REGENXBIO (RGNX) 10-K 1 Mar 21 2020 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2020	Feb. 25, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Trading Symbol	RGNX
Entity Registrant Name	REGENXBIO Inc.
Entity Central Index Key	0001590877
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		42,498,483
Entity Public Float			$ 977,613,237
Entity Interactive Data Current	Yes
Title of 12(b) Security	Common Stock, par value $0.0001 per share
Security Exchange Name	NASDAQ
Entity File Number	001-37553
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	47-1851754
Entity Address, Address Line One	9600 Blackwell Road
Entity Address, Address Line Two	Suite 210
Entity Address, City or Town	Rockville
Entity Address, State or Province	MD
Entity Address, Postal Zip Code	20850
City Area Code	240
Local Phone Number	552-8181
Document Annual Report	true
Document Transition Report	false
Documents Incorporated by Reference	Specified portions of the registrant’s definitive proxy statement with respect to the registrant’s 2021 Annual Meeting of Stockholders, which is to be filed pursuant to Regulation 14A within 120 days after the end of the registrant’s fiscal year ended December 31, 2020, are incorporated by reference into Part III of this Annual Report on Form 10-K.

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current assets
Cash and cash equivalents	$ 338,426	$ 69,514
Marketable securities	137,314	226,696
Accounts receivable (net of allowance of $7,678 as of December 31, 2020)	42,999	38,148
Prepaid expenses	10,505	6,475
Other current assets	1,953	4,199
Total current assets	531,197	345,032
Marketable securities	46,809	103,785
Accounts receivable	3,267	4,155
Property and equipment, net	56,467	28,973
Operating lease right-of-use assets	63,815	10,078
Restricted cash	1,330	1,330
Other assets	5,279	4,555
Total assets	708,164	497,908
Current liabilities
Accounts payable	10,622	6,409
Accrued expenses and other current liabilities	49,082	24,846
Deferred revenue	449
Operating lease liabilities	2,500	2,421
Liability related to sale of future royalties	18,794
Total current liabilities	81,447	33,676
Deferred revenue	3,783	3,333
Operating lease liabilities	70,153	8,874
Liability related to sale of future royalties	174,504
Other liabilities	524	1,828
Total liabilities	330,411	47,711
Commitments and contingencies (Note 8)
Stockholders’ equity
Preferred stock; $0.0001 par value; 10,000 shares authorized, and no shares issued and outstanding at December 31, 2020 and December 31, 2019
Common stock; $0.0001 par value; 100,000 shares authorized at December 31, 2020 and December 31, 2019; 37,476 and 36,992 shares issued and outstanding at December 31, 2020 and December 31, 2019, respectively	4	4
Additional paid-in capital	667,181	627,810
Accumulated other comprehensive income (loss)	(360)	205
Accumulated deficit	(289,072)	(177,822)
Total stockholders’ equity	377,753	450,197
Total liabilities and stockholders’ equity	$ 708,164	$ 497,908

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Statement Of Financial Position [Abstract]
Accounts receivable, allowance for credit loss, current	$ 7,678
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	37,476,000	36,992,000
Common stock, shares outstanding	37,476,000	36,992,000

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Income (Loss) - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues
Revenues	$ 154,567	$ 35,233	$ 218,505
Operating Expenses
Cost of revenues	35,714	8,241	9,640
Research and development	166,294	124,185	83,873
General and administrative	63,817	51,815	36,850
Provision for credit losses and other	7,975	(10)	42
Total operating expenses	273,800	184,231	130,405
Income (loss) from operations	(119,233)	(148,998)	88,100
Other Income
Interest income from licensing	4,271	2,951	8,946
Investment income	9,723	48,559	7,070
Interest expense	(771)
Total other income	13,223	51,510	16,016
Income (loss) before income taxes	(106,010)	(97,488)	104,116
Income Tax Benefit (Expense)	(5,240)	2,755	(4,179)
Net income (loss)	(111,250)	(94,733)	99,937
Other Comprehensive Income (Loss)
Unrealized gain (loss) on available-for-sale securities, net	(565)	885	(5)
Total other comprehensive income (loss)	(565)	885	(5)
Comprehensive income (loss)	$ (111,815)	$ (93,848)	$ 99,932
Net income (loss) per share:
Basic	$ (2.98)	$ (2.58)	$ 2.99
Diluted	$ (2.98)	$ (2.58)	$ 2.73
Weighted-average common shares outstanding:
Basic	37,281	36,690	33,427
Diluted	37,281	36,690	36,648
License and royalty [Member]
Revenues
Revenues	$ 154,567	$ 35,233	$ 218,505

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Additional paid in capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Balances at Dec. 31, 2017	$ 183,029	$ 3	$ 371,497	$ (715)	$ (187,756)
Balances (Shares) at Dec. 31, 2017		31,295
Adoption of ASU 2014-09 (Topic 606) | ASU 2014-09 [Member]	4,803				4,803
Issuance of common stock upon public offering, net of transaction costs	189,097	$ 1	189,096
Issuance of stock, net of transaction costs		3,105
Exercise of stock options	14,499		14,499
Exercise of stock options, Shares		1,683
Issuance of common stock under employee stock purchase plan	847		847
Issuance of common stock under employee stock purchase plan, shares		37
Stock-based compensation expense	16,641		16,641
Unrealized loss on available-for-sale securities, net	(5)			(5)
