487

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	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Summary Of Significant Accounting Policies [Abstract]
Cash and cash equivalents	$ 74,324	$ 55,135
Restricted cash	4,266	4,266
Total cash, cash equivalents and restricted cash shown in the statement of cash flows	$ 78,590	$ 59,401	$ 45,427	$ 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		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Summary Of Significant Accounting Policies [Abstract]
Balance at beginning of period	$ 3,735	$ 4,392	$ 4,482	$ 6,790
Amortization of Strimvelis loss provision	(84)		(898)	(2,018)
Foreign currency translation	(105)	191	(38)	(189)
Balance at end of period	$ 3,546	$ 4,583	$ 3,546	$ 4,583

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	6 Months Ended	9 Months Ended
	Jun. 30, 2020	Sep. 30, 2021
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common shares attributable to anti-dilutive shares	12,848,007	15,322,535
Share options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common shares attributable to anti-dilutive shares	11,980,433	14,659,035
Unvested Performance-based Restricted Share Units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common shares attributable to anti-dilutive shares	867,574	663,500

Fair Value Measurements and M_3

Fair Value Measurements and Marketable Securities - Additional Information (Details) $ in Thousands	Sep. 30, 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 Securities - Schedule of Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Cash equivalents
Total cash equivalents	$ 48,114	$ 12,650
Marketable securities
Total marketable securities	179,819	136,813
Total	227,933	149,463
Level 1
Cash equivalents
Total cash equivalents	24,014	6,650
Marketable securities
Total	24,014	6,650
Level 2
Cash equivalents
Total cash equivalents	24,100	6,000
Marketable securities
Total marketable securities	179,819	136,813
Total	203,919	142,813
Money Market Funds
Cash equivalents
Total cash equivalents	24,014	6,650
Money Market Funds | Level 1
Cash equivalents
Total cash equivalents	24,014	6,650
Corporate Bonds
Cash equivalents
Total cash equivalents	1,002	3,001
Marketable securities
Total marketable securities	102,877	93,358
Corporate Bonds | Level 2
Cash equivalents
Total cash equivalents	1,002	3,001
Marketable securities
Total marketable securities	102,877	93,358
Commercial Paper
Cash equivalents
Total cash equivalents	23,098	2,999
Marketable securities
Total marketable securities	76,942	40,458
Commercial Paper | Level 2
Cash equivalents
Total cash equivalents	23,098	2,999
Marketable securities
Total marketable securities	$ 76,942	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 Securities - Schedule of Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 203,967	$ 142,728
Gross Unrealized Gains	7	134
Gross Unrealized Losses	(55)	(49)
Fair Value	203,919	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	103,915	96,259
Gross Unrealized Gains	4	133
Gross Unrealized Losses	(41)	(32)
Fair Value	103,878	96,360
Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	100,052	43,469
Gross Unrealized Gains	3	1
Gross Unrealized Losses	(14)	(13)
Fair Value	$ 100,041	$ 43,457

Fair Value Measurements and M_6

Fair Value Measurements and Marketable Securities - Schedule of Available-for-Sale Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Due in one year	$ 192,325	$ 132,056
Due after one year through three years	11,594	10,757
Total	$ 203,919	$ 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	Sep. 30, 2021	Dec. 31, 2020
Prepaid Expense And Other Assets Current [Abstract]
Prepaid external research and development expenses	$ 3,165	$ 1,421
Inventories	1,712	665
Other prepayments	2,453	4,930
VAT receivable	1,189	2,780
Construction deposit - current	4,968	1,552
Non-trade receivables	4,475	2,017
Total prepaid expenses and other current assets	$ 17,962	$ 13,365

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Property Plant And Equipment [Line Items]
Property and equipment	$ 10,318	$ 9,005
Less: accumulated depreciation	(5,562)	(4,224)
Property and equipment, net	4,756	4,781
Lab Equipment
Property Plant And Equipment [Line Items]
Property and equipment	5,492	5,114
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment	2,445	2,522
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment	303	304
Office and Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment	1,172	763
Construction-In-Process
Property Plant And Equipment [Line Items]
Property and equipment	$ 906	$ 302

