Autolus Therapeutics (AUTL) 20-F 4 Mar 21 2020 FY Annual report (foreign) Financial data

Cover Page

Cover Page	12 Months Ended
	Dec. 31, 2020shares
Document Information [Line Items]
Document Type	20-F
Document Registration Statement	false
Document Annual Report	true
Current Fiscal Year End Date	--12-31
Document Period End Date	Dec. 31, 2020
Document Transition Report	false
Document Shell Company Report	false
Entity File Number	001-38547
Entity Registrant Name	Autolus Therapeutics plc
Entity Incorporation, State or Country Code	X0
Entity Address, Address Line One	Forest House
Entity Address, Address Line Two	58 Wood Lane
Entity Address, City or Town	London
Entity Address, Postal Zip Code	W12 7RZ
Entity Address, Country	GB
Title of 12(b) Security	American Depository Shares, each representing one ordinary share, nominal value $0.000042 per share
Trading Symbol	AUTL
Security Exchange Name	NASDAQ
Entity Common Stock, Shares Outstanding	52,346,231
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
ICFR Auditor Attestation Flag	false
Document Accounting Standard	U.S. GAAP
Entity Shell Company	false
Document Fiscal Period Focus	FY
Document Fiscal Year Focus	2020
Amendment Flag	false
Entity Central Index Key	0001730463
Business Contact
Document Information [Line Items]
Entity Address, Address Line One	58 Wood Lane
Entity Address, City or Town	London
Entity Address, Postal Zip Code	W12 7RZ
Entity Address, Country	GB
Contact Personnel Name	Christian Itin
City Area Code	+44 20
Local Phone Number	3829 6230
Contact Personnel Email Address	ir@autolus.com

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Current assets:
Cash	$ 153,299,000	$ 210,643,000
Restricted cash	786,000	787,000
Prepaid expenses and other current assets	42,899,000	37,826,000
Total current assets	196,984,000	249,256,000
Non-current assets:
Property and equipment, net	38,046,000	28,164,000
Prepaid expenses and other non-current assets	3,033,000	0
Right of use asset, net	51,637,000	23,409,000
Long-term deposits	2,625,000	2,040,000
Deferred tax asset	1,754,000	410,000
Intangible assets, net	158,000	254,000
Total assets	294,237,000	303,533,000
Current liabilities:
Accounts payable	2,263,000	1,075,000
Accrued expenses and other liabilities	27,781,000	21,398,000
Lease liability	3,590,000	2,511,000
Total current liabilities	33,634,000	24,984,000
Non-current liabilities:
Lease liability	50,571,000	23,710,000
Total liabilities	84,205,000	48,694,000
Shareholders' equity:
Additional paid-in capital	595,016,000	500,560,000
Accumulated other comprehensive loss	(5,861,000)	(8,691,000)
Accumulated deficit	(379,244,000)	(237,150,000)
Total shareholders' equity	210,032,000	254,839,000
Total liabilities and shareholders' equity	294,237,000	303,533,000
Ordinary shares
Shareholders' equity:
Shares value	3,000	2,000
Deferred Shares
Shareholders' equity:
Shares value	0	0
Deferred B shares
Shareholders' equity:
Shares value	118,000	118,000
Deferred C shares
Shareholders' equity:
Shares value	$ 0	$ 0

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parentheticals)	Dec. 31, 2020$ / sharesshares	Dec. 31, 2020£ / sharesshares	Dec. 31, 2019$ / sharesshares
Ordinary shares
Shareholders' equity:
Common Stock, Par or Stated Value Per Share | $ / shares	$ 0.000042		$ 0.000042
Common stock, shares authorized (shares)	200,000,000	200,000,000	200,000,000
Common stock, shares, issued (shares)	52,346,231	52,346,231	44,983,006
Common stock, shares outstanding (shares)	52,346,231	52,346,231	44,983,006
Deferred Shares
Shareholders' equity:
Common Stock, Par or Stated Value Per Share | (per share)	$ 0.00001	£ 0.00001	$ 0.00001
Common stock, shares authorized (shares)	34,425	34,425	34,425
Common stock, shares, issued (shares)	34,425	34,425	34,425
Common stock, shares outstanding (shares)	34,425	34,425	34,425
Deferred B shares
Shareholders' equity:
Common Stock, Par or Stated Value Per Share | (per share)	$ 0.00099	£ 0.00099	$ 0.00099
Common stock, shares authorized (shares)	88,893,548	88,893,548	88,893,548
Common stock, shares, issued (shares)	88,893,548	88,893,548	88,893,548
Common stock, shares outstanding (shares)	88,893,548	88,893,548	88,893,548
Deferred C shares
Shareholders' equity:
Common Stock, Par or Stated Value Per Share | (per share)	$ 0.000008	£ 0.000008	$ 0.000008
Common stock, shares authorized (shares)	1	1	1
Common stock, shares, issued (shares)	1	1	1
Common stock, shares outstanding (shares)	1	1	1

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($)	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019
Operating expenses:
Research and development	$ (17,713,000)	$ (36,150,000)	$ (134,888,000)	$ (105,418,000)
General and administrative	(7,593,000)	(22,790,000)	(34,972,000)	(39,452,000)
Loss on impairment of leasehold improvements	0	0	0	(4,102,000)
Total operating expenses, net	(25,010,000)	(57,533,000)	(168,145,000)	(146,064,000)
Other income (expense):
Interest income	660,000	1,532,000	536,000	2,542,000
Other income (expense)	1,097,000	3,970,000	1,352,000	4,514,000
Total other income, net	1,757,000	5,502,000	1,888,000	7,056,000
Net loss before income tax	(23,253,000)	(52,031,000)	(166,257,000)	(139,008,000)
Income tax benefit	2,605,000	7,280,000	24,163,000	15,159,000
Net loss attributable to ordinary shareholders	(20,648,000)	(44,751,000)	(142,094,000)	(123,849,000)
Other comprehensive (loss) income:
Foreign currency exchange translation adjustment	(5,568,000)	(6,071,000)	2,830,000	6,797,000
Total comprehensive loss	$ (26,216,000)	$ (50,822,000)	$ (139,264,000)	$ (117,052,000)
Basic and diluted net loss per ordinary share (usd per share)	$ (0.52)	$ (1.42)	$ (2.76)	$ (2.88)
Weighted-average basic and diluted ordinary shares (shares)	39,366,634	31,557,034	51,558,075	43,065,542
Grant
Revenues	$ 296,000	$ 1,407,000	$ 1,473,000	$ 2,908,000
License
Revenues	$ 0	$ 0	$ 242,000	$ 0

Consolidated Statements of Shar

Consolidated Statements of Shareholders’ Equity Statement - USD ($) $ in Thousands	Total	Ordinary shares	Common sharesOrdinary shares	Common sharesDeferred Shares	Common sharesDeferred B shares	Common sharesDeferred C Shares	Additional Paid in Capital	Accumulated other comprehensive loss	Accumulated deficit
Beginning balance at Sep. 30, 2017	$ 142,601		$ 1	$ 0	$ 0	$ 0	$ 194,351	$ (3,849)	$ (47,902)
Beginning balance (shares) at Sep. 30, 2017			29,962,742	0	0	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of ordinary shares, net of issuance costs	156,803		$ 1		$ 118		156,802
Issuance, net of issuance cost (shares)			10,183,440	34,425	88,893,548	1
Share-based compensation expense	6,765						6,765
Unrealized gain on foreign currency translation	(6,071)							(6,071)
Net loss	(44,751)								(44,751)
Ending, balance at Sep. 30, 2018	255,465		$ 2	$ 0	$ 118	$ 0	357,918	(9,920)	(92,653)
Ending balance (shares) at Sep. 30, 2018			40,146,182	34,425	88,893,548	1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of ordinary shares, net of issuance costs	3,393						3,393
Restricted shares - forfeited (shares)			(565)
Unrealized gain on foreign currency translation	(5,568)							(5,568)
Net loss	(20,648)								(20,648)
Ending, balance at Dec. 31, 2018	232,642		$ 2	$ 0	$ 118	$ 0	361,311	(15,488)	(113,301)
Ending balance (shares) at Dec. 31, 2018			40,145,617	34,425	88,893,548	1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of ordinary shares, net of issuance costs	108,815						108,815
Issuance, net of issuance cost (shares)			4,830,000
Share-based compensation expense	30,386						30,386
Restricted shares - forfeited (shares)			(4,335)
Exercise of stock options	$ 48						48
Exercise of stock options (shares)	11,724		11,724
Unrealized gain on foreign currency translation	$ 6,797							6,797
Net loss	(123,849)								(123,849)
Ending, balance at Dec. 31, 2019	254,839		$ 2	$ 0	$ 118	$ 0	500,560	(8,691)	(237,150)
Ending balance (shares) at Dec. 31, 2019			44,983,006	34,425	88,893,548	1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of ordinary shares, net of issuance costs	73,953		$ 1				73,952
Issuance, net of issuance cost (shares)			7,250,000
Share-based compensation expense	20,021						20,021
Restricted shares - forfeited (shares)			(1,969)
Exercise of stock options	$ 483						483
Exercise of stock options (shares)	115,194		115,194
Unrealized gain on foreign currency translation	$ 2,830							2,830
Net loss	(142,094)								(142,094)
Ending, balance at Dec. 31, 2020	$ 210,032		$ 3	$ 0	$ 118	$ 0	$ 595,016	$ (5,861)	$ (379,244)
Ending balance (shares) at Dec. 31, 2020		52,346,231	52,346,231	34,425	88,893,548	1

Consolidated Statement of Cash

Consolidated Statement of Cash Flows - USD ($)	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2018
Cash flows from operating activities:
Net loss	$ (20,648,000)	$ (44,751,000)	$ (142,094,000)	$ (123,849,000)	$ (44,751,000)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	645,000	1,709,000	5,658,000	4,611,000
Loss on disposal of fixed assets and intangible assets	394,000	8,000	0	43,000
Non-cash share-based compensation (net of amount capitalized)	3,393,000	6,765,000	20,107,000	30,212,000
Gain on lease incentive and reassessment	0	0	(1,335,000)	0
Loss on impairment of leasehold improvements	0	0	0	4,102,000
Gain on termination of operating lease	0	0	(160,000)	0
Deferred income tax	0	0	(1,344,000)	(399,000)
Changes in operating assets and liabilities
Prepaid expenses and other current assets	(3,521,000)	(7,132,000)	(2,660,000)	(21,008,000)
Prepaid expenses and other non-current assets	0	0	(2,637,000)	0
Long-term deposits	(1,285,000)	0	(508,000)	(702,000)
Accounts payable	(723,000)	838,000	1,439,000	2,551,000
Right of use assets, net	(723,000)	838,000	3,670,000	(1,451,000)
Accrued expenses and other liabilities	1,960,000	11,026,000	5,608,000	4,958,000
Lease liabilities	0	0	(3,502,000)	(552,000)
Net cash used in operating activities	(19,785,000)	(31,537,000)	(117,758,000)	(101,484,000)
Cash flows from investing activities:
Purchases of property and equipment	(4,422,000)	(9,119,000)	(14,681,000)	(18,341,000)
Purchase of intangible assets	0	(412,000)	0	(327,000)
Net cash used in investing activities	(4,422,000)	(9,531,000)	(14,681,000)	(18,668,000)
Cash flows from financing activities:
Proceeds of issuance of ordinary shares, net of issuance costs	0	156,920,000	74,415,000	108,863,000
Net cash provided by financing activities	0	156,920,000	74,415,000	108,863,000
Effect of exchange rate changes on cash	(5,327,000)	(5,833,000)	679,000	5,164,000
Net (decrease) increase in cash	(29,534,000)	110,019,000	(57,345,000)	(6,125,000)
Cash and restricted cash, beginning of period	247,089,000		211,430,000	217,555,000	137,070,000
Cash and restricted cash, end of period	217,555,000	247,089,000	154,085,000	211,430,000	247,089,000
Supplemental cash flow information
Cash paid for taxes	0	0	1,841,000	1,535,000
Supplemental non-cash flow information
Property and equipment purchases included in accounts payable or accrued	3,343,000	328,000	2,499,000	2,818,000
Leased assets terminated and obtained in exchange for operating lease liabilities, net	0	0	2,487,000	0
Reduction in right of use asset	0	0	0	1,919,000
Leased assets obtained in exchange for operating lease liabilities	0	0	26,956,000	0
Capitalized share-based compensation, net of forfeitures	0	0	(86,000)	174,000
Capitalized implementation costs included in accrued expenses	0	0	144,000	0
Reconciliation of cash and restricted cash reported within the consolidated balance sheets
Cash						$ 153,299,000	$ 210,643,000	$ 217,450,000	$ 246,984,000
Short-term restricted cash						786,000	787,000	105,000	105,000
Total cash and restricted cash	$ 247,089,000	$ 247,089,000	$ 211,430,000	$ 217,555,000	$ 247,089,000	$ 154,085,000	$ 211,430,000	$ 217,555,000	$ 247,089,000