Net income (loss)	99,937				99,937
Balances at Dec. 31, 2018	508,848	$ 4	592,580	(720)	(83,016)
Balances (Shares) at Dec. 31, 2018		36,120
Adoption of ASU 2014-09 (Topic 606) | ASU 2016-02 [Member]	(33)				(33)
Adoption of ASU 2014-09 (Topic 606) | ASU 2018-02 [Member]				40	(40)
Vesting of restricted stock units, Shares		40
Exercise of stock options	7,062		7,062
Exercise of stock options, Shares		796
Issuance of common stock under employee stock purchase plan	1,314		1,314
Issuance of common stock under employee stock purchase plan, shares		36
Stock-based compensation expense	26,854		26,854
Unrealized loss on available-for-sale securities, net	885			885
Net income (loss)	(94,733)				(94,733)
Balances at Dec. 31, 2019	450,197	$ 4	627,810	205	(177,822)
Balances (Shares) at Dec. 31, 2019		36,992
Exercise of stock options	5,623		5,623
Exercise of stock options, Shares		428
Issuance of common stock under employee stock purchase plan	1,799		1,799
Issuance of common stock under employee stock purchase plan, shares		55
Stock-based compensation expense	31,949		31,949
Unrealized loss on available-for-sale securities, net	(565)			(565)
Net income (loss)	(111,250)				(111,250)
Balances at Dec. 31, 2020	$ 377,753	$ 4	$ 667,181	$ (360)	$ (289,072)
Balances (Shares) at Dec. 31, 2020		37,476

Consolidated Statements of St_2

Consolidated Statements of Stockholders' Equity (Parenthetical) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)
Common Stock [Member]
Issuance of Stock, transaction costs	$ 12,728

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities
Net income (loss)	$ (111,250)	$ (94,733)	$ 99,937
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities
Stock-based compensation expense	31,949	26,854	16,641
Depreciation and amortization	8,407	7,152	3,982
Provision for credit losses	7,678	0	0
Net amortization of premiums (accretion of discounts) on marketable securities	1,083	(1,196)	755
Net realized and unrealized losses (gains) on marketable securities	(4,918)	(37,774)	39
Imputed interest income from licensing	(2,163)	(2,951)	(8,946)
Non-cash interest expense	771
Other non-cash adjustments	163	268	14
Changes in operating assets and liabilities
Accounts receivable	(9,898)	(8,622)	(16,803)
Prepaid expenses	(4,030)	(973)	(400)
Other current assets	2,341	(499)	(2,069)
Operating lease right-of-use assets	3,219	2,431
Other assets	399	(2,694)	(1,453)
Accounts payable	3,873	1,528	(218)
Accrued expenses and other current liabilities	17,400	6,882	7,582
Deferred revenue		(600)	3,933
Operating lease liabilities	2,139	(2,255)
Deferred rent			(32)
Other liabilities	(1,224)	(523)	1,686
Net cash provided by (used in) operating activities	(54,061)	(107,705)	104,648
Cash flows from investing activities
Purchases of marketable debt securities	123,041	190,735	445,829
Maturities of marketable debt securities	233,468	289,994	179,749
Sales of marketable debt securities	2,287
Sales of marketable equity securities	36,914	6,020
Purchases of property and equipment	(26,869)	(11,720)	(13,278)
Net cash provided by (used in) investing activities	122,759	93,559	(279,358)
Cash flows from financing activities
Proceeds from exercise of stock options	5,623	7,062	14,499
Proceeds from issuance of common stock under employee stock purchase plan	1,799	1,314	847
Proceeds from public offerings of common stock, net of underwriting discounts and commissions			189,716
Issuance costs for public offerings of common stock			(619)
Proceeds from sale of future royalties	196,000
Transaction costs for sale of future royalties	(3,208)
Net cash provided by financing activities	200,214	8,376	204,443
Net increase (decrease) in cash and cash equivalents and restricted cash	268,912	(5,770)	29,733
Cash and cash equivalents and restricted cash
Beginning of period	70,844	76,614	46,881
End of period	339,756	70,844	76,614
Supplemental cash flow information
Cash paid (received) for income taxes	(191)	904	3,443
Supplemental disclosures of non-cash investing and financing activities
Additions to property and equipment through accounts payable and accrued expenses	6,812	$ 1,572
Non-cash additions to property and equipment through tenant improvement allowance	2,263
Assets acquired under financing lease obligation			$ 5,854
Non-cash consideration received for licenses granted	1,123
Transaction costs for sale of future royalties in accounts payable and accrued expenses	$ 265

Nature of Business

Nature of Business	12 Months Ended
	Dec. 31, 2020
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of Business	1. Nature of Business REGENXBIO Inc. (the Company) is a clinical-stage biotechnology company seeking to improve lives through the curative potential of gene therapy. The Company’s proprietary adeno-associated virus (AAV) gene delivery platform (NAV Technology Platform) consists of exclusive rights to over 100 novel AAV vectors, including AAV7, AAV8, AAV9 and AAVrh10. The NAV® Technology Platform is being applied by the Company, as well as by third-party licensees (NAV Technology Licensees), in the development of a broad pipeline of product candidates in multiple therapeutic areas and in one commercially available product, Zolgensma®, which is marketed by a NAV Technology Licensee. The Company was formed in 2008 in the State of Delaware and is headquartered in Rockville, Maryland. As of December 