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Property Plant And Equipment [Abstract]
Depreciation expense	$ 500	$ 500		$ 1,500	$ 1,600
Impairment charges related to construction-in-process			$ 2,300	2,285
Impairment charges related to laboratory equipment			$ 800	$ 760

Intangible Assets, Net - Schedu

Intangible Assets, Net - Schedule of Intangible Assets Net of Accumulated Amortization (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Finite Lived Intangible Assets [Line Items]
Intangible Assets, Cost	$ 4,841	$ 3,076
Intangible Assets, Accumulated Amortized	(101)
Intangible Assets, Net	4,740	3,076
License Milestones
Finite Lived Intangible Assets [Line Items]
Intangible Assets, Cost	4,841	3,076
Intangible Assets, Accumulated Amortized	(101)
Intangible Assets, Net	$ 4,740	$ 3,076

Intangible Assets, Net - Additi

Intangible Assets, Net - Additional Information (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2021USD ($)
Finite Lived Intangible Assets [Line Items]
Intangible asset, useful life	12 years
Amortization of intangible assets	$ 101
Selling, General and Administrative Expenses
Finite Lived Intangible Assets [Line Items]
Amortization of intangible assets	$ 100

Intangible Assets, Net - Sche_2

Intangible Assets, Net - Schedule of Estimated Future Amortization For Intangible Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Goodwill And Intangible Assets Disclosure [Abstract]
2021 (October - December)	$ 103
2022	410
2023	410
2024	410
2025	410
Thereafter	2,997
Intangible Assets, Net	$ 4,740	$ 3,076

Other Assets - Schedule of Othe

Other Assets - Schedule of Other Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Other Assets Noncurrent Disclosure [Abstract]
Deferred tax assets	$ 4,468	$ 5,219
Deposits	996	1,144
Deferring financing costs	708	975
Other non-current assets	3,077	1,554
Construction deposits - long-term	2,975	6,572
Total other assets	$ 12,224	$ 15,464

Accrued Expenses and Other Cu_3

Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Accrued Liabilities And Other Liabilities [Abstract]
Accrued external research and development expenses	$ 11,599	$ 8,878
Accrued payroll and related expenses, including severance	10,100	11,881
Accrued professional fees	993	791
Accrued other	2,671	3,401
Accrued milestone payments	2,585	3,076
Strimvelis loss provision current	552	916
Total accrued expenses and other liabilities	$ 28,500	$ 28,943

Notes Payable - Additional Info

Notes Payable - Additional Information (Details) - USD ($)	1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	May 31, 2021	May 31, 2019	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Debt Instrument [Line Items]
Line of credit facility agreement date		May 31, 2019
Credit facility maximum borrowings	$ 100,000,000	$ 75,000,000
Proceeds from lines of credit					$ 3,000,000		$ 3,000,000
Credit facility, remaining borrowings	$ 67,000,000
Line of credit facility interest payments term	18 months	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	3.50%	4.50%
Final payment amount	$ 1,200,000	$ 1,100,000
Repayment of line of credit	$ 500,000
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			$ 700,000	$ 600,000	$ 1,800,000	$ 1,800,000
Debt instrument, effective annual interest rate					8.50%
LIBOR
Debt Instrument [Line Items]
Debt instrument, annual interest rate	5.95%	6.00%
Maximum
Debt Instrument [Line Items]
Line of credit facility amortization term	30 months
Minimum
Debt Instrument [Line Items]
Line of credit facility amortization term	18 months
Minimum | Second Tranche (Term Loan Available from July 1, 2022 to July 1, 2023)
Debt Instrument [Line Items]
Line of credit facility minimum cash financial covenant	$ 20,000,000
Minimum | Third Tranche (Term Loan Available from July 1, 2023 to July 1, 2024)
Debt Instrument [Line Items]
Line of credit facility minimum cash financial covenant	35,000,000
Initial Term Loan
Debt Instrument [Line Items]
Credit facility maximum borrowings	33,000,000	$ 25,000,000
Proceeds from lines of credit	33,000,000
Second Tranche (Term Loan Available from July 1, 2022 to July 1, 2023)
Debt Instrument [Line Items]
Credit facility maximum borrowings	$ 33,000,000
Line of Credit Facility, Covenant Terms					upon certain regulatory approvals and evidence of the Company having $100 million in cash and cash equivalent investments
Line of credit facility interest payments term	30 months
Second Tranche (Term Loan Available from July 1, 2022 to July 1, 2023) | Minimum
Debt Instrument [Line Items]
Cash and cash equivalent investments	$ 100,000,000
Third Tranche (Term Loan Available from July 1, 2023 to July 1, 2024)
Debt Instrument [Line Items]
Credit facility maximum borrowings	$ 34,000,000
Line of Credit Facility, Covenant Terms					upon evidence of the Company having $100 million in cash and cash equivalent investments and attaining a pre-specified trailing 12-month revenue target
Line of credit facility interest payments term	30 months
Third Tranche (Term Loan Available from July 1, 2023 to July 1, 2024) | Minimum
Debt Instrument [Line Items]
Cash and cash equivalent investments	$ 100,000,000