Nature of the Business

Nature of the Business	12 Months Ended
	Dec. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of the Business	Nature of the Business Autolus Therapeutics plc (the “Company”) is a biopharmaceutical company developing next-generation programmed T cell therapies for the treatment of cancer. Using its broad suite of proprietary and modular T cell programming technologies, the Company is engineering precisely targeted, controlled and highly active T cell therapies that are designed to better recognize cancer cells, break down their defense mechanisms and attack and kill these cells. The Company believes its programmed T cell therapies have the potential to be best-in-class and offer cancer patients substantial benefits over the existing standard of care, including the potential for cure in some patients. The Company is a public limited company incorporated in England and Wales. On June 22, 2018, the Company completed its initial public offering ("IPO") of American Depositary Shares (“ADSs”). In the IPO, the Company sold an aggregate of 10,147,059 ADSs representing the same number of ordinary shares, including 1,323,529 ADSs pursuant to the underwriters’ option to purchase additional ADSs, at a public offering price of $17.00 per ADS. Net proceeds were $156.5 million, after deducting underwriting discounts and commissions and offering expenses paid by the Company. On April 15, 2019, the Company completed an underwritten public offering of 4,830,000 ADSs representing 4,830,000 ordinary shares, at a public offering price of $24.00 per ADS, which includes an additional 630,000 ADSs issued upon the exercise in full of the underwriters’ option to purchase additional ADSs. Aggregate net proceeds to the Company, after underwriting discounts and offering expenses, were $108.8 million. On January 27, 2020, the Company completed an underwritten public offering of 7,250,000 ADSs representing 7,250,000 ordinary shares, at a public offering price of $11.00 per ADS. Aggregate net proceeds to the Company, after underwriting discounts, were $75.0 million. The Company is a continuation of Autolus Limited and its subsidiaries. In connection with the IPO, the Company completed a corporate reorganization, which has been accounted for as a combination of entities under common control. The corporate reorganization has been given retrospective effect in these financial statements and such financial statements represent the financial statements of Autolus Therapeutics plc. In connection with the corporate reorganization, outstanding restricted share awards and option grants of Autolus Limited were exchanged for share awards and option grants of Autolus Therapeutics plc with identical restrictions. The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, dependence on key personnel, protection of proprietary technology, compliance with government regulations and the ability to secure additional capital to fund operations. Product candidates currently under development will require significant additional research and development efforts, including preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of capital, adequate personnel and infrastructure and extensive compliance-reporting capabilities. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will realize revenue from its product sales. The Company has funded its operations primarily with proceeds from the sale of its equity securities. The Company has incurred recurring losses since its inception, including net losses of $142.1 million and $123.8 million for the years ended December 31, 2020 and 2019, $20.6 million for the three months ended December 31, 2018, and $44.8 million for the year ended September 30, 2018, respectively. In addition, as of December 31, 2020 and 2019, the Company had an accumulated deficit of $379.2 million and $237.2 million, respectively. The Company expects to continue to generate operating losses for the foreseeable future. The future viability of the Company beyond that point is dependent on its ability to raise additional capital to finance its operations. The Company’s inability to raise additional capital as and when needed could have a negative impact on its financial condition and ability to pursue its business strategies. There can be no assurances, however, that the current operating plan will be achieved or that additional funding will be available on terms acceptable to the Company, or at all. The Company believes the cash on hand at December 31, 2020 of $153.3 million, with the additional net proceeds from sale of ADSs under the Company’s at-the market facility program in January 2021 and its follow-on capital raise in February 2021 of $123.4 million, will be sufficient to fund the Company’s operations for at least 12 months from the issuance date of these financial statements.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies Basis of Presentation The accompanying consolidated financial statements include those of the Company, Autolus Limited, and its U.S. subsidiary, Autolus Inc., and have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”). All intercompany accounts and transactions have been eliminated upon consolidation. Use of Estimates The preparation of consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of income and expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual for research and development expenses, the fair value of ordinary shares, share-based compensation and income taxes. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results could differ materially from those estimates. Cash and Cash Equivalents The Company considers cash and cash equivalents in the consolidated financial statements to include cash at banks with a maturity of less than three months, which is subject to an insignificant risk of changes in value. Restricted Cash The Company entered into a lease that requires a letter of credit supported by $0.6 million deposit held by the Company's bank for the duration of the lease and a credit card arrangement that requires a security deposit of $0.2 million. 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 Company's consolidated statements of cash flows. Fair Value Measurements The carrying amounts reported in the balance sheets for cash, prepaid expenses and other assets, accounts payable and accrued expenses and other liabilities approximate their fair value because of the short-term nature of these instruments. Concentration of Credit Risk Financial instruments that subject the Company to credit risk consist primarily of cash and cash equivalents. The Company places cash and cash equivalents in established financial institutions. The Company has no significant off-balance-sheet risk or concentration of credit risk, such as foreign exchange contracts, options contracts, or other foreign hedging arrangements. Implementation Costs in a Cloud Computing Arrangement The Company’s cloud computing arrangements primarily comprise hosting arrangements which are service contracts, whereby the Company gains remote access to use enterprise software hosted by the vendor or another third party on an as-needed basis for a period of time in exchange for a subscription fee. Implementation costs for cloud computing arrangements are capitalized if certain criteria are met and consist of internal and external costs directly attributable to developing and configuring cloud computing software for its intended use. These capitalized implementation costs are presented in the condensed consolidated balance sheet in prepaid expenses and other assets, current and non-current, and are generally amortized over the fixed, non-cancellable term of the associated hosting arrangement on a straight-line basis. Property and Equipment Property and equipment are recorded at cost and depreciated or amortized using the straight-line method over the estimated useful lives of the respective assets. As of December 31, 2020 and 2019, the Company’s property and equipment consisted of office equipment, lab equipment, furniture and fixtures, and leasehold improvements. The office equipment has an estimated useful life of three years, lab equipment has an estimated useful life of five shorter of the lease term or the estimated useful life of the asset. Upon retirement or sale, the cost of assets disposed of, and the related accumulated depreciation, are removed from the accounts and any resulting gain or loss is included in the statement of operations and other comprehensive loss. Repairs and maintenance expenditures, which are not considered improvements and do not extend the useful life of property and equipment, are expensed as incurred. The Company evaluates an asset for potential impairment when events or changes in circumstances indicate the carrying value of the asset may not be recoverable. Recoverability is measured by comparing the book value of the asset to the expected future net undiscounted cash flows that the asset is expected to generate. If such asset is considered to be impaired, the impairment to be recognized is measured by the amount by which the book value of the asset exceeds the fair value. The Company did not recognize an impairment nor a disposal for the year ended December 31, 2020, but did recognize a loss on impairment of $4.1 million for the year ended December 31, 2019 related to leasehold improvements, as the Company chose to discontinue the fit-out of the Company's manufacturing capacity at the Enfield, U.K. facility. There were no impairments or disposals for the three months ended December 31, 2018, and an asset disposal of less than $10,000 for the year ended September 30, 2018. The Company routinely evaluates the useful life attributed to its assets. During the second quarter ended June 30, 2019, the Company determined that the useful lives of certain lab equipment should be increased from five-years to ten-years based on expectation of future usability. The Company accounted for this as a change in estimate that was applied prospectively, effective April 1, 2019. This change in useful life resulted in a reduction of depreciation expense of $0.3 million, and an increase in basic and diluted earnings per share of $0.01, for the year ended December 31, 2019. Leases Effective January 1, 2019, the Company adopted Accounting Standards Codification (“ASC”), Topic 842, Leases (“ASC 842”), using the required modified retrospective approach and utilizing the effective date as its date of initial application, for which prior periods are presented in accordance with the previous guidance in ASC 840, Leases. At the inception of an arrangement, the Company determines whether the arrangement is or contains a lease based on the unique facts and circumstances present. Most leases with a term greater than one year are recognized on the balance sheet as right-of-use assets, lease liabilities and, if applicable, long-term lease liabilities. The Company has elected not to recognize on the balance sheet leases with terms of one year or less. Operating lease liabilities and their corresponding right-of-use assets are recorded based on the present value of lease payments over the expected remaining lease term. However, certain adjustments to the right-of-use asset may be required for items such as incentives received, initial direct costs, or prepayments. The interest rate implicit in lease contracts is typically not readily determinable. As a result, the Company utilizes its incremental borrowing rates, which are the rates incurred to borrow on a collateralized basis over a similar term an amount equal to the lease payments in a similar economic environment. In accordance with the guidance in ASC 842, 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, consumables, etc.), and non-components ( e.g. , property taxes, insurance, etc.). Then the fixed and in-substance fixed contract consideration (including any related to non-components) must be allocated based on the respective relative 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. Entities may elect the practical expedient to 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. For new and amended leases beginning in 2019, the Company elected the practical expedients to account for the lease and non-lease components for leases for classes of all underlying assets and allocate all of the contract consideration to the lease component only. The Company determined the underlying lease to be the predominant component, and therefore, the entire agreement was accounted for under ASC 842. Intangible Assets Subject to Amortization The Company’s intangible assets have been related to acquired software licenses with finite lives are amortized over their useful lives and reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of the asset may not be recoverable. If any indicators were present, the Company would test for recoverability by comparing the carrying amount of the asset to the net undiscounted cash flows expected to be generated from the asset. If those net undiscounted cash flows do not exceed the carrying amount (i.e., the asset is not recoverable), the Company would perform the next step, which is to determine the fair value of the asset and record an impairment loss, if any. The Company evaluates the useful lives for these intangible assets each reporting period to determine whether events and circumstances warrant a revision in their remaining useful lives. The Company did not recognize an impairment loss in the years ended December 31, 2020 and 2019. The Company recognized an impairment loss of $0.4 million for the three months ended December 31, 2018 related to software which the Company elected to discontinue. The Company did not recognize an impairment loss in the year ended September 30, 2018. Segment Information Operating segments are defined as components of an enterprise about which separate discrete information is available for evaluation by the chief operating decision maker in deciding how to allocate resources and assess performance. The Company and the Company’s chief operating decision maker, the Company’s Chief Executive Officer, view the Company’s operations and manages its business as a single operating segment, which is the business of developing and commercializing gene therapies. The Company operates in two geographic regions: the United Kingdom and the United States. A majority of the Company’s assets are held in the United Kingdom. Deferred Rent and Lease Incentives Prior to the adoption of ASC 842, rent expense and lease incentives from operating leases were recognized on a straight-line basis over the lease term. The Company has operating leases that include rent escalation payment terms and a rent-free period. Deferred rent represents the difference between actual operating lease payments and straight-line rent expense over the term of the lease. Upon adoption of ASC 842, the Company no longer records or presents deferred rent. 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, depreciation expense, third-party license fees, external costs of outside vendors engaged to conduct clinical development activities, clinical trials, costs to manufacture clinical trial materials and certain tax credits associated with research and development activities. The Company recorded the U.K.'s research and development expenditure credit ("RDEC") of $0.1 million and $0.2 million for the years ended December 31, 2020 and 2019, $37,000 for the three months ended December 31, 2018, and $0.2 million for the year ended September 30, 2018, respectively, as reductions of research and development expenses within the Company’s statement of operations and comprehensive loss. Accrued Research and Development Expenses As part of the process of preparing consolidated financial statements, the Company is required to estimate accruals for research and development expenses. This process involves reviewing and identifying services which have been performed by third parties on the Company’s behalf and determining the value of these services. In addition, the Company makes estimates of costs incurred to date but not yet invoiced, in relation to external clinical research organizations and clinical site costs. The Company analyzes the progress of clinical trials, including levels of patient enrollment; invoices received and contracted costs, when evaluating the adequacy of the accrued liabilities for research and development. The Company makes judgments and estimates in determining the accrued balance in any accounting period. Share-Based Compensation The Company recognizes compensation expense for equity awards based on the grant date fair value of the award. The Company recognizes share-based compensation expense for awards granted to employees that have a graded vesting schedule based on a service condition only on a straight-line basis over the requisite service period for each separately vesting portion of the award as if the award was, in substance, multiple awards (the “graded-vesting attribution method”), based on the estimated grant date fair value for each separately vesting tranche. For equity awards with a graded vesting schedule and a combination of service and performance conditions, the Company recognizes share-based compensation expense using a graded-vesting attribution method over the requisite service period when the achievement of a performance-based milestone is probable, based on the relative satisfaction of the performance condition as of the reporting date. The Company accounts for forfeitures as they occur. For share-based awards granted to consultants and non-employees, compensation expense is recognized using the graded-vesting attribution method over the period during which services are rendered by such consultants and non-employees until completed. The measurement date for employee awards is the date of grant, and share-based compensation costs are recognized as expense over the employees’ requisite service period, which is the vesting period, on an accelerated basis. In the year ended December 31, 2019 the Company adopted Accounting Standards Update (“ASU”) No. 2018-07, “Compensation —Stock Compensation (Topic 718): Improvements to Non-employee Share-Based Payment Accounting” (“ASU No. 2018-07”), prior to which the measurement date for non-employee awards was generally the date the services were completed, resulting in financial reporting period adjustments to share-based compensation during the vesting terms for changes in the fair value of the awards. After the adoption of ASU No. 2018-07, the measurement date for non-employee awards is the later of the adoption date of ASU No. 2018-07 or the date of grant, without changes in the fair value of the award. The fair value of each share option grant is estimated on the date of grant using the Black-Scholes option pricing model. See Note 8 for the Company’s assumptions used in connection with option grants made during the periods covered by these consolidated financial statements. Assumptions used in the option pricing model include the following: Expected volatility. We lack company-specific historical and implied volatility information for our ADSs for expected terms greater than 2.5 years. Therefore, we use a combination of the historical volatility of our ADSs and also the expected share volatility based on the historical volatility of publicly traded peer companies and expect to continue to do so until such time as we have adequate historical data regarding the volatility of our own traded security price Expected term . The expected term of the Company’s share options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. Risk-free interest rate . The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods that are approximately equal to the expected term of the award. Expected dividend. Expected dividend yield of zero is based on the fact that the Company has never paid cash dividends on ordinary shares and does not expect to pay any cash dividends in the foreseeable future. Fair value of ordinary shares. Options granted after the Company’s IPO are issued at the fair market value of the Company’s ADS at the date the grant is approved by the Board. Prior to the IPO, the Company calculated the fair value of its ordinary shares in accordance with the guidelines in the American Institute of Certified Public Accountants’ Accounting and Valuation Guide, Valuation of Privately-Held-Company Equity Securities Issued as Compensation . The Company’s valuations of ordinary shares were prepared using a market approach, based on precedent transactions in the shares, to estimate the Company’s total equity value using the option-pricing method (“OPM”), which used a combination of market approaches and an income approach to estimate the Company’s enterprise value. The OPM derives an equity value such that the value indicated is consistent with the investment price, and it provides an allocation of this equity value to each class of the Company’s securities. The OPM treats the various classes of shares as call options on the total equity value of a company, with exercise prices based on the value thresholds at which the allocation among the various holders of a company’s securities changes. Under this method, each class of shares has value only if the funds available for distribution to shareholders exceed the value of the share liquidation preferences of the class or classes of shares with senior preferences at the time of the liquidity event. Key inputs and assumptions used in the OPM calculation include the following: Expected volatility. The Company applied re-levered equity volatility based on the historical unlevered and re-levered equity volatility of publicly traded peer companies. Expected dividend. Expected dividend yield of zero is based on the fact that the Company has never paid cash dividends on ordinary shares and does not expect to pay any cash dividends in the foreseeable future. Expected term . The expected term of the option or the estimated time until a liquidation event. Risk-free interest rate . The risk-free interest rate is determined by reference to the U.S. Treasury yield curve for the period commensurate with the expected of the exit event. When considering the fair value of options granted in the period prior to the IPO, the Company considered probability-weighted scenarios based on the relative likelihoods of completing the IPO and remaining a privately-held company. In the IPO scenarios, the fair value was calculated by dividing the total estimated equity value by the number of fully diluted ordinary shares outstanding, and then discounting the implied per-share value at a rate intended to approximate the Company's cost of equity between share option grant date and the expected IPO date. The stay-private scenario utilized an OPM "Backsolve" calculation to estimate its equity value implied by the purchase price of the series A preference shares in September 2017. In March and May 2018, the Company issued share option grants to employees that applied a 50% and 80% probability weighting of an IPO, respectively, to the fair value of the underlying ordinary share utilized in the Black-Scholes option pricing model. Foreign Currency Translation The Company maintains its financial statements in its functional currency, which is the pounds sterling. Monetary assets and liabilities denominated in currencies other than the functional currency are translated into the functional currency at rates of exchange prevailing at the balance sheet dates. Non-monetary assets and liabilities denominated in foreign currencies are translated into the functional currency at the exchange rates prevailing at the date of the transaction. Exchange gains or losses arising from foreign currency transactions are included in the determination of net income (loss) for the respective periods. The Company recorded foreign exchange loss of $0.2 million for the year ended December 31, 2020, foreign exchange gains of $4.6 million, $1.1 million, and $4.0 million for the year ended December 31, 2019, the three months ended December 31, 2018, and the year ended September 30, 2018, respectively, which are included in other income in the statements of operations and comprehensive loss. For financial reporting purposes, the financial statements of the Company have been translated into U.S. dollars. Assets and liabilities have been translated at the exchange rates at the balance sheet dates, while revenue and expenses are translated at the average exchange rates over the reporting period and shareholders’ equity amounts are translated based on historical exchange rates as of the date of each transaction. Translation adjustments are not included in determining net income (loss) but are included in foreign exchange adjustment to other comprehensive loss, a component of shareholders’ equity. Patent Costs The Company expenses patent prosecution and related legal costs as they are incurred and classifies such costs as general and administrative expenses in the accompanying statements of operations and comprehensive loss. The Company recorded patent expenses of $2.1 million and $2.0 million for the years ended December 31, 2020 and 2019, $0.2 million for the three months ended December 31, 2018, and $1.0 million for the year ended September 30, 2018, respectively. Grant Income The Company has received research grants under which it is reimbursed for specific research and development activities. Payments received are recognized as income in the statements of operations and comprehensive loss over the period in which the Company recognizes the related costs. At the time the Company recognizes grant income, it has complied with the conditions attached to it and the receipt of the reimbursement is reasonably assured. The Company has received grants from the U.K. government, which are repayable under certain circumstances, including breach or noncompliance. For grants with refund provisions, the Company reviews the grant to determine the likelihood of repayment. If the likelihood of repayment of the grant is determined to be remote, then the grant is recognized as grant income. The Company has determined that the likelihood of any repayment events included in its current grants is remote. License Revenue The Company accounts for its revenues pursuant to the provisions of Accounting Standards Codification (“ASC”) Topic 606, Revenue from Contracts with Customers (“ASC Topic 606”). The Company has no products approved for commercial sale and has not generated any revenue from commercial product sales. The total revenue to date has been generated from a license agreement with an investee company of one our affiliates. The terms of the agreement include a non-refundable license fee, payments based upon achievement of clinical development and regulatory objectives, and royalties on product sales. In determining the appropriate amount of revenue to be recognized as the Company fulfills its obligations under its agreements, the Company performs the following steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations, including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations based on estimated selling prices; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. License Fees and Multiple Element Arrangements If a license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company recognizes revenues from non-refundable, upfront fees allocated to the license at such time as the license is transferred to the licensee and the licensee is able to use, and benefit from, the license. For licenses that are bundled with other promises, the Company utilizes judgment to assess the nature of the combined performance obligations to determine whether the combined performance obligations are satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue from non-refundable, upfront fees. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Appropriate methods of measuring progress include output methods and input methods. In determining the appropriate method for measuring progress, the Company considers the nature of service that the Company promises to transfer to the customer. When the Company decides on a method of measurement, the Company will apply that single method of measuring progress for each performance obligation satisfied over time and will apply that method consistently to similar performance obligations and in similar circumstances. Contingent Research Milestone Payments ASC Topic 606 constrains the amount of variable consideration included in the transaction price in that either all, or a portion, of an amount of variable consideration should be included in the transaction price. The variable consideration amount should be included only to the extent that it is probable that a significant reversal in the amount of cumulative revenue recognized will not occur when the uncertainty associated with the variable consideration is subsequently resolved. The assessment of whether variable consideration should be constrained is largely a qualitative one that has two elements: the likelihood of a change in estimate, and the magnitude thereof. Variable consideration is not constrained if the potential reversal of cumulative revenue recognized is not significant, for example. If the consideration in a contract includes a variable amount, the Company will estimate the amount of consideration in exchange for transfer of promised goods or services. The consideration also can vary if the Company’s entitlement to the consideration is contingent on the occurrence or non-occurrence of a future event. The Company considers contingent research milestone payments to fall under the scope of variable consideration, which should be estimated for revenue recognition purposes at the inception of the contract and reassessed ongoing at the end of each reporting period. The Company assesses whether contingent research milestones should be considered variable consideration that should be constrained and thus not part of the transaction price. This includes an assessment of the probability that all or some of the milestone revenue could be reversed when the uncertainty around whether or not the achievement of each milestone is resolved, and the amount of reversal could be significant. GAAP provides factors to consider when assessing whether variable consideration should be constrained. All of the factors should be considered, and no factor is determinate. The Company considers all relevant factors. For the year ended December 31, 2020, the Company has not recognized any variable consideration with regards to the development milestones which are included in the license agreement that was executed in the third quarter of the year. This is due to the fact that those development milestones have not yet been met and the recognition of the related revenue is not yet probable. Income Taxes The Company accounts for income taxes under the asset and liability method which includes the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the Company’s financial statements. Under this approach, deferred taxes are recorded for the future tax consequences expected to occur when the reported amounts of assets and liabilities are recovered or paid. The provision for income taxes represents income taxes paid or payable for the current year plus deferred taxes. Deferred taxes result from differences between the financial statements and tax bases of the Company’s assets and liabilities, and are adjusted for changes in tax rates and tax law when changes are enacted. The effects of future changes in income tax laws or rates are not anticipated. The Company is subject to income taxes in the United Kingdom and the United States. The calculation of the Company’s tax provision involves the application of United Kingdom tax law and requires judgement and estimates. The Company evaluates the realizability of its deferred tax assets at each reporting date, and establishes a valuation allowance when it is more likely than not that all or a portion of its deferred tax assets will not be realized. The ultimate realization of deferred tax assets is dependent upon the generation of future taxable income of the same character and in the same jurisdiction. The Company considers all available positive and negative evidence in making this assessment, including, but not limited to, the scheduled reversal of deferred tax liabilities, projected future taxable income, and tax planning strategies. In circumstances where there is sufficient negative evidence indicating that the Company’s deferred tax assets are not more likely than not realizable, the Company establishes a valuation allowance. The Company uses a two-step approach for recognizing and measuring uncertain tax positions. The first step is to evaluate tax positions taken or expected to be taken in a tax return by assessing whether they are more likely than not sustainable, based solely on their technical merits, upon examination, and including resolution of any related appeals or litigation process. The second step is to measure the associated tax benefit or each position as the largest amount that the Company believes is more likely than not realizable. Differences between the amount of tax benefits taken or expected to be taken in the Company’s income tax returns and the amount of tax benefits recognized in the its financial statements represent the Company’s unrecognized income tax benefits, which it either records as a liability or reduction of deferred tax assets. Income Tax Credit The Company benefits from the U.K. research and development tax credit regime under both the small and medium sized enterprise, or SME, scheme and by claiming an RDEC in respect of grant funded projects. Under the SME regime, a portion of the Company’s losses can be surrendered for a cash rebate of up to 33.35% of eligible expenditures. Such credits are accounted for within the tax provision in the year in which the expenditures were incurred. Comprehensive Loss The Company follows the provisions of the Financial Accounting Standards Board (“FASB”) Accounting Standards Codification (“ASC”) Topic 220, Comprehensive Income , which establishes standards for the reporting and display of comprehensive income and its components. Comprehensive gain or loss is defined to include all changes in equity during a period except those resulting from investments by owners and distributions to owners. The Company recorded a gain of $2.8 million and $6.8 million for the years ended December 31, 2020 and 2019, a loss of $5.6 million for the three months ended December 31, 2018, and a loss $6.1 million for the year ended September 30, 2018, respectively to foreign currency translation. Net Loss per Share Basic and diluted net loss per ordinary share is determined by dividing net loss by the weighted average number of ordinary shares outstanding during the period. For all periods presented, the preferred shares and outstanding but unvested restricted shares and share options have been excluded from the calculation, because their effects would be anti-dilutive. Therefore, the weighted average shares outstanding used to calculate both basic and diluted loss per share are the same for all periods presented. The following potentially dilutive securities have been excluded from the calculation of diluted net loss per share due to their