31, 2020, the Company had generated an accumulated deficit of $289.1 million since inception. As the Company has incurred cumulative losses since inception, transition to recurring profitability is dependent upon achieving a level of revenues adequate to support the Company’s cost structure, which depends heavily on the successful development, approval and commercialization of its product candidates. The Company may never achieve recurring profitability, and unless and until it does, the Company will continue to need to raise additional capital, to the extent possible. As of December 31, 2020, the Company had cash, cash equivalents and marketable securities of $522.5 million, which management believes is sufficient to fund operations for at least the next 12 months from the date these consolidated financial statements were issued.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation and Principles of Consolidation The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (GAAP) and include the accounts of the Company and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. Foreign Currency Transactions The functional currency of the Company and its consolidated subsidiaries is the U.S. dollar. Transaction gains and losses that arise from exchange rate fluctuations on transactions denominated in currencies other than the U.S. dollar are included in results of operations as incurred. Use of Estimates The preparation of the consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses and related disclosure of contingent assets and liabilities for the periods presented. Management bases its estimates on historical experience and on various other factors that it believes are reasonable under the circumstances, the results of which form the basis for making judgments about the carrying value of assets and liabilities, and other reported amounts, that are not readily apparent from other sources. Actual results may differ materially from these estimates. Significant estimates are used in the following areas, among others: license and royalty revenue, the allowance for credit losses, accrued research and development expenses and other accrued liabilities, stock-based compensation expense, income taxes and the fair value of financial instruments. The Company is actively monitoring the impact of the COVID-19 pandemic on its business, results of operations and financial condition. The full extent to which the COVID-19 pandemic will directly or indirectly impact the Company’s business, results of operations and financial condition in the future is unknown at this time and will depend on future developments that are highly unpredictable. The most significant estimates affecting the Company’s consolidated financial statements that may be impacted by the COVID-19 pandemic are related to the Company’s assessment of credit losses on accounts receivable, contract assets and available-for-sale debt securities. Reclassifications Certain amounts reported in prior periods have been reclassified to conform to current period financial statement presentation. These reclassifications are not material and have no effect on previously reported financial position, results of operations and cash flows. Segment and Geographical Information Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker (CODM), or decision-making group, in making decisions on how to allocate resources and assess performance. The Company’s CODM, its Chief Executive Officer, views its operations and manages its business as one operating segment. The Company’s revenues consist of license and royalty revenue. For the year ended December 31, 2020, 80% of the Company’s revenues were attributed to the U.S. and no other countries were attributed 10% or more of the Company’s revenues. For the year ended December 31, 2019, 90% of the Company’s revenues were attributed to the U.S. and no other countries were attributed 10% or more of the Company’s revenues. For the year ended December 31, 2018, 99% of the Company’s revenue were attributed to the U.S. The country of origin for license revenue is determined based on the country of domicile of the licensee. The country of origin for royalty revenue is determined based on the location of the underlying net sales of licensed products. The substantial majority of the Company’s assets reside in the U.S. Cash and Cash Equivalents The Company considers all highly liquid investments purchased with original maturities of 90 days or less at acquisition to be cash equivalents. Restricted Cash Restricted cash includes money market mutual funds used to collateralize irrevocable letters of credit as required by the Company’s lease agreements. The following table provides a reconciliation of cash and cash equivalents and restricted cash as reported on the consolidated balance sheets to the total of these amounts as reported at the end of the period in the consolidated statements of cash flows (in thousands): December 31, 2020 December 31, 2019 December 31, 2018 Cash and cash equivalents $ 338,426 $ 69,514 $ 75,561 Restricted cash 1,330 1,330 1,053 Total cash and cash equivalents and restricted cash $ 339,756 $ 70,844 $ 76,614 Marketable Securities Marketable securities consist of available-for-sale debt securities and equity securities and are carried at fair