Notes Payable - Summary of Note

Notes Payable - Summary of Notes Payable (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Long Term Notes And Loans [Abstract]
Notes payable, net of issuance costs	$ 32,647	$ 24,659
Less: current portion		(4,861)
Notes payable, net of current portion	32,647	19,798
Accretion related to final payment	139	406
Notes payable, long term	$ 32,786	$ 20,204

Notes Payable - Summary of Esti

Notes Payable - Summary of Estimated Future Principal Payments Due (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Long Term Debt By Maturity [Abstract]
2022	$ 785
2023	9,429
2024	9,429
2025	9,429
Thereafter	5,083
Total	34,155
Less unamortized portion of final payment	(1,016)
Less unamortized debt issuance costs	(353)
Notes payable, long term	$ 32,786	$ 20,204

Share-Based Compensation - Addi

Share-Based Compensation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Apr. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted-average grant date fair value of share options granted				$ 3.19
Number of shares granted				47,500
Compensation cost recognized		$ 5,389	$ 5,663	$ 17,198	$ 21,221
Total unrecognized compensation cost of stock option		45,100		$ 45,100
Compensation cost expected to be recognized weighted average period				2 years 10 months 24 days
Total unrecognized compensation cost		3,300		$ 3,300
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]
Maximum aggregate total fair value of outstanding RSUs		$ 3,300		3,300
Compensation cost recognized				$ 300
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
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
2018 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Remaining shares available for issuance		6,429,862		6,429,862
2018 ESPP
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Remaining shares available for issuance		1,341,016		1,341,016
Inducement Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Remaining shares available for issuance		721,500		721,500

Share-Based Compensation - Summ

Share-Based Compensation - Summary of Option Activity (Details) - $ / shares	9 Months Ended
	Sep. 30, 2021
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Options, Outstanding at December 31, 2020	13,895,643
Number of Options, Granted	8,104,206
Number of Options, Exercised	(1,691,435)
Number of Options, Forfeited	(2,467,243)
Number of Options, Outstanding at June 30, 2021	17,841,171
Number of Options, Vested and expected to vest, as of June 30, 2021	17,841,171
Number of Options, Exercisable, June 30, 2021	7,334,390
Weighted Average Exercise Price, Outstanding at December 31, 2020	$ 7.96
Weighted Average Exercise Price, Granted	4.89
Weighted Average Exercise Price, Exercised	1.62
Weighted Average Exercise Price, Forfeited	10.92
Weighted Average Exercise Price, Outstanding at June 30, 2021	6.76
Weighted Average Exercise Price, Vested and expected to vest, as of June 30, 2021	6.76
Weighted Average Exercise Price, Exercisable, June 30, 2021	$ 6.95