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets	12 Months Ended
	Dec. 31, 2020
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid Expenses and Other Current Assets	Prepaid Expenses and Other Current Assets Prepaid expenses and other current assets consisted of the following (in thousands): December 31, 2020 2019 Research and development claims receivable $ 24,711 $ 27,567 Prepayments 10,206 7,023 VAT receivable 3,124 1,928 Lease incentive receivable 1,237 — Other asset — 279 Grant income receivable 414 547 Other receivable 199 482 Deferred cost 3,008 — Total prepaid expenses and other current assets $ 42,899 $ 37,826 The decrease to research and development claims receivable is due the fact that as of December 31, 2020 the research and development claim receivable is solely for the year ended December 31, 2020; however, the research and development claims receivable as of December 31, 2019 were related to a claim receivable for both the 15-month period ended December 31, 2019 and the year ended September 30, 2018. The increase in prepayments in the year ended December 31, 2020 is primarily related to $1.9 million of prepayments for facilities and manufacturing and $1.4 million for prepayments of clinical trial costs.

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2020
Property, Plant and Equipment [Abstract]
Property and Equipment, Net	Property and Equipment, Net Property and equipment, net consisted of the following (in thousands): December 31, 2020 2019 Lab equipment $ 23,491 $ 18,214 Office equipment 2,928 2,211 Furniture and fixtures 1,340 1,301 Leasehold improvements 10,629 10,316 Assets under construction 14,321 4,687 Less: accumulated depreciation (14,663) (8,565) Total property and equipment, net $ 38,046 $ 28,164

Intangible Asset

Intangible Asset	12 Months Ended
	Dec. 31, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible Asset	Intangible Asset The following table summarizes the carrying amount of the Company's intangible assets, net of accumulated amortization (in thousands): December 31, 2020 Estimated Life Cost Accumulated Amortization Net Software license 3 $ 291 $ (133) $ 158 Amortization expense for the years ended December 31, 2020 and 2019 was $91,000 and $47,000, respectively. Assuming no changes in the gross cost basis of intangible assets, the total estimated amortization expense for finite-lived intangible assets is approximately $92,000 and $66,000 for each of the years ending December 31, 2021 through December 31, 2022.

Accrued Expenses and Other Liab

Accrued Expenses and Other Liabilities	12 Months Ended
	Dec. 31, 2020
Payables and Accruals [Abstract]
Accrued Expenses and Other Liabilities	Accrued Expenses and Other Liabilities Accrued expenses and other liabilities consisted of the following (in thousands): December 31, 2020 2019 Compensation and benefits 8,732 $ 6,568 Research and development costs 15,343 10,449 UCLB milestone 205 663 Professional fees 3,005 2,611 Corporate tax — 391 Other liabilities 496 716 Total accrued expenses and other liabilities $ 27,781 $ 21,398 Other liabilities primarily consisted of the business rates for UK property in the amount of $0.4 million and $0.2 million as of December 31, 2020 and 2019, respectively. As of December 31, 2019, other liabilities also consisted of the current portion of the lease incentive liability of $0.3 million.

Shareholders_ Equity

Shareholders’ Equity	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Shareholders’ Equity	Shareholders’ Equity Ordinary Shares Each holder of ordinary shares is entitled to one vote per ordinary share and to receive dividends when and if such dividends are recommended by the board of directors and declared by the shareholders. As of December 31, 2020, the Company has not declared any dividends. Effective from June 26, 2018, the board of directors has the authority to allot new ordinary shares or to grant rights to subscribe for or to convert any security into ordinary shares in the Company up to a maximum aggregate nominal amount of $8,400. This authority runs for five years and will expire on June 26, 2023. Effective from June 26, 2018, the board also has the authority to allot ordinary shares for cash or to grant rights to subscribe for or to convert any security into ordinary shares in the Company without first offering them to existing shareholders in proportion to their existing holdings up to an aggregate maximum nominal amount of $8,400. This authority runs for five years and will expire on June 26, 2023. As of December 31, 2020, the Company’s issued capital share consisted of 52,346,231 ordinary shares, with a nominal value of $0.000042 per share, (ii) 34,425 deferred shares, with a nominal value of £0.00001 per share, (iii) 88,893,548 B deferred shares, with a nominal value of £0.00099 per share and (iv) 1 C deferred share, with a nominal value of £0.000008. Each issued share has been fully paid. Initial Public Offering and Impact of Corporate Reorganization On June 18, 2018, Autolus Therapeutics Limited re-registered as a public limited company and its name was changed from Autolus Therapeutics Limited to Autolus Therapeutics plc (see Note 1). On June 26, 2018, the Company closed its IPO. In the IPO, the Company sold an aggregate of 10,147,059 ADSs representing the same number of ordinary shares at a public offering price of $17.00 per ADS, which included the full exercise by the underwriters of their option to purchase additional ADSs. Net proceeds were approximately $156.5 million, after deducting underwriting discounts, and commissions and offering expenses paid by the Company of $16.0 million. Upon the closing of the IPO, each separate class of ordinary shares of Autolus Therapeutics plc was converted into a single class of ordinary shares of Autolus Therapeutics plc as described further below. Prior to the Company’s June 2018 reorganization and IPO, the Company had issued series A preferred shares, ordinary B shares, and ordinary C shares to fund its operations and upon the completion of the IPO, the different classes of shares were converted into a single class of ordinary shares on a 3.185-for-1 basis and created various classes of deferred shares. The following deferred share classes were created: Deferred Shares - The 34,425 deferred shares, aggregate nominal value less than $1.00, existed in Autolus Limited and were re-created in Autolus Therapeutics plc as part of the share exchange to place Autolus Therapeutics as the ultimate parent entity. The Company was required to replicate the shares to ensure the existing share has the correct nominal value to ensure stamp duty mirroring relief is available on the subsequent share for share exchange. These deferred shares have no voting rights. Deferred B Shares - The deferred shares were the product of the reorganization of the series A preferred shares and ordinary B shares into ordinary shares. The nominal residual value was utilized by management as the required £50,000 of share capital to re-register Autolus Therapeutics Limited as Autolus Therapeutics plc. The resulting 88,893,548 deferred shares, aggregate nominal value of $118,000, is presented as a separate class of equity on the balance sheet and statement of shareholder’s equity. These deferred B shares have no voting rights. Deferred C Share - The deferred share, nominal value less than $1.00, was created when the shares in the Company were redenominated from pounds sterling to U.S. Dollars as part of the capital reduction to deal with rounding issues that would otherwise have unbalanced the company’s nominal share capital. This deferred C share has no voting rights. Open Market Sale Agreement In September 2020, the Company entered into an Open Market Sale Agreement, or the Sales Agreement, with Jefferies LLC, or Jefferies, under which the Company may, at its option, offer and sell ADSs having an aggregate offering price of up to $100.0 million from time to time through Jefferies, acting as sales agent. Any such sales made through Jefferies can be made by any method that is deemed an “at-the-market offering” as defined in Rule 415 promulgated under the Securities Act, or in other transactions pursuant to an effective shelf registration statement on Form F-3. The Company agreed to pay Jefferies a commission of 3.0% of the gross proceeds of any sales of ADSs sold pursuant to the Sales Agreement.