value. Marketable debt securities with remaining maturity dates exceeding 12 months which are not intended to be sold prior to maturity for use in current operations are classified as non-current assets. Marketable equity securities are classified as current assets. Unrealized gains and losses on available-for-sale debt securities, net of any related tax effects, are excluded from results of operations and are included in other comprehensive income (loss) and reported as a separate component of stockholders’ equity until realized. The Company uses the aggregate portfolio approach to release the tax effects of unrealized gains and losses on available-for-sale debt securities in accumulated other comprehensive income (loss). Purchase premiums and discounts on marketable debt securities are amortized or accreted into the cost basis over the life of the related security as adjustments to the yield using the effective-interest method. Interest income is recognized when earned. Unrealized gains and losses on marketable equity securities are included in results of operations as investment income. Realized gains and losses from the sale or maturity of marketable securities are based on the specific identification method and are included in results of operations as investment income. At each reporting date, the Company evaluates available-for-sale debt securities which have an amortized cost basis in excess of the fair value of the security to determine if the unrealized loss or any potential credit losses should be recognized in results of operations . If the Company does not have the intent and ability to hold the security until recovery of the unrealized loss, the difference between the fair value and amortized cost basis of the security is charged to results of operations resulting in a new amortized cost basis of the security. If the Company has the intent and ability to hold the security until recovery of the unrealized loss, the security is evaluated for potential credit losses. If a credit loss is deemed to exist, the credit loss is recognized in results of operations and an allowance for credit losses is recorded against the amortized cost basis of the security. In determining whether a credit loss exists related to impaired available-for-sale debt securities, the Company considers, among other factors, the extent of the unrealized loss relative to the amortized cost basis, the credit rating of the issuer and any recent changes thereto, current and expected future economic conditions, and any adverse events or other changes in circumstances that have occurred which may indicate a potential credit loss. The Company did not record an allowance for credit losses on its available-for-sale debt securities as of December 31 , 2020 or 2019 . Accounts Receivable Accounts receivable primarily consist of consideration due to the Company resulting from its license agreements with NAV Technology Licensees. Accounts receivable include amounts invoiced to licensees as well as rights to consideration which have not yet been invoiced, including unbilled royalties, and for which payment is conditional solely upon the passage of time. If a licensee elects to terminate a license prior to the end of the license term, the licensed intellectual property is returned to the Company and any accounts receivable from the licensee which are not contractually payable to the Company are charged off as a reduction of license revenue in the period of the termination. Accounts receivable which are not expected to be received by the Company within 12 months from the reporting date are stated net of a discount to present value and recorded as non-current assets on the consolidated balance sheets. The present value discount is recognized as a reduction of revenue in the period in which the accounts receivable are initially recorded and is accreted as interest income from licensing over the term of the receivables. Accounts receivable are stated net of an allowance for credit losses, if deemed necessary based on the Company’s evaluation of collectability and potential credit losses. Management assesses the collectability of its accounts receivable using the specific identification of account balances, and considers the credit quality and financial condition of its significant customers, historical information regarding credit losses and the Company’s evaluation of current and expected future economic conditions. If necessary, an allowance for credit losses is recorded against accounts receivable such that the carrying value of accounts receivable reflects the net amount expected to be collected. Accounts receivable balances are written off against the allowance for credit losses when the potential for collectability is considered remote. Please refer to Note 10 for further information regarding the allowance for credit losses related to accounts receivable. Concentrations of Credit Risk and Off-balance Sheet Risk Cash and cash equivalents, marketable debt securities and accounts receivable are financial instruments that are potentially subject to concentrations of credit risk. The Company’s cash and cash equivalents are deposited in accounts at multiple financial institutions, and amounts may exceed federally insured limits. The Company believes it is not exposed to significant