Share-Based Compensation - Su_2

Share-Based Compensation - Summary of Award Activity (Details)	9 Months Ended
	Sep. 30, 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	47,500
RSUs, Vested	(131,334)
RSUs, Forfeited	(114,000)
RSUs, Unvested and outstanding at June 30, 2021	446,166
Weighted Average Grant Date Fair Value, Unvested and outstanding at December 31, 2020 | $ / shares	$ 8.75
Weighted Average Grant Date Fair Value, Granted | $ / shares	4.94
Weighted Average Grant Date Fair Value, Vested | $ / shares	9.15
Weighted Average Grant Date Fair Value, Forfeited | $ / shares	9.94
Weighted Average Grant Date Fair Value, Unvested and outstanding at June 30, 2021 | $ / shares	$ 7.89
Performance-based Restricted Share Units ("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	(51,500)
RSUs, Unvested and outstanding at June 30, 2021	322,833
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	47,500
RSUs, Vested	(41,667)
RSUs, Forfeited	(62,500)
RSUs, Unvested and outstanding at June 30, 2021	123,333

Share-Based Compensation - Shar

Share-Based Compensation - Share-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation	$ 5,389	$ 5,663	$ 17,198	$ 21,221
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation	2,096	2,617	7,106	8,398
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total share-based compensation	$ 3,293	$ 3,046	$ 10,092	$ 12,823

Restructuring Charges - Additio

Restructuring Charges - Additional Information (Details) - USD ($)	May 04, 2020	Dec. 31, 2020	May 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Restructuring Cost And Reserve [Line Items]
Restructuring charges				$ (9,000)		$ 0	$ 1,575,000
Construction-in-process, impairment charges					$ 2,300,000	2,285,000
Operating lease right-of-use-assets		$ 29,815,000				26,267,000
Right of use asset, impairment charge						$ 2,605,000
Research and Development Expense
Restructuring Cost And Reserve [Line Items]
Employee termination benefits							1,100,000
Selling, General and Administrative Expenses
Restructuring Cost And Reserve [Line Items]
Employee termination benefits							$ 500,000
Menlo Park, California
Restructuring Cost And Reserve [Line Items]
Number of positions eliminated, percent	25.00%
Fremont, California
Restructuring Cost And Reserve [Line Items]
Construction-in-process, impairment charges			$ 2,300,000
Operating lease right-of-use-assets			$ 13,800,000
Discount rate			4.60%
Fair value of ROU asset			$ 11,200,000
Right of use asset, impairment charge		$ 0	2,600,000
Fremont, California | Minimum
Restructuring Cost And Reserve [Line Items]
Undiscounted cash flow estimate			11,700,000
Fremont, California | Maximum
Restructuring Cost And Reserve [Line Items]
Undiscounted cash flow estimate			$ 19,100,000

Restructuring Charges - Summary

Restructuring Charges - Summary of Cash Restructuring Charges (Details) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Restructuring And Related Activities [Abstract]
Balance at beginning of period	$ 841,000
Charged to expense	(9,000)	$ 0	$ 1,575,000
Payments made	(511,000)		(1,254,000)
Balance at end of period	$ 321,000		$ 321,000

Restructuring Charges - Summa_2

Restructuring Charges - Summary of Impairment of Long Lived Assets (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Jun. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Restructuring And Related Activities [Abstract]
Operating lease right-of-use asset		$ 2,605
Construction-in-process	$ 2,300	2,285
Laboratory equipment, Asset write-downs	$ 800	760
Charge taken to research and development expense		$ 5,650	$ 5,650

License Agreements - Additional

License Agreements - Additional Information (Details) € in Millions, £ in Millions, $ in Millions	1 Months Ended				3 Months Ended	9 Months Ended
	Apr. 30, 2020USD ($)shares	May 31, 2019USD ($)	May 31, 2019EUR (€)	Apr. 30, 2018Program	Jun. 30, 2018USD ($)	Sep. 30, 2021GBP (£)shares	Sep. 30, 2021EUR (€)shares	Dec. 31, 2020shares	Nov. 30, 2018shares	Sep. 30, 2018shares	Nov. 30, 2016shares
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Ordinary Shares, Issued | shares						124,770,519	124,770,519	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%
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 | €							€ 31
Upfront payment description						The Company is also required to make milestone payments contingent upon achievement of certain development, regulatory and commercial milestones. Additionally, the Company will be required to pay Telethon a tiered mid-single to low-double digit royalty percentage on annual net sales of licensed products.
Upfront and milestone payments | €			€ 15
In-process research and development expense for upfront and milestone payments | $		$ 17.2
Oxford BioMedica License, Development and Supply Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Ordinary Shares, Issued | shares	75,413								150,826	150,826	588,220
Milestone payment | $	$ 0.8