Share Based Compensation

Share Based Compensation	12 Months Ended
	Dec. 31, 2020
Share-based Payment Arrangement [Abstract]
Share Based Compensation	Share Based Compensation In February 2017, the Company’s board of directors adopted the 2017 Share Option Plan, or the 2017 Plan. The 2017 Plan was set to expire on February 21, 2027. The 2017 Plan provided for the grant of potentially tax-favored Enterprise Management Incentives, or EMI, options to the Company's U.K. employees and for the grant of options to its U.S. employees. In June 2018, as part of the Company's reorganization and IPO, the Company’s board of directors and shareholders approved the 2018 Equity Incentive Plan, or the 2018 Plan. The initial maximum number of ordinary shares that may be issued under the 2018 Plan was 3,281,622. This number consists of 3,025,548 new ordinary shares and 256,074 ordinary shares that would have otherwise remained available for future grants under the 2017 Plan. The number of ordinary shares reserved for issuance under the 2018 Plan will automatically increase on October 1st of each year, for a period of not more than ten years, commencing on October 1, 2018 and ending on (and including) October 1, 2027, by an amount equal to the lesser of (i) 4% of the total number of ordinary shares outstanding on September 30th of the same calendar year or (ii) such fewer number of ordinary shares as the board of directors may designate prior to the applicable October 1st date. The updated maximum number of ordinary shares that may be issued under the 2018 Plan is 8,778,719 as of December 31, 2020. The total Shares issued under the 2018 Plan may be authorized but unissued shares, shares purchased on the open market, treasury shares or ADSs. No more than 14,000,000 shares may be issued under the 2018 Plan upon the exercise of incentive share options. Options granted under the 2018 Plan and 2017 Plan, as well as restricted shares granted as employee incentives, typically vest over a four-year service period with 25% of the award vesting on the first anniversary of the commencement date and the balance vesting monthly over the remaining three years, unless the award contains specific performance vesting provisions. For equity awards issued that have both a performance vesting condition and a services condition, once the performance criteria is achieved, the awards are then subject to a four-year service vesting with 25% of the award vesting on the first anniversary of the performance condition being achieved and the balance vesting monthly over the remaining three years. Options granted under the 2018 Plan and 2017 Plan generally expire 10 years from the date of grant. For certain senior members of management and directors, the board of directors has approved an alternative vesting schedule. Restricted stock units awarded in December 2019 vest over a 3-year service period with 50% of the award vesting one-and-half years from commencement date and the remaining 50% of the award vesting at the end of the third year. No restricted stock units were awarded in the year ended December 31, 2020. Share Option Valuation The assumptions (see Note 2) used in the Black-Scholes option pricing model to determine the fair value of the share options granted to employees and directors during the years ended December 31, 2020 and 2019, the three months ended December 31, 2018, and the year ended September 30, 2018 were as follows: Year Ended December 31, Three Months Ended December 31, Year Ended September 30, 2020 2019 2018 2018 Expected option life (years) 5.27 to 6.08 5.27 to 6.08 6.08 6.08 Risk-free interest rate 0.31% to 1.66% 1.39% to 2.66% 2.70% to 3.09% 2.61% to 3.00% Expected volatility 76.38% to 81.45% 72.30% to 76.22% 69.52% to 71.26% 68.15% to 72.99% Expected dividend yield —% —% —% —% Share Options The table below summarizes activity for the years ended December 31, 2020 and 2019. Number of Weighted- Weighted- Aggregate Outstanding as of December 31, 2018 3,711,274 $ 19.25 9.47 $ 51,464 Granted 2,438,145 15.22 — — Exercised (11,724) 3.94 — $ 198 Forfeited (201,456) 19.72 — — Outstanding as of December 31, 2019 5,936,239 $ 17.71 9.02 $ 11,873 Granted 474,225 11.61 — — Exercised (115,194) 4.07 — — Forfeited (683,841) 20.06 — — Outstanding as of December 31, 2020 5,611,429 17.19 7.96 $ 4,262 Exercisable as of December 31, 2020 2,664,362 $ 17.36 7.36 $ 3,432 Vested and expected to vest as of December 31, 2020 5,611,429 $ 17.19 7.96 $ 4,262 The aggregate intrinsic value of share options is calculated as the difference between the exercise price of the share options and the fair value of the Company’s restricted ordinary shares for those share options that had exercise prices lower than the fair value of the Company’s restricted ordinary shares. The weighted average grant-date fair value of share options granted was $7.82, $10.07, $20.04, and $8.55 per share for the years ended December 31, 2020 and 2019, for the three months ended December 31, 2018, and for the year ended September 30, 2018, respectively. As of December 31, 2020, the total unrecognized compensation expense related to unvested options was $13.2 million, which the Company expects to recognize over a weighted average vesting period of 2.57 years. Restricted Ordinary Shares The assumptions (Note 2) used in the OPM to determine the fair value of the ordinary shares for the following dates are as follows: March 2, 2016 April 26, 2017 September 25, 2017 March 31, 2018 May 31, 2018 Expected term 2.8 years 1.2 years 0.8 years 1.8 years 1.8 years Risk-free interest rate 1.0 % 1.0 % 1.3 % 2.1 % 2.1 % Expected volatility 73.2 % 76.6 % 71.0 % 71 % 71 % Expected dividend yield 0.0 % 0.0 % 0.0 % 0.0 % 0.0 % A summary of the changes in the Company’s restricted ordinary shares during the years ended December 31, 2020 and 2019 are as follows and reflect the conversion of ordinary shares in the current and previous years. Number of Weighted average grant date fair value Unvested and outstanding at December 31, 2018 708,834 $ 4.10 Granted 0 0.00 Vested (389,755) 4.31 Forfeited (4,335) 4.43 Unvested and outstanding at December 31, 2019 314,744 4.22 Granted — 0 Vested (222,392) 4.24 Forfeited (1,969) 4.44 Unvested and outstanding at December 31, 2020 90,383 4.17 As of December 31, 2020, there was unrecognized compensation of $0.04 million, which will be recognized over 0.59 years. Restricted Stock Units A restricted stock unit (“RSU”) represents the right to receive one of the Company’s ADSs upon vesting of the RSU. The fair value of each RSU is based on the closing price of the Company’s ADSs on the date of grant. The Company grants RSUs with service conditions that vest over 3-year service period with 50% of the award vesting one-and-half years from commencement date and the remaining 50% of the award vesting at the end of the third year. During the year ended December 31, 2020, the Company did not grant RSUs under the 2018 Plan. The following is a summary of RSU activity for the 2018 Plan for the years ended December 31, 2020 and 2019, respectively: Number of Weighted average Unvested and outstanding at December 31, 2018 — $ — Granted 500,000 12.09 Vested — — Forfeited — — Unvested and outstanding at December 31, 2019 500,000 12.09 Granted — — Vested — — Forfeited (85,000) 12.09 Unvested and outstanding at December 31, 2020 415,000 $ 12.09 As of December 31, 2020, there was $2.4 million of unrecognized compensation costs related to unvested RSUs, which are expected to be recognized over a weighted average period of 1.96 years. Share-based Compensation Expense Share-based compensation expense recorded as research and development and general and administrative expenses is as follows (in thousands): Year Ended December 31, Three Months December 31, Years Ended September 30, 2020 2019 2018 2018 Research and development $ 12,992 $ 17,761 $ 1,906 $ 3,116 General and administrative $ 7,115 12,451 1,487 3,649 Capitalized to fixed assets $ (86) 174 — — Total share-based compensation $ 20,021 $ 30,386 $ 3,393 $ 6,765

License Agreements

License Agreements	12 Months Ended
	Dec. 31, 2020
License Agreements [Abstract]
License Agreements	License Agreements University College of London Business Ltd. (UCLB) License In September 2014, the Company entered into an exclusive license agreement (the “License”) with UCL Business Ltd. (“UCLB”), the technology transfer company of University College London (“UCL”), to obtain licenses to certain technology rights in the field of cancer therapy and diagnosis. In March 2016, the License was amended to include additional rights. As part of the consideration for the License in September 2014, the Company issued 1,497,643 ordinary shares to UCLB. The Company paid upfront fees of $0.3 million and issued an additional 313,971 ordinary shares to UCLB when the License was amended in March 2016. In March 2018, the License was further amended and restated to include a license to the Company's product candidate, AUTO1, for which UCL is conducting Phase 1 clinical trials of AUTO1 in pediatric and adult ALL patients. The Company paid an upfront fee of £1.5 million for consideration for the amended and restated License and paid the additional £0.35 million in connection with UCLB's transfer of clinical data to the Company in December 2020. No equity was issued as part of the upfront fee consideration. Additionally, the Company may be obligated to make payments to UCLB under the amended and restated License upon the initiation of certain clinical activities in an aggregate amount of £0.18 million, the receipt of specified regulatory approvals in an aggregate amount of £37.5 million, the start of commercialization in an aggregate amount of £18 million, and the achievement of net sales levels in an aggregate amount of £51 million, as well as royalty payments based on possible future sales resulting from the utilization of the licensed technologies. On a per-product basis, these milestone payments range from £1 million to £18.5 million, depending on which T cell programming modules are used in the product achieving the milestone. Upon commercialization of any of the Company’s products that use the in-licensed patent rights, the Company will be obligated to pay UCLB a flat royalty for each licensed product ranging from the low- to mid-single digits, depending on which technologies are deployed in the licensed product, based on worldwide annual net sales of each licensed product, subject to certain reductions, including for the market entry of competing products and for loss of patent coverage of licensed products. The Company may deduct from the royalties payable to UCLB one-half of any payments made to a third party to obtain a license to such third party’s intellectual property that is necessary to exploit any licensed products. Once net sales of a licensed product have reached a certain specified threshold, the Company may exercise an option to buy out UCLB’s rights to the remaining milestone payments, royalty payments, and sublicensing revenue payments for such licensed product, on terms to be negotiated at the time. The License expires on a product-by-product and country-by-country basis upon the expiration of the royalty term with respect to each product in each country. The Company may unilaterally terminate the license agreement for any reason upon advance notice to UCLB. Either party may terminate the License for the uncured material breach by the other party or for the insolvency of the other party. If UCLB terminates the License following the Company’s insolvency or the Company’s material breach of the License, or if the Company terminates the License unilaterally, all rights and licenses granted to the Company will terminate, and all patent rights and know-how transferred to the Company pursuant to the License will revert back to UCLB, unless and to the extent the Company has exercised its option to acquire ownership of the licensed patent rights. In addition, UCLB has the right to negotiate with the Company for the grant of an exclusive license to the Company’s improvements to the T cell programming modules the Company has licensed on terms to be agreed upon at the time. Noile-Immune Biotech Inc. In November 2019, the Company entered into an exclusive license agreement with Noile-Immune Biotech Inc. ("Noile") under which the Company will have the right to develop CAR T cell therapies incorporating Noile’s PRIME (proliferation-inducing and migration-enhancing) technology. The PRIME technology is designed to improve proliferation and trafficking into solid tumors of both engineered CAR T cells as well as the patient’s own T cells. The Company paid an upfront fee and may be obligated to make additional payments to Noile upon the achievement of development milestones and receipt of regulatory approvals product sale milestones, as well as royalty payments based on possible future sales resulting from the utilization of the licensed technology.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The Company recorded an income tax benefit of $24.2 million, $15.2 million, $2.6 million, and $7.3 million for the years ended December 31, 2020 and 2019, the three months ended December 31, 2018, and the year ended September 30, 2018, respectively. A reconciliation of income tax expense (benefit) at the statutory corporate income tax rate to the income tax expense (benefit) at the Company’s effective income tax rates is as follows (in thousands): Year ended December 31, Three months ended December 31, Year ended September 30, 2020 2019 2018 2018 Net loss before taxes $ (166,257) $ (139,008) $ (23,253) $ (52,031) U.K. statutory tax rate 19.0 % 19.0 % 19.0 % 19.0 % Income tax benefit at U.K. statutory tax rate (31,589) (26,411) (4,423) (9,886) Tax incentives / credits (23,076) (16,312) (3,234) (7,296) Non-deductible expenses 797 4,048 127 1,553 Adjustments in respect of prior years (1,088) 96 265 (13) Operating losses 28,672 21,643 3,605 7,317 Tax on property, plant, equipment and intangibles 547 267 140 233 Other, net 1,552 1,510 915 812 Foreign rate differential 22 $ — — $ — Total income tax benefit $ (24,163) $ (15,159) $ (2,605) $ (7,280) Current income tax benefit (22,819) (14,749) (2,605) (7,280) Deferred income tax benefit (1,344) (410) — — Effective rate of income tax 14.5 % 10.9 % 11.2 % 14.0 % The effective tax rates in the above table for the years ended December 31, 2020 and 2019, the three months ended December 31, 2018, and the year ended September 30, 2018 is lower than the main rate of U.K. tax primarily due to administration of the U.K. research and development tax credit, which is included within the tax incentive/credits line in the table above. Deferred tax assets and liabilities consisted of the following at December 31, 2020 and 2019 (in thousands): December 31, 2020 2019 Deferred tax assets: Other differences $ 14,135 $ 1,522 Tax losses 40,221 19,624 Fixed assets 3,934 896 Total deferred tax assets 58,290 22,042 Valuation allowances (56,536) (21,632) Net deferred tax asset (liability) $ 1,754 $ 410 Deferred tax assets resulting from loss carry forwards, fixed assets and retirement benefits, with total deferred tax assets increasing by $1.3 million in 2020. The Company has recorded a valuation allowance against the net deferred tax asset where the recoverability due to future taxable profits is unknown. The $1.8 million deferred tax asset balance is related to the Company's U.S. entity. At December 31, 2020, the Company had U.K. trading losses carry forward of $211.7 million. These losses are carried forward indefinitely under local law, but are subject to numerous utilization criteria and restrictions. As required by the authoritative guidance on accounting for income taxes, the Company evaluates the realizability of deferred tax assets at each reporting date. Accounting for income taxes guidance requires that a valuation allowance be established when it is more likely than not that all or a portion of the deferred tax assets will not be realized. In circumstances where this is sufficient negative evidence indicating that the deferred tax assets are not more likely than not realizable, the Company establishes a valuation allowance. The Company recorded valuation allowances in the amounts of $56.5 million and $21.6 million at December 31, 2020 and 2019, respectively.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies License Agreement The Company has entered into an exclusive license agreement, as amended, with UCLB (see Note 9). In connection with the UCLB license agreement, the Company is required to make annual license payments and may be required to make payments upon the achievement of specified milestones. The Company has estimated the probability of the Company achieving each potential milestone in accordance with ASC 450, Contingencies . The Company concluded that, as of December 31, 2020, there were no other milestones for which the likelihood of achievement was probable. Legal Proceedings From time to time, the Company may be a party to litigation or subject to claims incident to the ordinary course of business. Regardless of the outcome, litigation can have an adverse impact on the Company because of defense and settlement costs, diversion of management resources and other factors. The Company was not a party to any litigation and did not have contingency reserves established for any liabilities as of December 31, 2020. Leases The Company leases certain office space, laboratory space, and equipment. At the inception of an arrangement, the Company determines whether the arrangement is or contains a lease based on the unique facts and circumstances present. The Company does not recognize right-of-use assets or lease liabilities for leases determined to have a term of 12 months or less. Many of the Company's leases contain variable non-lease components such as maintenance, taxes, insurance, and similar costs for the spaces it occupies. For new and amended leases beginning in 2019, the Company has elected the practical expedient not to separate these non-lease components of leases for classes of all underlying assets and instead account for them as a single lease component for all leases. The Company recognizes on a straight-lines basis the net fixed payments of operating leases over the lease term. Variable executory costs, as it relates to net leases, are excluded from the calculation of the lease liability and the Company expenses the variable lease payments in the period in which it incurs the obligation to pay such variable amounts and will be included in variable lease costs in the leases footnote disclosure. These variable lease payments are not included in the Company's calculation of its right-of-use assets or lease liabilities. In adopting ASC 842, in the year ended December 31, 2019, the Company elected the package of practical expedients which, among other things, allowed it to retain the classification of its leases in place at the effective date of ASC 842. The Company’s corporate headquarters are located in London, United Kingdom. As of December 31, 2020, and 2019, the Company leased space at this location from Imperial (Forest House) Limited under a ten year lease, the term of which commenced in September 2015. The lease included an option for the Company to lease additional space within a 15-month period, which the Company exercised in October 2016. The exercise of the option resulted in a separate new lease with a concurrent term through August 2025. The Company and the landlord has the option to early terminate both leases in September 2020 and the landlord has the option to give notice to terminate the lease from September 2020 onward. The Company has measured its right-of-use assets and lease liabilities based on lease terms ending in September 2025. The landlord exercised its option to give notice in September 2020 to terminate the Forest House lease and pay the Company a break-lease payment fee in September 2021. The Company recorded a $1.3 million gain upon the notice of lease termination and lease incentive receivable of $1.2 million in the third quarter of 2020. In addition to base rent, the Company is obligated to pay its proportionate share of building operating expenses and real estate taxes in excess of base year amounts. These costs are considered to be variable lease payments and are not included in the determination of the lease’s right-of-use asset or lease liability. Prior to the lease commencement date of the Forest House leases, the Company, in conjunction with the landlord, made improvements to the leased space. The total cost of these improvements was funded by the landlord, a portion of the cost will be reimbursed by the Company over the term of the leases. The total cost of the improvements was capitalized as leasehold improvements on the Company’s balance sheet, with an offset to long-term lease incentive obligation for the portion funded by the landlord and other long-term payables for the portion to be repaid to the landlord. The lease related to this facility is classified as an operating lease. In September 2017, the Company executed an arrangement with Catapult Limited to lease a manufacturing suite at the Cell and Gene Therapy Catapult manufacturing center in Stevenage, United Kingdom for a term through May 2021, at which time the Company has the option to renew or terminate the lease. The lease related to this facility is classified as an operating lease. The lease has a six-month rent-free period. In addition to base rent, the Company is obligated to pay its proportionate share of building operating expenses and real estate taxes in excess of base year amounts. These costs are considered to be variable lease payments and are not included in the determination of the lease’s right-of-use asset or lease liability. In December 2018, the Company executed an additional lease arrangement for additional manufacturing space for a term through September 2023, at which time the Company has the option to renew or terminate the lease. In June 2018, the Company signed a binding letter of intent to enter into a lease for office and laboratory space in White City, London. The letter of intent required the Company to enter into a ten-year lease provided that the landlord completed the required leasehold improvements described in the agreement. The leasehold improvements were completed and the lease commenced in January 2019. The Company has the option to terminate the lease in November 2026. In addition to base rent, the Company is obligated to pay its proportionate share of building operating expenses and real estate taxes in excess of base year amounts. These costs are considered to be variable lease payments and are not included in the determination of the lease’s right-of-use asset or lease liability. The lease agreement includes an option to lease additional space. As of December 31, 2020, the Company capitalized $6.7 million as leasehold improvements. In September 2018, the Company signed a binding letter of intent to enter into a lease for manufacturing space in Enfield, United Kingdom. The letter of intent required the Company to enter into a 15-year lease provided that the landlord completed the required leasehold improvements described in the agreement. The Company executed lease agreements for three manufacturing space units, each for 15-year lease terms. The leases commenced in February 2019 with option to terminate the lease in February 2029. In addition to base rent, the Company is obligated to pay its proportionate share of building operating expenses and real estate taxes in excess of base year amounts. These costs are considered to be variable lease payments and are not included in the determination of the lease’s right-of-use asset or lease liability. The Company expensed $4.1 million of leasehold improvements from assets under construction as of December 31, 2019 as a result of discontinuing the fit-out of the manufacturing facility. The Company reduced the right-of-use asset and lease liability based on the contractual option termination date. The Company is actively seeking to sub-lease or assign the lease arrangements to a third party. The Company completed an asset impairment analysis of the right-of-use lease concluding the undiscounted cash flows exceeded the carrying value as of December 31, 2020. In October 2018, the Company executed an agreement to sublease office space in Rockville, Maryland for a term through October 2021. The Company then terminated the sublease in February 2020 and immediately entered into a five-year lease for the same space with the landlord. As a result of the sublease termination, the Company recognized a $0.2 million gain in other income (expense) in the first quarter of 2020. T he lease related to this facility is classified as an operating lease. The Company is obligated to pay its proportionate share of building operating expenses and real estate taxes in excess of base year amounts. These costs are considered to be variable lease payments and are not included in the determination of the lease’s right-of-use asset or lease liability. In January 2019, the Company executed a lease agreement to lease additional office and manufacturing space in Rockville, Maryland. The lease agreement required the Company to enter into a lease provided that the landlord completes the required leasehold improvements described in the agreement. The lease commenced in August 2020 for a term through June 2036. The lease related to this facility is classified as an operating lease . The Company has capitalize d $2.4 million in leasehold improvements as assets under construction as of December 31, 2020. In May 2020, the Company executed an arrangement with Catapult Limited to lease a manufacturing suite at the Cell and Gene Therapy Catapult manufacturing center in Stevenage, United Kingdom for a term through April 2024. The lease related to this facility is classified as an operating lease. In addition to base rent, the Company is obligated to pay its proportionate share of building operating expenses and real estate taxes in excess of base year amounts. These costs are considered to be variable lease payments and are not included in the determination of the lease’s right-of-use asset or lease liability. The Company identified and assessed the following significant assumptions in recognizing its right-of-use assets and corresponding lease liabilities during the adoption of ASC 842, in the year ended December 31, 2019: • As the Company's leases do not provide an implicit rate, it estimated the incremental borrowing rate for each lease based on a yield curve analysis, utilizing the interest rate derived from the fair value analysis of its existing leases and adjusting it for factors that appropriately reflect the profile of secured borrowing over the lease term. For leases existing as of the adoption date, the Company has utilized its incremental borrowing rate based on the remaining lease term as of the adoption date. For leases that commenced after the adoption date, the Company determined the incremental borrowing rate based on the lease term as determined at the commencement date of the lease. • The expected lease terms include both contractual lease periods and, when applicable, cancellable option periods where failure to exercise such options would result in an economic penalty. • Since the Company elected to account for each lease component and its associated non-lease components as a single combined lease component, all contract consideration was allocated to the combined lease component. Leases (in thousands) Balance Sheet Classification As of December 31, 2020 2019 Assets Operating lease assets Operating right-of-use assets $ 51,637 $ 23,409 Liabilities Current Operating lease liabilities Current liabilities: Operating lease liabilities $ 3,590 $ 2,511 Noncurrent Operating lease liabilities Non-current liabilities: Operating lease liabilities $ 50,571 $ 23,710 Total lease liabilities $ 54,161 $ 26,221 Year Ended December 31, Lease cost (in thousands) Statement of Operations Classification 2020 2019 Operating lease cost Operating expenses: research and development $ 5,266 $ 3,156 Variable lease cost Operating expenses: research and development 1,212 1,222 Operating lease cost Operating expenses: general and administrative 1,380 1,241 Variable lease cost Operating expenses: general and administrative 270 250 Short term lease costs 282 270 Total lease cost $ 8,410 $ 6,139 December 31, Other information (in thousands) 2020 2019 Cash paid for amounts included in the measurement of lease liabilities Operating cash flows from operating leases $ 4,896 $ 2,415 Right-of-use assets obtained in exchange for new operating lease liabilities $ 30,786 $ 26,124 Weighted-average remaining lease term — operating leases (years) 11.2 7.7 Weighted-average discount rate — operating leases 9.0 % 6.8 % Future fixed payments for non-cancellable operating leases in effect as of December 31, 2020 are payable as follows: Operating Leases Maturity of lease liabilities for the years ending December 31, (in thousands) 2021 $ 7,026 2022 9,112 2023 8,644 2024 8,150 2025 7,044 Thereafter 51,259 Total lease payments 91,235 Less: imputed interest (37,074) Present value of lease liabilities $ 54,161 The Company recognized rent expense on a straight-line basis over the respective lease period and has recorded deferred rent for rent expense incurred but not yet paid prior to the adoption of ASC 842. The Company recorded rent expense totaling $0.5 million for the three months ended December 31, 2018, and $0.9 million for the year ended September 30, 2018, respectively.