credit risk due to the financial strength of the depository institutions in which the cash and cash equivalents are held. The Company’s marketable debt securities consist of investment grade securities and may be subject to concentrations of credit risk. The Company has adopted an investment policy which limits potential concentrations of investments and establishes minimum acceptable credit ratings, thereby reducing credit risk exposure. With the exception of accounts receivable from Abeona Therapeutics Inc. (Abeona), as discussed further in Note 10, the Company believes that it is not exposed to significant credit risk related to accounts receivable due to the credit quality and history of collections from its significant customers, and the Company is unaware of any concentrations of credit risk related to accounts receivable from significant customers with deteriorated credit quality. The Company has no financial instruments with off-balance sheet risk of loss. The following table summarizes those customers who represented at least 10% of revenues or total net accounts receivable for the periods presented: Revenues Accounts Receivable, Net Years Ended December 31, December 31, 2020 2019 2018 2020 2019 Customer A 94 % 69 % 81 % 44 % 28 % Customer B * * 16 % 48 % 62 % Customer C * 13 % * * * Customer D * 10 % * * * * Represented less than 10% Leases Effective January 1, 2019, the Company adopted Accounting Standards Update (ASU) 2016-02, Leases Leases Under Topic 842, the Company classifies its leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the Company. Lease classification is evaluated at the inception of the lease agreement. Regardless of classification, the Company records a right-of-use asset and a lease liability for all leases with a term greater than 12 months. All of the Company’s leases as of December 31, 2020 and 2019 have been classified as operating leases. Operating lease expense is recognized on a straight-line basis over the term of the lease, with the exception of variable lease expenses which are recognized as incurred. The Company identifies leases in its contracts if the contract conveys the right to control the use of identified property, plant or equipment for a period of time in exchange for consideration. The Company does not allocate lease consideration between lease and nonlease components and records a lease liability equal to the present value of the remaining fixed consideration under the lease. The interest rates implicit in the Company’s leases are generally not readily determinable. Accordingly, the Company uses its estimated incremental borrowing rate at the commencement date of the lease to determine the present value discount of the lease liability. The Company estimates its incremental borrowing rate for each lease based on an evaluation of its expected credit rating and the prevailing market rates for collateralized debt in a similar economic environment with similar payment terms and maturity dates commensurate with the term of the lease. The right-of-use asset for each lease is equal to the lease liability, adjusted for unamortized initial direct costs and lease incentives and prepaid or accrued rent. Initial direct costs of entering into a lease are included in the right-of-use asset and amortized as lease expense over the term of the lease. Lease incentives, such as tenant improvements allowances, are recorded as a reduction of the right-of-use asset and amortized as a reduction of lease expense over the term of the lease. The Company excludes options to extend or terminate leases from the calculation of the lease liability unless it is reasonably certain the option will be exercised. Property and Equipment Property and equipment is stated at cost less accumulated depreciation and amortization. Maintenance and repairs that do not improve or extend the lives of the respective assets are expensed to operations as incurred. Upon disposal, the related cost and accumulated depreciation is removed from the accounts and any resulting gain or loss is included in the results of operations. Depreciation and amortization is calculated using the straight-line method over the estimated useful lives of the assets, which are as follows: Estimated Useful Life Computer equipment and software 3 years Furniture and fixtures 5 years Laboratory and manufacturing equipment 5 to 15 years Leasehold improvements Shorter of lease term or estimated useful life Impairment of Long-lived Assets The Company evaluates its long-lived assets for impairment when events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Recoverability is measured by comparison of the book values of the assets to estimated future net undiscounted cash flows that the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the book value of the assets exceed their fair value, which is measured based on the projected discounted future net cash flows arising from the assets. No impairment losses on long-lived assets were recorded during the years ended December 31, 2020, 2019 and 2018. Non-marketable Equity Securities The Company’s non-marketable equity securities consist of equity investments in other entities in which the Company’s ownership interest is below 20% and the Company does not have significant influence over the operations of the entity, or