Collaboration Agreement with _3

Collaboration Agreement with Pharming Group N.V. - Additional Information (Details) - USD ($)	Jul. 01, 2021	Mar. 31, 2021	Sep. 30, 2020	Sep. 30, 2021
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total consideration		$ 143,711,000
Shares issued as part of license agreement			$ 791,000
Strategic Collaboration Agreement with Pharming
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Cash upfront payment received	$ 10,000,000
Non-refundable up-front payment	10,000,000
Strategic Collaboration Agreement with Pharming | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments receivable	189,500,000
Share Purchase Agreement with Pharming
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total consideration	$ 7,500,000
Issuance of ordinary shares	1,227,738
Common stock consideration	$ 4,100,000
Premium consideration	3,400,000
Pharming Agreements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total upfront payments	17,500,000
Upfront payments attributed to equity sold	4,100,000
Upfront payments attributed to research activities	13,400,000
Shares issued as part of license agreement	4,100,000
Excess proceeds from fair value of shares issued allocated to collaboration agreement	$ 3,400,000
Collaborative Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contract liabilities				$ 12,900,000
Collaborative Arrangement | Accounts Receivable
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accounts receivable				$ 400,000

Collaboration Agreement with _4

Collaboration Agreement with Pharming Group N.V. - Summary of Research and Development Costs Incurred and Collaboration Revenue Recognized (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenues	$ 1,192	$ 1,998	$ 1,192	$ 2,595
Collaborative Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenues	492		492
Collaborative Arrangement | Reimbursement Revenue
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenues	428		428
Collaborative Arrangement | Upfront and Milestone Payment Revenue
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenues	$ 64		$ 64

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Tax Disclosure [Abstract]
Income tax expense (benefit)	$ (1,133)	$ (541)	$ 704	$ (816)

Commitments and Contingencies -

Commitments and Contingencies - Schedule of Annual Commitments Associated with the Contract (Details) $ in Thousands	Sep. 30, 2021USD ($)
Other Commitments [Line Items]
2021 (October - December)	$ 6,650
2022	14,710
2023	14,579
2024	14,579
2025	7,290
Total commitment	57,808
Product Manufacturing Commitments
Other Commitments [Line Items]
2021 (October - December)	1,416
2022	2,744
2023	3,186
2024	3,186
2025	1,593
Total commitment	12,125
Dedicated Manufacturing And Development Resources
Other Commitments [Line Items]
2021 (October - December)	5,234
2022	8,750
2023	8,177
2024	8,177
2025	4,089
Total commitment	34,427
Exclusive Transduction Suites
Other Commitments [Line Items]
2022	3,216
2023	3,216
2024	3,216
2025	1,608
Total commitment	$ 11,256

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Annual Commitments Associated with the Contract (Parenthetical) (Details)	Sep. 30, 2021
Commitments And Contingencies Disclosure [Abstract]
Exchange rate, Euro to U.S. Dollar	1.18

Employee Benefit Plans - Additi

Employee Benefit Plans - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Compensation And Retirement Disclosure [Abstract]
Maximum annual contribution employer matches per employee, percent			6.00%
Contribution expenses	$ 0.5	$ 0.3	$ 1.4	$ 1.3

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Details)	1 Months Ended	3 Months Ended		9 Months Ended
	Apr. 30, 2018Program	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Dec. 31, 2020USD ($)
Related Party Transaction [Line Items]
Research and development		$ 20,846,000	$ 14,678,000	$ 63,631,000	$ 71,082,000
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, accrued expenses						$ 100,000
Related party transaction expense				0
Payments on accounts payable				100,000	$ 5,800,000
GSK | GSK Asset Purchase and License Agreement
Related Party Transaction [Line Items]
Research and development				$ 1,300,000