Employee Benefit Plans

Employee Benefit Plans	12 Months Ended
	Dec. 31, 2020
Retirement Benefits [Abstract]
Employee Benefit Plans	Employee Benefit Plans In the United Kingdom, the Company makes contributions to private defined benefit pension schemes on behalf of its employees. The Company expensed $1.3 million, $1.0 million, $0.2 million, and $0.5 million in contributions in the years ended December 31, 2020 and 2019, three months ended December 2018, and the year ended September 30, 2018, respectively. In the United States, the Company established a defined contribution savings plan under Section 401(k) of the Internal Revenue Code subsequent to September 30, 2018. The plan covers substantially all U.S. employees who meet minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pre-tax basis. The Company matches employee contributions up to four percent of the employee’s annual salary. The Company expensed $0.3 million, $0.2 million and $18,000 in contributions in the years ended December 31, 2020 and 2019 and the three months ended December 31, 2018, respectively. No matching contributions were recorded for the year ended September 30, 2018. The Company pays all administrative fees related to the Plan.

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2020
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events The Company evaluated subsequent events through March 4, 2021 the date on which these financial statements were issued. In January 2021, the Company announced the prioritization of the AUTO1 program and its intention to seek a partner for the AUTO3 program before progressing AUTO3 into the next phase of development. The Company also announced an adjustment of its workforce and infrastructure footprint during the first quarter of 2021, which will involve an overall reduction in headcount of approximately 20%. The company expects to realize cash savings, on an annualized basis, of approximately $15 million per annum once the operational changes are fully implemented. In the short-term, we expect there to be an increase in the first half of 2021 to both research and development expenses and selling, general, and administrative expenses due to restructuring charges of approximately $2.5 million, combined. In January 2021, the Company sold an aggregate of 1.7 million ADSs under the Sales Agreement, resulting in net proceeds of $15.3 million. On February 12, 2021, the Company completed an underwritten public offering of 14,285,715 ADSs, which includes the full exercise by the underwriters to purchase an additional 2,142,857 ADSs, at a public offering price of $7.00 per ADS. Aggregate net proceeds to the Company, after underwriting discounts, were $108.1 million. .