for which the equity securities are not common stock or in-substance common stock. The Company’s non-marketable equity securities do not have readily determinable fair values and are measured at cost less impairment, adjusted for observable price changes for identical or similar investments of the same issuer. Please refer to Note 4 for further information on non-marketable equity securities. Fair Value of Financial Instruments The Company is required to disclose information on all assets and liabilities reported at fair value that enables an assessment of the inputs used in determining the reported fair values. ASC 820, Fair Value Measurements and Disclosures • Level 1—Valuations based on unadjusted quoted prices in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date. • Level 2—Valuations based on quoted prices for similar assets or liabilities in markets that are not active or for which all significant inputs are observable, either directly or indirectly. • Level 3—Valuations that require inputs that reflect the Company’s own assumptions that are both significant to the fair value measurement and unobservable. To the extent that valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. The fair values of the Company’s Level 2 instruments are based on quoted market prices or broker or dealer quotations for similar assets. These investments are initially valued at the transaction price and subsequently valued utilizing third party pricing providers or other market observable data. Please refer to Note 4 for further information on the fair value measurement of the Company’s financial instruments. Liability Related to Sale of Future Royalties As discussed in Note 7, the Company recorded a liability for the net proceeds received from the sale of its Zolgensma royalty payments to entities managed by Healthcare Royalty Management, LLC (collectively, HCR). The liability is accounted for as debt since the return to HCR is explicitly capped under the royalty purchase agreement, and is amortized over the estimated life of the arrangement using the effective interest method. The total amount of royalty payments received by HCR under the agreement, less the net proceeds received by the Company, is recorded as non-cash interest expense over the life of the arrangement. The Company estimates the effective interest rate based on its estimate of total royalty payments to be received by HCR under the agreement. The Company reassesses these estimates at each reporting date and adjusts the effective interest rate and amortization of the liability on a prospective basis as necessary. Due to its continuing involvement in the underlying license agreement with Novartis Gene Therapies, Inc. (formerly AveXis, Inc.), the Company continues to recognize royalty revenue on net sales of Zolgensma and records the royalty payments to HCR as a reduction of the liability when paid. As such payments are made to HCR, the balance of the liability will be effectively repaid over the life of the royalty purchase agreement. The portion of the liability related to the sale of future royalties which is expected to be amortized within 12 months of the reporting date is recorded as a current liability, with the remaining portion of the liability recorded as a non-current liability. Revenue Recognition The Company recognizes revenue in accordance with ASC 606, Revenue from Contracts with Customers The Company applies the five-step model to contracts that are within the scope of Topic 606 only when it is probable that the Company will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, for contracts within the scope of Topic 606, the Company assesses the goods or services promised within each contract and determine those that are performance obligations and whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to respective performance obligations when (or as) the respective performance obligations are satisfied. The Company evaluates its contracts with customers for the presence of significant financing components. If a significant financing component is identified in a contract and provides a financing benefit to the customer, the transaction price for the contract is adjusted to account for the financing portion of the arrangement, which is recognized as interest income over the financing term using the effective interest method. In determining the appropriate interest rates for significant financing components, the Company evaluates the credit profile of the customer and prevailing market interest rates and selects an interest rate in which it believes would be charged to the customer in a separate financing arrangement over a similar financing term. License and Royalty Revenue The Company licenses its NAV Technology Platform to other biotechnology and pharmaceutical companies. The terms of the licenses vary, and licenses may be exclusive or non-exclusive and may be sublicensable by the licensee. Licenses may grant intellectual property rights for purposes of internal and preclinical research and development only, or may include the rights, or options to obtain future rights, to commercialize drug therapies for specific diseases using the Company’s NAV Technology Platform. License agreements generally have a term at least equal to the life of the underlying patents, but