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements include those of the Company, Autolus Limited, and its U.S. subsidiary, Autolus Inc., and have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”). All intercompany accounts and transactions have been eliminated upon consolidation.
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of income and expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, the accrual for research and development expenses, the fair value of ordinary shares, share-based compensation and income taxes. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results could differ materially from those estimates.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers cash and cash equivalents in the consolidated financial statements to include cash at banks with a maturity of less than three months, which is subject to an insignificant risk of changes in value.
Restricted Cash	Restricted Cash The Company entered into a lease that requires a letter of credit supported by $0.6 million deposit held by the Company's bank for the duration of the lease and a credit card arrangement that requires a security deposit of $0.2 million. 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 Company's consolidated statements of cash flows.
Fair Value Measurements	Fair Value Measurements The carrying amounts reported in the balance sheets for cash, prepaid expenses and other assets, accounts payable and accrued expenses and other liabilities approximate their fair value because of the short-term nature of these instruments.
Concentration of Credit Risk	Concentration of Credit Risk Financial instruments that subject the Company to credit risk consist primarily of cash and cash equivalents. The Company places cash and cash equivalents in established financial institutions. The Company has no significant off-balance-sheet risk or concentration of credit risk, such as foreign exchange contracts, options contracts, or other foreign hedging arrangements.
Implementation Costs in a Cloud Computing Arrangement	Implementation Costs in a Cloud Computing Arrangement The Company’s cloud computing arrangements primarily comprise hosting arrangements which are service contracts, whereby the Company gains remote access to use enterprise software hosted by the vendor or another third party on an as-needed basis for a period of time in exchange for a subscription fee. Implementation costs for cloud computing arrangements are capitalized if certain criteria are met and consist of internal and external costs directly attributable to developing and configuring cloud computing software for its intended use. These capitalized implementation costs are presented in the condensed consolidated balance sheet in prepaid expenses and other assets, current and non-current, and are generally amortized over the fixed, non-cancellable term of the associated hosting arrangement on a straight-line basis.
Property and Equipment	Property and Equipment Property and equipment are recorded at cost and depreciated or amortized using the straight-line method over the estimated useful lives of the respective assets. As of December 31, 2020 and 2019, the Company’s property and equipment consisted of office equipment, lab equipment, furniture and fixtures, and leasehold improvements. The office equipment has an estimated useful life of three years, lab equipment has an estimated useful life of five
Leases, Deferred Rent and Lease Incentives	Leases Effective January 1, 2019, the Company adopted Accounting Standards Codification (“ASC”), Topic 842, Leases (“ASC 842”), using the required modified retrospective approach and utilizing the effective date as its date of initial application, for which prior periods are presented in accordance with the previous guidance in ASC 840, Leases. At the inception of an arrangement, the Company determines whether the arrangement is or contains a lease based on the unique facts and circumstances present. Most leases with a term greater than one year are recognized on the balance sheet as right-of-use assets, lease liabilities and, if applicable, long-term lease liabilities. The Company has elected not to recognize on the balance sheet leases with terms of one year or less. Operating lease liabilities and their corresponding right-of-use assets are recorded based on the present value of lease payments over the expected remaining lease term. However, certain adjustments to the right-of-use asset may be required for items such as incentives received, initial direct costs, or prepayments. The interest rate implicit in lease contracts is typically not readily determinable. As a result, the Company utilizes its incremental borrowing rates, which are the rates incurred to borrow on a collateralized basis over a similar term an amount equal to the lease payments in a similar economic environment. In accordance with the guidance in ASC 842, 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, consumables, etc.), and non-components ( e.g. , property taxes, insurance, etc.). Then the fixed and in-substance fixed contract consideration (including any related to non-components) must be allocated based on the respective relative 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. Entities may elect the practical expedient to 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. For new and amended leases beginning in 2019, the Company elected the practical expedients to account for the lease and non-lease components for leases for classes of all underlying assets and allocate all of the contract consideration to the lease component only. The Company determined the underlying lease to be the predominant component, and therefore, the entire agreement was accounted for under ASC 842.
Intangible Assets Subject to Amortization	Intangible Assets Subject to Amortization     The Company’s intangible assets have been related to acquired software licenses with finite lives are amortized over their useful lives and reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of the asset may not be recoverable. If any indicators were present, the Company would test for recoverability by comparing the carrying amount of the asset to the net undiscounted cash flows expected to be generated from the asset. If those net undiscounted cash flows do not exceed the carrying amount (i.e., the asset is not recoverable), the Company would perform the next step, which is to determine the fair value of the asset and record an impairment loss, if any. The Company evaluates the useful lives for these intangible assets each reporting period to determine whether events and circumstances warrant a revision in their remaining useful lives.
Segment Information	Segment Information Operating segments are defined as components of an enterprise about which separate discrete information is available for evaluation by the chief operating decision maker in deciding how to allocate resources and assess performance. The Company and the Company’s chief operating decision maker, the Company’s Chief Executive Officer, view the Company’s operations and manages its business as a single operating segment, which is the business of developing and commercializing gene therapies. The Company operates in two geographic regions: the United Kingdom and the United States. A majority of the Company’s assets are held in the United Kingdom.
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, depreciation expense, third-party license fees, external costs of outside vendors engaged to conduct clinical development activities, clinical trials, costs to manufacture clinical trial materials and certain tax credits associated with research and development activities. Accrued Research and Development Expenses As part of the process of preparing consolidated financial statements, the Company is required to estimate accruals for research and development expenses. This process involves reviewing and identifying services which have been performed by third parties on the Company’s behalf and determining the value of these services. In addition, the Company makes estimates of costs incurred to date but not yet invoiced, in relation to external clinical research organizations and clinical site costs. The Company analyzes the progress of clinical trials, including levels of patient enrollment; invoices received and contracted costs, when evaluating the adequacy of the accrued liabilities for research and development. The Company makes judgments and estimates in determining the accrued balance in any accounting period.
Share-based Compensation	Share-Based Compensation The Company recognizes compensation expense for equity awards based on the grant date fair value of the award. The Company recognizes share-based compensation expense for awards granted to employees that have a graded vesting schedule based on a service condition only on a straight-line basis over the requisite service period for each separately vesting portion of the award as if the award was, in substance, multiple awards (the “graded-vesting attribution method”), based on the estimated grant date fair value for each separately vesting tranche. For equity awards with a graded vesting schedule and a combination of service and performance conditions, the Company recognizes share-based compensation expense using a graded-vesting attribution method over the requisite service period when the achievement of a performance-based milestone is probable, based on the relative satisfaction of the performance condition as of the reporting date. The Company accounts for forfeitures as they occur. For share-based awards granted to consultants and non-employees, compensation expense is recognized using the graded-vesting attribution method over the period during which services are rendered by such consultants and non-employees until completed. The measurement date for employee awards is the date of grant, and share-based compensation costs are recognized as expense over the employees’ requisite service period, which is the vesting period, on an accelerated basis. In the year ended December 31, 2019 the Company adopted Accounting Standards Update (“ASU”) No. 2018-07, “Compensation —Stock Compensation (Topic 718): Improvements to Non-employee Share-Based Payment Accounting” (“ASU No. 2018-07”), prior to which the measurement date for non-employee awards was generally the date the services were completed, resulting in financial reporting period adjustments to share-based compensation during the vesting terms for changes in the fair value of the awards. After the adoption of ASU No. 2018-07, the measurement date for non-employee awards is the later of the adoption date of ASU No. 2018-07 or the date of grant, without changes in the fair value of the award. The fair value of each share option grant is estimated on the date of grant using the Black-Scholes option pricing model. See Note 8 for the Company’s assumptions used in connection with option grants made during the periods covered by these consolidated financial statements. Assumptions used in the option pricing model include the following: Expected volatility. We lack company-specific historical and implied volatility information for our ADSs for expected terms greater than 2.5 years. Therefore, we use a combination of the historical volatility of our ADSs and also the expected share volatility based on the historical volatility of publicly traded peer companies and expect to continue to do so until such time as we have adequate historical data regarding the volatility of our own traded security price Expected term . The expected term of the Company’s share options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. Risk-free interest rate . The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods that are approximately equal to the expected term of the award. Expected dividend. Expected dividend yield of zero is based on the fact that the Company has never paid cash dividends on ordinary shares and does not expect to pay any cash dividends in the foreseeable future. Fair value of ordinary shares. Options granted after the Company’s IPO are issued at the fair market value of the Company’s ADS at the date the grant is approved by the Board. Prior to the IPO, the Company calculated the fair value of its ordinary shares in accordance with the guidelines in the American Institute of Certified Public Accountants’ Accounting and Valuation Guide, Valuation of Privately-Held-Company Equity Securities Issued as Compensation . The Company’s valuations of ordinary shares were prepared using a market approach, based on precedent transactions in the shares, to estimate the Company’s total equity value using the option-pricing method (“OPM”), which used a combination of market approaches and an income approach to estimate the Company’s enterprise value. The OPM derives an equity value such that the value indicated is consistent with the investment price, and it provides an allocation of this equity value to each class of the Company’s securities. The OPM treats the various classes of shares as call options on the total equity value of a company, with exercise prices based on the value thresholds at which the allocation among the various holders of a company’s securities changes. Under this method, each class of shares has value only if the funds available for distribution to shareholders exceed the value of the share liquidation preferences of the class or classes of shares with senior preferences at the time of the liquidity event. Key inputs and assumptions used in the OPM calculation include the following: Expected volatility. The Company applied re-levered equity volatility based on the historical unlevered and re-levered equity volatility of publicly traded peer companies. Expected dividend. Expected dividend yield of zero is based on the fact that the Company has never paid cash dividends on ordinary shares and does not expect to pay any cash dividends in the foreseeable future. Expected term . The expected term of the option or the estimated time until a liquidation event. Risk-free interest rate . The risk-free interest rate is determined by reference to the U.S. Treasury yield curve for the period commensurate with the expected of the exit event.
Foreign Currency Translation	Foreign Currency Translation The Company maintains its financial statements in its functional currency, which is the pounds sterling. Monetary assets and liabilities denominated in currencies other than the functional currency are translated into the functional currency at rates of exchange prevailing at the balance sheet dates. Non-monetary assets and liabilities denominated in foreign currencies are translated into the functional currency at the exchange rates prevailing at the date of the transaction. Exchange gains or losses arising from foreign currency transactions are included in the determination of net income (loss) for the respective periods. The Company recorded foreign exchange loss of $0.2 million for the year ended December 31, 2020, foreign exchange gains of $4.6 million, $1.1 million, and $4.0 million for the year ended December 31, 2019, the three months ended December 31, 2018, and the year ended September 30, 2018, respectively, which are included in other income in the statements of operations and comprehensive loss. For financial reporting purposes, the financial statements of the Company have been translated into U.S. dollars. Assets and liabilities have been translated at the exchange rates at the balance sheet dates, while revenue and expenses are translated at the average exchange rates over the reporting period and shareholders’ equity amounts are translated based on historical exchange rates as of the date of each transaction. Translation adjustments are not included in determining net income (loss) but are included in foreign exchange adjustment to other comprehensive loss, a component of shareholders’ equity.
Patent Costs	Patent CostsThe Company expenses patent prosecution and related legal costs as they are incurred and classifies such costs as general and administrative expenses in the accompanying statements of operations and comprehensive loss.
Grant Income	Grant Income The Company has received research grants under which it is reimbursed for specific research and development activities. Payments received are recognized as income in the statements of operations and comprehensive loss over the period in which the Company recognizes the related costs. At the time the Company recognizes grant income, it has complied with the conditions attached to it and the receipt of the reimbursement is reasonably assured. The Company has received grants from the U.K. government, which are repayable under certain circumstances, including breach or noncompliance. For grants with refund provisions, the Company reviews the grant to determine the likelihood of repayment. If the likelihood of repayment of the grant is determined to be remote, then the grant is recognized as grant income. The Company has determined that the likelihood of any repayment events included in its current grants is remote.
License Revenue	License Revenue The Company accounts for its revenues pursuant to the provisions of Accounting Standards Codification (“ASC”) Topic 606, Revenue from Contracts with Customers (“ASC Topic 606”). The Company has no products approved for commercial sale and has not generated any revenue from commercial product sales. The total revenue to date has been generated from a license agreement with an investee company of one our affiliates. The terms of the agreement include a non-refundable license fee, payments based upon achievement of clinical development and regulatory objectives, and royalties on product sales. In determining the appropriate amount of revenue to be recognized as the Company fulfills its obligations under its agreements, the Company performs the following steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations, including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations based on estimated selling prices; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. License Fees and Multiple Element Arrangements If a license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company recognizes revenues from non-refundable, upfront fees allocated to the license at such time as the license is transferred to the licensee and the licensee is able to use, and benefit from, the license. For licenses that are bundled with other promises, the Company utilizes judgment to assess the nature of the combined performance obligations to determine whether the combined performance obligations are satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue from non-refundable, upfront fees. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Appropriate methods of measuring progress include output methods and input methods. In determining the appropriate method for measuring progress, the Company considers the nature of service that the Company promises to transfer to the customer. When the Company decides on a method of measurement, the Company will apply that single method of measuring progress for each performance obligation satisfied over time and will apply that method consistently to similar performance obligations and in similar circumstances. Contingent Research Milestone Payments ASC Topic 606 constrains the amount of variable consideration included in the transaction price in that either all, or a portion, of an amount of variable consideration should be included in the transaction price. The variable consideration amount should be included only to the extent that it is probable that a significant reversal in the amount of cumulative revenue recognized will not occur when the uncertainty associated with the variable consideration is subsequently resolved. The assessment of whether variable consideration should be constrained is largely a qualitative one that has two elements: the likelihood of a change in estimate, and the magnitude thereof. Variable consideration is not constrained if the potential reversal of cumulative revenue recognized is not significant, for example. If the consideration in a contract includes a variable amount, the Company will estimate the amount of consideration in exchange for transfer of promised goods or services. The consideration also can vary if the Company’s entitlement to the consideration is contingent on the occurrence or non-occurrence of a future event. The Company considers contingent research milestone payments to fall under the scope of variable consideration, which should be estimated for revenue recognition purposes at the inception of the contract and reassessed ongoing at the end of each reporting period. The Company assesses whether contingent research milestones should be considered variable consideration that should be constrained and thus not part of the transaction price. This includes an assessment of the probability that all or some of the milestone revenue could be reversed when the uncertainty around whether or not the achievement of each milestone is resolved, and the amount of reversal could be significant. GAAP provides factors to consider when assessing whether variable consideration should be constrained. All of the factors should be considered, and no factor is determinate. The Company considers all relevant factors.
Income Taxes and Income Tax Credit	Income Taxes The Company accounts for income taxes under the asset and liability method which includes the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the Company’s financial statements. Under this approach, deferred taxes are recorded for the future tax consequences expected to occur when the reported amounts of assets and liabilities are recovered or paid. The provision for income taxes represents income taxes paid or payable for the current year plus deferred taxes. Deferred taxes result from differences between the financial statements and tax bases of the Company’s assets and liabilities, and are adjusted for changes in tax rates and tax law when changes are enacted. The effects of future changes in income tax laws or rates are not anticipated. The Company is subject to income taxes in the United Kingdom and the United States. The calculation of the Company’s tax provision involves the application of United Kingdom tax law and requires judgement and estimates. The Company evaluates the realizability of its deferred tax assets at each reporting date, and establishes a valuation allowance when it is more likely than not that all or a portion of its deferred tax assets will not be realized. The ultimate realization of deferred tax assets is dependent upon the generation of future taxable income of the same character and in the same jurisdiction. The Company considers all available positive and negative evidence in making this assessment, including, but not limited to, the scheduled reversal of deferred tax liabilities, projected future taxable income, and tax planning strategies. In circumstances where there is sufficient negative evidence indicating that the Company’s deferred tax assets are not more likely than not realizable, the Company establishes a valuation allowance. The Company uses a two-step approach for recognizing and measuring uncertain tax positions. The first step is to evaluate tax positions taken or expected to be taken in a tax return by assessing whether they are more likely than not sustainable, based solely on their technical merits, upon examination, and including resolution of any related appeals or litigation process. The second step is to measure the associated tax benefit or each position as the largest amount that the Company believes is more likely than not realizable. Differences between the amount of tax benefits taken or expected to be taken in the Company’s income tax returns and the amount of tax benefits recognized in the its financial statements represent the Company’s unrecognized income tax benefits, which it either records as a liability or reduction of deferred tax assets. Income Tax Credit The Company benefits from the U.K. research and development tax credit regime under both the small and medium sized enterprise, or SME, scheme and by claiming an RDEC in respect of grant funded projects. Under the SME regime, a portion of the Company’s losses can be surrendered for a cash rebate of up to 33.35% of eligible expenditures. Such credits are accounted for within the tax provision in the year in which the expenditures were incurred.
Comprehensive Loss	Comprehensive Loss The Company follows the provisions of the Financial Accounting Standards Board (“FASB”) Accounting Standards Codification (“ASC”) Topic 220, Comprehensive Income
Net Loss per Share	Net Loss per Share Basic and diluted net loss per ordinary share is determined by dividing net loss by the weighted average number of ordinary shares outstanding during the period. For all periods presented, the preferred shares and outstanding but unvested restricted shares and share options have been excluded from the calculation, because their effects would be anti-dilutive. Therefore, the weighted average shares outstanding used to calculate both basic and diluted loss per share are the same for all periods presented.
Recently issued accounting pronouncements adopted	Recently issued accounting pronouncements adopted In August 2018, the FASB issued ASU 2018-15, Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract, a new standard on a customer's accounting for implementation, set-up, and other upfront costs incurred in a cloud computing arrangement ("CCA") that aligns the requirements for capitalizing implementation costs in a CCA service contract with existing internal-use software guidance. The standard is effective for interim and annual periods beginning after December 15, 2019, with early adoption permitted, and can be adopted prospectively or retrospectively. The Company adopted the new standard on January 1, 2020 on a prospective basis. The Company’s CCAs are service contracts for the hosting of software primarily related to recording and tracking information related to its clinical trials, including but not limited to patient data and clinical manufacturing. The capitalized implementation costs are presented in the condensed consolidated balance sheet in prepaid expenses and other assets, current and non-current. The deferred implementation costs will be expensed over the term of the hosting arrangement, which is the non-cancelable term of the arrangement plus any reasonably certain renewal periods. As of December 31, 2020, $1.0 million was recorded in prepaid expenses and other assets, current and $1.1 million was recorded to prepaid expenses and other assets, non-current as deferred implementation costs. During the year ended December 31, 2020, $0.3 million of deferred implementation costs were expensed. Recently issued accounting pronouncements not adopted Other accounting standards that have been issued by the FASB or other standards-setting bodies that do not require adoption until a future date are not expected to have a material impact on the Company's financial statements upon adoption.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Schedule of Antidilutive Securities	The following potentially dilutive securities have been excluded from the calculation of diluted net loss per share due to their anti-dilutive effect: Year Ended December 31, Three Months Ended December 31, Years Ended September 30, 2020 2019 2018 2018 Unvested restricted shares and units 505,383 814,744 708,834 815,632 Incentive share options 5,611,429 5,963,239 3,711,274 2,065,481 Total 6,116,812 6,777,983 4,420,108 2,881,113

Prepaid Expenses and Other Cu_2

Prepaid Expenses and Other Current Assets (Tables)	12 Months Ended
	Dec. 31, 2020
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Schedule of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets consisted of the following (in thousands): December 31, 2020 2019 Research and development claims receivable $ 24,711 $ 27,567 Prepayments 10,206 7,023 VAT receivable 3,124 1,928 Lease incentive receivable 1,237 — Other asset — 279 Grant income receivable 414 547 Other receivable 199 482 Deferred cost 3,008 — Total prepaid expenses and other current assets $ 42,899 $ 37,826

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2020
Property, Plant and Equipment [Abstract]
Property and Equipment, Net	Property and equipment, net consisted of the following (in thousands): December 31, 2020 2019 Lab equipment $ 23,491 $ 18,214 Office equipment 2,928 2,211 Furniture and fixtures 1,340 1,301 Leasehold improvements 10,629 10,316 Assets under construction 14,321 4,687 Less: accumulated depreciation (14,663) (8,565) Total property and equipment, net $ 38,046 $ 28,164

Intangible Asset (Tables)

Intangible Asset (Tables)	12 Months Ended
	Dec. 31, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
Summary of Finite Lived intangible Asset	The following table summarizes the carrying amount of the Company's intangible assets, net of accumulated amortization (in thousands): December 31, 2020 Estimated Life Cost Accumulated Amortization Net Software license 3 $ 291 $ (133) $ 158

Accrued Expenses and Other Li_2

Accrued Expenses and Other Liabilities (Tables)	12 Months Ended
	Dec. 31, 2020
Payables and Accruals [Abstract]
Schedule of Accrued Liabilities	Accrued expenses and other liabilities consisted of the following (in thousands): December 31, 2020 2019 Compensation and benefits 8,732 $ 6,568 Research and development costs 15,343 10,449 UCLB milestone 205 663 Professional fees 3,005 2,611 Corporate tax — 391 Other liabilities 496 716 Total accrued expenses and other liabilities $ 27,781 $ 21,398

Share Based Compensation (Table

Share Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2020
Share-based Payment Arrangement [Abstract]
Schedule of Stock Options Valuation Assumptions	The assumptions (see Note 2) used in the Black-Scholes option pricing model to determine the fair value of the share options granted to employees and directors during the years ended December 31, 2020 and 2019, the three months ended December 31, 2018, and the year ended September 30, 2018 were as follows: Year Ended December 31, Three Months Ended December 31, Year Ended September 30, 2020 2019 2018 2018 Expected option life (years) 5.27 to 6.08 5.27 to 6.08 6.08 6.08 Risk-free interest rate 0.31% to 1.66% 1.39% to 2.66% 2.70% to 3.09% 2.61% to 3.00% Expected volatility 76.38% to 81.45% 72.30% to 76.22% 69.52% to 71.26% 68.15% to 72.99% Expected dividend yield —% —% —% —%
Stock Option Rollforward	The table below summarizes activity for the years ended December 31, 2020 and 2019. Number of Weighted- Weighted- Aggregate Outstanding as of December 31, 2018 3,711,274 $ 19.25 9.47 $ 51,464 Granted 2,438,145 15.22 — — Exercised (11,724) 3.94 — $ 198 Forfeited (201,456) 19.72 — — Outstanding as of December 31, 2019 5,936,239 $ 17.71 9.02 $ 11,873 Granted 474,225 11.61 — — Exercised (115,194) 4.07 — — Forfeited (683,841) 20.06 — — Outstanding as of December 31, 2020 5,611,429 17.19 7.96 $ 4,262 Exercisable as of December 31, 2020 2,664,362 $ 17.36 7.36 $ 3,432 Vested and expected to vest as of December 31, 2020 5,611,429 $ 17.19 7.96 $ 4,262
Schedule of Restricted Stock Valuation Assumptions	The assumptions (Note 2) used in the OPM to determine the fair value of the ordinary shares for the following dates are as follows: March 2, 2016 April 26, 2017 September 25, 2017 March 31, 2018 May 31, 2018 Expected term 2.8 years 1.2 years 0.8 years 1.8 years 1.8 years Risk-free interest rate 1.0 % 1.0 % 1.3 % 2.1 % 2.1 % Expected volatility 73.2 % 76.6 % 71.0 % 71 % 71 % Expected dividend yield 0.0 % 0.0 % 0.0 % 0.0 % 0.0 %
Schedule of Nonvested Share Activity	A summary of the changes in the Company’s restricted ordinary shares during the years ended December 31, 2020 and 2019 are as follows and reflect the conversion of ordinary shares in the current and previous years. Number of Weighted average grant date fair value Unvested and outstanding at December 31, 2018 708,834 $ 4.10 Granted 0 0.00 Vested (389,755) 4.31 Forfeited (4,335) 4.43 Unvested and outstanding at December 31, 2019 314,744 4.22 Granted — 0 Vested (222,392) 4.24 Forfeited (1,969) 4.44 Unvested and outstanding at December 31, 2020 90,383 4.17 During the year ended December 31, 2020, the Company did not grant RSUs under the 2018 Plan. The following is a summary of RSU activity for the 2018 Plan for the years ended December 31, 2020 and 2019, respectively: Number of Weighted average Unvested and outstanding at December 31, 2018 — $ — Granted 500,000 12.09 Vested — — Forfeited — — Unvested and outstanding at December 31, 2019 500,000 12.09 Granted — — Vested — — Forfeited (85,000) 12.09 Unvested and outstanding at December 31, 2020 415,000 $ 12.09
Schedule of Allocated Share Based Compensation	Share-based compensation expense recorded as research and development and general and administrative expenses is as follows (in thousands): Year Ended December 31, Three Months December 31, Years Ended September 30, 2020 2019 2018 2018 Research and development $ 12,992 $ 17,761 $ 1,906 $ 3,116 General and administrative $ 7,115 12,451 1,487 3,649 Capitalized to fixed assets $ (86) 174 — — Total share-based compensation $ 20,021 $ 30,386 $ 3,393 $ 6,765

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Schedule of Reconciliation of Income Tax Expense (Benefit)	A reconciliation of income tax expense (benefit) at the statutory corporate income tax rate to the income tax expense (benefit) at the Company’s effective income tax rates is as follows (in thousands): Year ended December 31, Three months ended December 31, Year ended September 30, 2020 2019 2018 2018 Net loss before taxes $ (166,257) $ (139,008) $ (23,253) $ (52,031) U.K. statutory tax rate 19.0 % 19.0 % 19.0 % 19.0 % Income tax benefit at U.K. statutory tax rate (31,589) (26,411) (4,423) (9,886) Tax incentives / credits (23,076) (16,312) (3,234) (7,296) Non-deductible expenses 797 4,048 127 1,553 Adjustments in respect of prior years (1,088) 96 265 (13) Operating losses 28,672 21,643 3,605 7,317 Tax on property, plant, equipment and intangibles 547 267 140 233 Other, net 1,552 1,510 915 812 Foreign rate differential 22 $ — — $ — Total income tax benefit $ (24,163) $ (15,159) $ (2,605) $ (7,280) Current income tax benefit (22,819) (14,749) (2,605) (7,280) Deferred income tax benefit (1,344) (410) — — Effective rate of income tax 14.5 % 10.9 % 11.2 % 14.0 %
Schedule of Deferred Tax Assets and Liabilities	Deferred tax assets and liabilities consisted of the following at December 31, 2020 and 2019 (in thousands): December 31, 2020 2019 Deferred tax assets: Other differences $ 14,135 $ 1,522 Tax losses 40,221 19,624 Fixed assets 3,934 896 Total deferred tax assets 58,290 22,042 Valuation allowances (56,536) (21,632) Net deferred tax asset (liability) $ 1,754 $ 410

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Assets And Liabilities, Lessee	Leases (in thousands) Balance Sheet Classification As of December 31, 2020 2019 Assets Operating lease assets Operating right-of-use assets $ 51,637 $ 23,409 Liabilities Current Operating lease liabilities Current liabilities: Operating lease liabilities $ 3,590 $ 2,511 Noncurrent Operating lease liabilities Non-current liabilities: Operating lease liabilities $ 50,571 $ 23,710 Total lease liabilities $ 54,161 $ 26,221
Lease Cost	Year Ended December 31, Lease cost (in thousands) Statement of Operations Classification 2020 2019 Operating lease cost Operating expenses: research and development $ 5,266 $ 3,156 Variable lease cost Operating expenses: research and development 1,212 1,222 Operating lease cost Operating expenses: general and administrative 1,380 1,241 Variable lease cost Operating expenses: general and administrative 270 250 Short term lease costs 282 270 Total lease cost $ 8,410 $ 6,139 December 31, Other information (in thousands) 2020 2019 Cash paid for amounts included in the measurement of lease liabilities Operating cash flows from operating leases $ 4,896 $ 2,415 Right-of-use assets obtained in exchange for new operating lease liabilities $ 30,786 $ 26,124 Weighted-average remaining lease term — operating leases (years) 11.2 7.7 Weighted-average discount rate — operating leases 9.0 % 6.8 %
Lease Maturity Schedule	Future fixed payments for non-cancellable operating leases in effect as of December 31, 2020 are payable as follows: Operating Leases Maturity of lease liabilities for the years ending December 31, (in thousands) 2021 $ 7,026 2022 9,112 2023 8,644 2024 8,150 2025 7,044 Thereafter 51,259 Total lease payments 91,235 Less: imputed interest (37,074) Present value of lease liabilities $ 54,161

Nature of the Business (Details

Nature of the Business (Details) - USD ($) $ / shares in Units, $ in Thousands	Feb. 12, 2021	Jan. 07, 2021	Jun. 26, 2020	Jan. 27, 2020	Apr. 15, 2019	Jun. 22, 2018	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Sep. 30, 2018	Jun. 26, 2018
Class of Stock
Net loss							$ (20,648)	$ (44,751)	$ (142,094)	$ (123,849)	$ (44,751)
Accumulated deficit									(379,244)	(237,150)
Cash							$ 217,450	$ 246,984	$ 153,299	$ 210,643	$ 246,984
Subsequent Events
Class of Stock
Number of shares issued in transaction (shares)		1,700,000
Proceeds of issuance of ordinary shares, net of issuance costs		$ 15,300
Ordinary shares | Subsequent Events
Class of Stock
Number of shares issued in transaction (shares)	14,285,715
Per share price of issuance (usd per share)	$ 7
Proceeds of issuance of ordinary shares, net of issuance costs	$ 123,400
Ordinary shares | IPO
Class of Stock
Number of shares issued in transaction (shares)			10,147,059
Per share price of issuance (usd per share)												$ 17
Proceeds of issuance of ordinary shares, net of issuance costs						$ 156,500
Ordinary shares | Public Stock Offering
Class of Stock
Number of shares issued in transaction (shares)				7,250,000	4,830,000
Per share price of issuance (usd per share)				$ 11	$ 24
Proceeds of issuance of ordinary shares, net of issuance costs				$ 75,000	$ 108,800
Ordinary shares | Public Stock Offering | Subsequent Events
Class of Stock
Proceeds of issuance of ordinary shares, net of issuance costs	$ 108,100
Ordinary shares | Options
Class of Stock
Number of shares issued in transaction (shares)					630,000	1,323,529
Ordinary shares | Options | Subsequent Events
Class of Stock
Number of shares issued in transaction (shares)	2,142,857

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2018
Restricted Cash and Cash Equivalents Items
Restricted cash	$ 786	$ 787	$ 105	$ 105
Letter of credit collateral
Restricted Cash and Cash Equivalents Items
Restricted cash	600
Security deposit
Restricted Cash and Cash Equivalents Items
Restricted cash	$ 200

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Property and Equipment (Details) - USD ($)	3 Months Ended		9 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment
Fixed asset impairment		$ 0	$ 0	$ 0	$ 4,102,000
Decrease in depreciation expense		$ (645,000)	$ (1,709,000)	$ (5,658,000)	$ (4,611,000)
Increase in basic and diluted net loss per ordinary share (usd per share)		$ (0.52)	$ (1.42)	$ (2.76)	$ (2.88)
Change in fixed asset assumptions
Property, Plant and Equipment
Decrease in depreciation expense				$ 300,000
Increase in basic and diluted net loss per ordinary share (usd per share)				$ 0.01
Maximum
Property, Plant and Equipment
Fixed asset impairment			$ 10,000
Office equipment
Property, Plant and Equipment
Property, plant and equipment useful life				3 years
Lab equipment | Minimum
Property, Plant and Equipment
Property, plant and equipment useful life	5 years			5 years
Lab equipment | Maximum
Property, Plant and Equipment
Property, plant and equipment useful life	10 years			10 years
Furniture and fixtures
Property, Plant and Equipment
Property, plant and equipment useful life				5 years

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Intangible Assets Subject to Amortization (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets
Impairment of finite lived intangible assets	$ 0	$ 0	$ 0
Software
Finite-Lived Intangible Assets
Impairment of finite lived intangible assets				$ 400,000

Summary of Significant Accoun_7

Summary of Significant Accounting Policies - Segment Information (Details)	12 Months Ended
	Dec. 31, 2020regionsegment
Accounting Policies [Abstract]
Number of reportable segments | segment	1
Number of geographic regions | region	2

Summary of Significant Accoun_8

Summary of Significant Accounting Policies - Research and Development (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Research and development tax credits	$ 37	$ 200	$ 100	$ 200

Summary of Significant Accoun_9

Summary of Significant Accounting Policies - Share-Based Compensation (Details)	May 31, 2018	Mar. 31, 2018
Accounting Policies [Abstract]
Probability weighting applied to IPO	80.00%	50.00%

Summary of Significant Accou_10

Summary of Significant Accounting Policies - Foreign Currency Translation (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Foreign currency translation gain (loss)	$ 1.1	$ 4	$ 0.2	$ 4.6

Summary of Significant Accou_11

Summary of Significant Accounting Policies - Patent Costs (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Patent costs	$ 0.2	$ 1	$ 2.1	$ 2

Summary of Significant Accou_12

Summary of Significant Accounting Policies - Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Sep. 30, 2018
Accounting Policies [Abstract]
Foreign currency exchange translation adjustment	$ (5,568)	$ (6,071)	$ 2,830	$ 6,797	$ (6,071)

Summary of Significant Accou_13

Summary of Significant Accounting Policies - Schedule of Anti-dilutive Shares (Details) - shares	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive shares (shares)	4,420,108	2,881,113	6,116,812	6,777,983
Unvested restricted shares and units
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive shares (shares)	708,834	815,632	505,383	814,744
Incentive share options
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive shares (shares)	3,711,274	2,065,481	5,611,429	5,963,239

Summary of Significant Accou_14

Summary of Significant Accounting Policies - Recent Accounting Pronouncements Adopted (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)
Accounting Policies [Abstract]
Deferred implementation cost, current	$ 1
Deferred implementation cost, non-current	1.1
Deferred Implementation Costs	$ 0.3

Prepaid Expenses and Other Cu_3

Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Research and development claims receivable	$ 24,711	$ 27,567
Prepayments	10,206	7,023
VAT receivable	3,124	1,928
Lease incentive receivable	1,237	0
Other asset	0	279
Grant income receivable	414	547
Other receivable	199	482
Deferred cost	3,008	0
Total prepaid expenses and other current assets	$ 42,899	$ 37,826

Prepaid Expenses and Other Cu_4

Prepaid Expenses and Other Current Assets - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid Facility And Manufacturing	$ 1.9
Prepaid Clinical Trials	1.4
Capitalized implementation costs	1
Non-refundable upfront costs related to future payments	$ 2

Property and Equipment, Net (De

Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment
Less: accumulated depreciation	$ (14,663)	$ (8,565)
Total property and equipment, net	38,046	28,164
Lab equipment
Property, Plant and Equipment
Property and equipment, gross	23,491	18,214
Office equipment
Property, Plant and Equipment
Property and equipment, gross	2,928	2,211
Furniture and fixtures
Property, Plant and Equipment
Property and equipment, gross	1,340	1,301
Leasehold improvements
Property, Plant and Equipment
Property and equipment, gross	10,629	10,316
Assets under construction
Property, Plant and Equipment
Property and equipment, gross	$ 14,321	$ 4,687

Property and Equipment, Net - D

Property and Equipment, Net - Depreciation (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Abstract]
Depreciation	$ 645	$ 1,709	$ 5,658	$ 4,611

Intangible Asset (Details)

Intangible Asset (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets
Net	$ 158	$ 254
Amortization expense	91	$ 47,000
Amortization expense, 2020	92
Amortization expense, 2021	$ 66
Software license
Finite-Lived Intangible Assets
Estimated Life (years)	3 years
Cost	$ 291
Accumulated Amortization	(133)
Net	$ 158

Accrued Expenses and Other Li_3

Accrued Expenses and Other Liabilities (Details) £ in Thousands, $ in Thousands	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Mar. 31, 2018GBP (£)
Payables and Accruals [Abstract]
Compensation and benefits	$ 8,732	$ 6,568
Research and development costs	15,343	10,449
UCLB milestone	205	663	£ 350
Professional fees	3,005	2,611
Corporate tax	0	391
Other liabilities	496	716
Total accrued expenses and other liabilities	27,781	21,398
Accrued property tax	$ 400	200
Current portion of long term debt and leases		$ 300

Shareholders_ Equity - Ordinary

Shareholders’ Equity - Ordinary Shares (Details)	Jun. 26, 2020	Dec. 31, 2020$ / sharesshares	Dec. 31, 2020£ / sharesshares	Dec. 31, 2019$ / sharesshares	Jun. 26, 2018USD ($)
Class of Stock
Authorized grant, life	5 years
Ordinary shares
Class of Stock
Maximum nominal shares authorized for grant | $					$ 8,400
Common stock, shares, issued (shares)		52,346,231	52,346,231	44,983,006
Common stock, par value (usd/gbp per share) | $ / shares		$ 0.000042		$ 0.000042
Deferred Shares
Class of Stock
Common stock, shares, issued (shares)		34,425	34,425	34,425
Common stock, par value (usd/gbp per share) | (per share)		$ 0.00001	£ 0.00001	$ 0.00001
Deferred B shares
Class of Stock
Common stock, shares, issued (shares)		88,893,548	88,893,548	88,893,548
Common stock, par value (usd/gbp per share) | (per share)		$ 0.00099	£ 0.00099	$ 0.00099
Deferred C shares
Class of Stock
Common stock, shares, issued (shares)		1	1	1
Common stock, par value (usd/gbp per share) | (per share)		$ 0.000008	£ 0.000008	$ 0.000008

Shareholders_ Equity - Initial

Shareholders’ Equity - Initial Public Offering and Impact of Corporate Reorganization (Details)	Jun. 26, 2020shares	Jun. 26, 2018USD ($)$ / sharesshares	Dec. 31, 2018USD ($)	Sep. 30, 2018USD ($)	Dec. 31, 2020USD ($)shares	Dec. 31, 2019USD ($)	Jun. 26, 2018GBP (£)shares
Class of Stock
Proceeds of issuance of ordinary shares, net of issuance costs			$ 0	$ 156,920,000	$ 74,415,000	$ 108,863,000
Payment of stock issuance cost		$ 16,000,000
Equity conversion ratio	3.185
Ordinary shares
Class of Stock
Shares outstanding (shares) | shares					52,346,231
Shares value					$ 3,000	2,000
Ordinary shares | IPO
Class of Stock
Number of shares issued in transaction (shares) | shares	10,147,059
Per share price of issuance (usd per share) | $ / shares		$ 17
Proceeds of issuance of ordinary shares, net of issuance costs		$ 156,500,000
Deferred Shares
Class of Stock
Shares outstanding (shares) | shares		34,425					34,425
Shares value		$ 1			0	0
Deferred B shares
Class of Stock
Shares outstanding (shares) | shares		88,893,548					88,893,548
Shares value		$ 118,000			118,000	118,000
Share capital registration threshold | £							£ 50,000
Deferred C shares
Class of Stock
Shares value		$ 1			$ 0	$ 0

Shareholders_ Equity - Open Mar

Shareholders’ Equity - Open Market Sales Agreement (Details) $ in Millions	Sep. 18, 2020USD ($)
Class of Stock
Commission, percentage of gross sales price	0.030
Ordinary shares
Class of Stock
Sale of stock, authorized amount	$ 100