are terminable at the option of the licensee. Consideration payable to the Company under its license agreements may include: (i) up-front and annual fees, (ii) option fees to acquire additional licenses, (iii) milestone payments based on the achievement of certain development and sales-based milestones by licensees, (iv) sublicense fees and (v) royalties on sales of licensed products. The Company’s license agreements are accounted for as contracts with customers within the scope of Topic 606. At the inception of each license agreement, the Company determines the contract term for purposes of applying the requirements of Topic 606. Licenses are generally terminable at the option of the licensee with advance notice to the Company. For each license granted, including licenses granted upon the exercise of license options, the Company evaluates these termination rights to determine whether a substantive termination penalty would be incurred by the licensee upon termination. If the licensee incurs a substantive termination penalty upon termination, the contract term for revenue recognition purposes is generally equal to the stated term of the license, which is the life of the underlying licensed patents. Alternatively, if the licensee does not incur a substantive termination penalty upon termination, the contract term for revenue recognition purposes may be shorter than the stated term of the license, in which case the termination rights may be accounted for as contract renewal options. The determination of whether a substantive termination penalty is associated with the termination rights requires significant judgment. In making this determination, the Company considers, among other things, the nature of the intellectual property rights that would be returned to the Company upon termination, including the exclusivity of the licensed rights and the stage of development of the licensed products, the payment terms, including the amount and timing of non-refundable or guaranteed payments, and the business purpose of the termination rights granted to the licensee. Generally, the most significant judgment in determining whether a substantive termination penalty exists relates to the amount of any up-front or guaranteed non-refundable payments relative to the amount of annual payments that may be avoided by the licensee upon termination of the license. The Company considers all of the facts and circumstances relevant to each license when making this determination. Performance obligations under the Company’s license agreements may include (i) the delivery of intellectual property licenses, (ii) options granted to licensees to acquire additional licenses, to the extent the options represent material rights to the licensee, and (iii) research and development services to be performed by the Company related to licensed products. At the inception of each license agreement which contains options for the licensee to acquire additional licenses, or contract renewal options, the Company evaluates the options to determine whether they provide material rights to the licensee. In making this determination, the Company considers whether the options are priced at a discount to the standalone selling price for the underlying licenses. If an option is priced at a discount to the standalone selling price for the underlying license, the option is considered to be a material right to the licensee and is accounted for as a separate performance obligation under the current license agreement. At the inception of each license agreement which contains performance obligations for research and development services, the Company evaluates whether the license is distinct from the research and development services, which requires judgment. In making this determination, the Company considers, among other things, the stage of development of the licensed products and whether the research and development services will significantly impact further development of the licensed products. If it is determined that the license if not distinct from the research and development services, the license is combined with the research and development services into a single performance obligation. The Company evaluates the transaction price of its license agreements at the inception of each agreement and at each reporting date. The transaction price includes the fixed consideration payable to the Company during the contract term, as well as any variable consideration to the extent that it is probable that a significant reversal of revenue will not occur in the future. Fixed consideration under the license agreements includes up-front and annual fees payable during the contract term. Variable consideration under the license agreements includes development and sales-based milestone payments, sublicense fees and royalties on sales of licensed products. Consideration contingent upon the exercise of options by a licensee is excluded from the transaction price and not accounted for as part of the license agreement until the option is exercised. The transaction price for each license agreement is allocated to the underlying performance obligations based on their relative standalone selling prices and recognized as revenue when (or as) the performance obligations are satisfied. Consideration allocated