Share Based Compensation - 2018

Share Based Compensation - 2018 Equity Incentive Plan (Narratives) (Details) - shares	1 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2020
Stock options
Share-based Compensation Arrangement by Share-based Payment Award
Share based compensation term (years)		10 years
Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award
Vesting period (years)		3 years
Vesting Group One
Share-based Compensation Arrangement by Share-based Payment Award
Vesting period (years)		4 years
Award vesting percentage		25.00%
Vesting Group One | Performance Based
Share-based Compensation Arrangement by Share-based Payment Award
Vesting period (years)		4 years
Award vesting percentage		25.00%
Vesting Group One | Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award
Award vesting percentage		50.00%
Vesting Group Two
Share-based Compensation Arrangement by Share-based Payment Award
Vesting period (years)		3 years
Vesting Group Two | Performance Based
Share-based Compensation Arrangement by Share-based Payment Award
Vesting period (years)		3 years
Vesting Group Two | Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award
Award vesting percentage		50.00%
2018 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award
Shares authorized for distribution (shares)	14,000,000
Increase in reserved shares term	10 years
Periodical increase in authorized shares (percentage)	4.00%
2018 Equity Incentive Plan | Ordinary shares
Share-based Compensation Arrangement by Share-based Payment Award
Shares authorized for distribution (shares)	3,281,622
2018 Equity Incentive Plan - New | Ordinary shares
Share-based Compensation Arrangement by Share-based Payment Award
Shares authorized for distribution (shares)	3,025,548	8,778,719
2018 Equity Incentive Plan - Shares Available Under 2017 Plan | Ordinary shares
Share-based Compensation Arrangement by Share-based Payment Award
Shares authorized for distribution (shares)	256,074

Share Based Compensation - Opti

Share Based Compensation - Options Valuation Assumptions (Details) - Stock options	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology
Expected term (in years)	6 years 29 days	6 years 29 days
Risk-free interest rate minimum (percentage)	2.70%	3.00%	0.31%	1.39%
Risk-free interest rate maximum (percentage)	3.09%	2.61%	1.66%	2.66%
Expected volatility minimum (percentage)	69.52%	68.15%	76.38%	72.30%
Expected volatility maximum (percentage)	71.26%	72.99%	81.45%	76.22%
Expected dividend yield (percentage)	0.00%	0.00%	0.00%	0.00%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology
Expected term (in years)			5 years 3 months 7 days	5 years 3 months 7 days
Maximum
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology
Expected term (in years)			6 years 29 days	6 years 29 days

Share Based Compensation - Shar

Share Based Compensation - Share Options Rollforward (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding
Beginning balance (shares)	5,936,239	3,711,274
Granted (shares)	474,225	2,438,145
Exercised (shares)	(115,194)	(11,724)
Forfeited (shares)	(683,841)	(201,456)
Ending balance (shares)	5,611,429	5,936,239	3,711,274
Shares Exercisable (shares)	2,664,362
Vested and expected to vest (shares)	5,611,429
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price
Beginning balance (usd per share)	$ 17.71	$ 19.25
Granted (usd per share)	11.61	15.22
Exercised (usd per share)	4.07	3.94
Forfeited (usd per share)	20.06	19.72
Ending balance (usd per share)	17.19	$ 17.71	$ 19.25
Exercisable (usd per share)	17.36
Vested or expected to vest (usd per share)	$ 17.19
Weighted- Average Remaining Contractual Term (Years)
Options outstanding (weighted- average remaining contractual term years)	7 years 11 months 15 days	9 years 7 days	9 years 5 months 19 days
Exercisable (weighted average remaining contractual term years)	7 years 4 months 9 days
Vested and expected to vest (weighted- average remaining contractual term years)	7 years 11 months 15 days
Aggregate Intrinsic Value
Options outstanding (aggregate intrinsic value)	$ 4,262	$ 11,873	$ 51,464
Exercised (aggregate intrinsic value)	0	$ 198
Options exercisable (aggregate intrinsic value)	3,432
Options vested and expected to vest (aggregate intrinsic value)	$ 4,262

Share Based Compensation - Sh_2

Share Based Compensation - Share Options (Narratives) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award
Unrecognized stock based compensation			$ 13.2
Weighted average vesting period			2 years 6 months 25 days
Stock options
Share-based Compensation Arrangement by Share-based Payment Award
Weighted average grant date fair value (usd per share)	$ 20.04	$ 8.55	$ 7.82	$ 10.07

Share Based Compensation - Rest

Share Based Compensation - Restricted Ordinary Shares Valuation Assumptions (Details) - Restricted Ordinary Shares	May 31, 2018	Mar. 31, 2018	Sep. 25, 2017	Apr. 26, 2017	Mar. 02, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology
Expected term (in years)	1 year 9 months 18 days	1 year 9 months 18 days	9 months 18 days	1 year 2 months 12 days	2 years 9 months 18 days
Risk-free interest rate (percentage)	2.10%	2.10%	1.30%	1.00%	1.00%
Expected volatility (percentage)	71.00%	71.00%	71.00%	76.60%	73.20%
Expected dividend yield (percentage)	0.00%	0.00%	0.00%	0.00%	0.00%

Share Based Compensation - Re_2

Share Based Compensation - Restricted Ordinary Shares Rollforward (Details) - Restricted Ordinary Shares - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Number of restricted shares
Unvested and outstanding beginning balance (shares)	314,744	708,834
Granted (shares)	0	0
Vested (shares)	(222,392)	(389,755)
Forfeited (shares)	(1,969)	(4,335)
Unvested and outstanding ending balance (shares)	90,383	314,744
Weighted average grant date fair value
Unvested and outstanding beginning balance (usd per share)	$ 4.22	$ 4.10
Granted (usd per share)	0	0
Vested (usd per share)	4.24	4.31
Forfeited (usd per share)	4.44	4.43
Unvested and outstanding ending balance (usd per share)	$ 4.17	$ 4.22

Share Based Compensation - Re_3

Share Based Compensation - Restricted Ordinary Shares (Narratives) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)
Share-based Compensation Arrangement by Share-based Payment Award
Weighted average vesting period	2 years 6 months 25 days
Restricted Ordinary Shares
Share-based Compensation Arrangement by Share-based Payment Award
Unrecognized stock based compensation	$ 40
Weighted average vesting period	7 months 2 days

Share Based Compensation - Re_4

Share Based Compensation - Restricted Stock Unites (Narratives) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award
Weighted average vesting period	2 years 6 months 25 days
Vesting Group One
Share-based Compensation Arrangement by Share-based Payment Award
Vesting period (years)	4 years
Award vesting percentage	25.00%
Vesting Group Two
Share-based Compensation Arrangement by Share-based Payment Award
Vesting period (years)	3 years
Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award
Vesting period (years)	3 years
Granted (shares)	0	500,000
Unrecognized stock based compensation	$ 2.4
Weighted average vesting period	1 year 11 months 15 days
Restricted stock units | Vesting Group One
Share-based Compensation Arrangement by Share-based Payment Award
Award vesting percentage	50.00%
Restricted stock units | Vesting Group Two
Share-based Compensation Arrangement by Share-based Payment Award
Award vesting percentage	50.00%

Share Based Compensation - Re_5

Share Based Compensation - Restricted Stock Units Rollforward (Details) - Restricted stock units - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Number of restricted shares
Unvested and outstanding beginning balance (shares)	500,000	0
Granted (shares)	0	500,000
Vested (shares)	0	0
Forfeited (shares)	(85,000)	0
Unvested and outstanding ending balance (shares)	415,000	500,000
Weighted average grant date fair value
Unvested and outstanding beginning balance (usd per share)	$ 12.09	$ 0
Granted (usd per share)	0	12.09
Vested (usd per share)	0	0
Forfeited (usd per share)	12.09	0
Unvested and outstanding ending balance (usd per share)	$ 12.09	$ 12.09

Share Based Compensation - Comp

Share Based Compensation - Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award
Total share-based compensation	$ 3,393	$ 6,765	$ 20,021	$ 30,386
Research and development
Share-based Compensation Arrangement by Share-based Payment Award
Total share-based compensation	1,906	3,116	12,992	17,761
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award
Total share-based compensation	1,487	3,649	7,115	12,451
Capitalized to fixed assets
Share-based Compensation Arrangement by Share-based Payment Award
Total share-based compensation	$ 0	$ 0	$ (86)	$ 174

License Agreements (Details)

License Agreements (Details) £ in Thousands, $ in Thousands	1 Months Ended
	Mar. 31, 2018GBP (£)	Mar. 31, 2016USD ($)shares	Sep. 30, 2014shares	Dec. 31, 2020USD ($)	Dec. 31, 2020GBP (£)	Dec. 31, 2019USD ($)
Schedule Of License Agreements
Shares issued (shares) | shares		313,971
Accrued license fees payable	£ 350			$ 205		$ 663
Product Sales | Minimum
Schedule Of License Agreements
Conditional payment					£ 1,000
Product Sales | Maximum
Schedule Of License Agreements
Conditional payment					18,500
UCLB
Schedule Of License Agreements
Shares issued (shares) | shares			1,497,643
Payment of upfront fees	£ 1,500	$ 300
UCLB | Clinical Activities
Schedule Of License Agreements
Conditional payment					180
UCLB | Regulatory Approval
Schedule Of License Agreements
Conditional payment					37,500
UCLB | Product Sales
Schedule Of License Agreements
Conditional payment | $				$ 18,000
UCLB | Net Sales Levels
Schedule Of License Agreements
Conditional payment					£ 51,000

Income Taxes - (Narratives) (De

Income Taxes - (Narratives) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income tax expense (benefit)	$ (2,605)	$ (7,280)	$ (24,163)	$ (15,159)
Increase in deferred tax assets			1,300
Deferred tax asset			1,754	410
Operating loss carryforward, foreign			211,700
Deferred tax asset valuation allowances			$ 56,536	$ 21,632

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Income Tax Expense (Benefit) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Net loss before taxes	$ (23,253)	$ (52,031)	$ (166,257)	$ (139,008)
U.K. statutory tax rate (percentage)	19.00%	19.00%	19.00%	19.00%
Income tax benefit at U.K. statutory tax rate	$ (4,423)	$ (9,886)	$ (31,589)	$ (26,411)
Tax incentives / credits	(3,234)	(7,296)	(23,076)	(16,312)
Non-deductible expenses	127	1,553	797	4,048
Adjustments in respect of prior years	265	(13)	(1,088)	96
Operating losses	3,605	7,317	28,672	21,643
Tax on property, plant, equipment and intangibles	140	233	547	267
Other, net	915	812	1,552	1,510
Foreign rate differential	0	0	22	0
Total income tax benefit	(2,605)	(7,280)	(24,163)	(15,159)
Current income tax benefit	(2,605)	(7,280)	(22,819)	(14,749)
Deferred income tax benefit	$ 0	$ 0	$ (1,344)	$ (410)
Effective rate of income tax (percentage)	11.20%	14.00%	14.50%	10.90%

Income Taxes - Deferred Tax Ass

Income Taxes - Deferred Tax Assets Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Other differences	$ 14,135	$ 1,522
Tax losses	40,221	19,624
Fixed assets	3,934	896
Total deferred tax assets	58,290	22,042
Valuation allowances	(56,536)	(21,632)
Net deferred tax asset (liability)	$ 1,754	$ 410

Commitments and Contingencies -

Commitments and Contingencies - (Narratives) (Details) $ in Thousands	1 Months Ended		3 Months Ended			9 Months Ended	12 Months Ended
	Oct. 31, 2018	Sep. 30, 2017	Sep. 30, 2020USD ($)	Mar. 31, 2020USD ($)	Dec. 31, 2018USD ($)	Sep. 30, 2018USD ($)unit	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Jun. 30, 2018
Loss Contingencies
Gain on termination of operating lease					$ 0	$ 0	$ 160	$ 0
Lease incentive receivable			$ 1,200
Rent expense					$ 500	$ 900
Imperial (Forest House) Limited
Loss Contingencies
Gain on termination of operating lease			$ 1,300
Imperial (Forest House) Limited | Building
Loss Contingencies
Lease term (in years)							10 years
Imperial (Forest House) Limited | Additional Space
Loss Contingencies
Lease term (in years)							15 months
Catapult Manufacturing Center | Building
Loss Contingencies
Rent free period (in months)		6 months
Laboratory Space In White City, London
Loss Contingencies
Leasehold improvements, gross							$ 6,700
Laboratory Space In White City, London | Building
Loss Contingencies
Lease term (in years)									10 years
Manufacturing Space In Enfield, United Kingdom | Building
Loss Contingencies
Lease term (in years)						15 years
Number of units (units) | unit						3
Leasehold Improvements								$ 4,100
Office Space In Rockville, Maryland | Building
Loss Contingencies
Gain on termination of operating lease				$ 200
Operating lease rent free period	5 years
Office And Manufacturing Space in Rockville, Maryland
Loss Contingencies
Leasehold improvements, gross							$ 2,400

Commitments and Contingencies_2

Commitments and Contingencies - Lease Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets
Operating lease assets	$ 51,637	$ 23,409
Current
Lease liability	3,590	2,511
Noncurrent
Operating lease liabilities	50,571	23,710
Total lease liabilities	$ 54,161	$ 26,221
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:OtherAssets	us-gaap:OtherAssets
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:LiabilitiesCurrentAbstract	us-gaap:LiabilitiesCurrentAbstract
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:OperatingLeaseLiabilityNoncurrent	us-gaap:OperatingLeaseLiabilityNoncurrent

Commitments and Contingencies_3

Commitments and Contingencies - Lease Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Lease, Cost
Short term lease costs	$ 282	$ 270
Total lease cost	8,410	6,139
Research and development
Lease, Cost
Operating lease cost	5,266	3,156
Variable lease cost	1,212	1,222
General and administrative
Lease, Cost
Operating lease cost	1,380	1,241
Variable lease cost	$ 270	$ 250

Commitments and Contingencies_4

Commitments and Contingencies - Other Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Cash paid for amounts included in the measurement of lease liabilities
Operating cash flows from operating leases	$ 4,896	$ 2,415
Leased assets obtained in exchange for operating lease liabilities	$ 30,786	$ 26,124
Weighted-average remaining lease term — operating leases (years)	11 years 2 months 12 days	7 years 8 months 12 days
Weighted-average discount rate — operating leases (percent)	9.00%	6.80%

Commitments and Contingencies_5

Commitments and Contingencies - Lease Maturity Schedule (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Lessee, Operating Lease, Liability, Payment, Due [Abstract]
2021	$ 7,026
2022	9,112
2023	8,644
2024	8,150
2025	7,044
Thereafter	51,259
Total lease payments	91,235
Less: imputed interest	(37,074)
Present value of lease liabilities	$ 54,161	$ 26,221

Employee Benefit Plans (Details

Employee Benefit Plans (Details) - USD ($)	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019
Retirement Benefits [Abstract]
Employer contributions to defined benefit pension schemes	$ 200,000	$ 500,000	$ 1,300,000	$ 1,000,000
Employer contribution (percent)			4.00%
Employer contributions	$ 18,000	$ 0	$ 300,000	$ 200,000

Subsequent Events (Details)

Subsequent Events (Details) - USD ($) $ / shares in Units, $ in Millions	Feb. 12, 2021	Jan. 07, 2021	Jan. 27, 2020	Apr. 15, 2019	Jun. 22, 2018	Jan. 31, 2021	Jun. 30, 2021
Ordinary shares | Options
Subsequent Event
Number of shares issued in transaction (shares)				630,000	1,323,529
Ordinary shares | Public Stock Offering
Subsequent Event
Number of shares issued in transaction (shares)			7,250,000	4,830,000
Proceeds of issuance of ordinary shares, net of issuance costs			$ 75	$ 108.8
Per share price of issuance (usd per share)			$ 11	$ 24
Subsequent Events
Subsequent Event
Restructuring and related cost, number of positions eliminated, period expected percent						20.00%
Number of shares issued in transaction (shares)		1,700,000
Proceeds of issuance of ordinary shares, net of issuance costs		$ 15.3
Cash savings, annual basis per annum						$ 15
Subsequent Events | Forecast
Subsequent Event
Restructuring and related cost, incurred cost							$ 2.5
Subsequent Events | Ordinary shares
Subsequent Event
Number of shares issued in transaction (shares)	14,285,715
Proceeds of issuance of ordinary shares, net of issuance costs	$ 123.4
Per share price of issuance (usd per share)	$ 7
Subsequent Events | Ordinary shares | Options
Subsequent Event
Number of shares issued in transaction (shares)	2,142,857
Subsequent Events | Ordinary shares | Public Stock Offering
Subsequent Event
Proceeds of issuance of ordinary shares, net of issuance costs	$ 108.1