to performance obligations for the delivery of an intellectual property license is recognized as revenue in full upon the delivery of the license to the licensee. Consideration allocated to performance obligations for license options is recognized as revenue in full upon the earlier of the option exercise or expiration. The exercise of a license option by a licensee is accounted for as a new license for revenue recognition purposes. Consideration allocated to performance obligations for research and development services is recognized as revenue as the services are performed by the Company. Up-front and annual licenses fees payable to the Company over the contract term of each license are included in the transaction price, and the portion of this consideration that is allocated to the performance obligation for the delivery of the intellectual property license is recognized as revenue in full upon the delivery of the license to the licensee. If annual license fees are payable to the Company in periods beyond 12 months from the delivery of the license, a significant financing component is deemed to exist which provides a financing benefit to the licensee. If a significant financing component is identified, the Company adjusts the transaction price for the license to include only the present value of the annual license fees payable to the Company over the contract term. The discounted portion of the license fees is recognized as interest income from licensing over the financing period of the license. Development milestone payments are payable to the Company upon the achievement of specified development milestones by licensees. At the inception of each license agreement that contains development milestone payments, the Company evaluates whether the milestones are considered probable of achievement and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal will not occur in the future, milestone payments are included in the transaction price and recognized as revenue upon the delivery of the license. Milestone payments contingent on the achievement of development milestones that are not within the control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved and are excluded from the transaction price until the milestone is achieved. At each reporting date, the Company re-evaluates the probability of achievement of each outstanding development milestone and, if necessary, adjusts the transaction price for any milestones for which the probability of achievement has changed due to current facts and circumstances. Any such adjustments are recorded on a cumulative catch-up basis and recognized as revenue in the period of the adjustment. Royalties on sales of licensed products, sales-based milestone payments and sublicense fees based on the receipt of certain fees by licensees from any sublicensees are excluded from the transaction price of each license and recognized as revenue in the period that the related sales or sublicenses occur, provided that the associated license has been delivered to the licensee. Royalty revenue to date consists primarily of royalties on net sales of Zolgensma, which is a licensed product under the Company’s license agreement with Novartis Gene Therapies, Inc. (formerly AveXis, Inc.) (Novartis Gene Therapies), a wholly owned subsidiary of Novartis AG (Novartis), for the development and commercialization of treatments for spinal muscular atrophy (SMA). The Company recognizes royalty revenue from net sales of Zolgensma in the period in which the underlying products are sold by Novartis Gene Therapies, which in certain cases may require the Company to estimate royalty revenue for periods of net sales which have not yet been reported to the Company. Sales-based milestone payments related to net sales of Zolgensma are recognized as royalty revenue in the period in which the milestone is achieved. The Company receives payments from licensees based on the billing schedules established in each license agreement. Amounts recognized as revenue which have not yet been received from licensees, including unbilled royalties, are recorded as accounts receivable when the Company’s rights to the consideration are conditional solely upon the passage of time. Amounts recognized as revenue which have not yet been received from licensees are recorded as contract assets when the Company’s rights to the consideration are not unconditional. Contract assets are recorded as other current assets on the consolidated balance sheets. If a licensee elects to terminate a license prior to the end of the license term, the licensed intellectual property is returned to the Company and any consideration recorded as accounts receivable or contract assets which is not contractually payable by the licensee is charged off as a reduction of license revenue in the period of the termination. Amounts received by the Company prior to the delivery of underlying performance obligations are deferred and recognized as revenue upon the satisfaction of the performance obligations by the Company. Deferred revenue which is not expected to be recognized within 12 months from the reporting date is recorded as non-current on the consolidated balance sheets. Cost of Revenues Cost of revenues consists primarily of sublicense fees, milestone payments and royalties on net sales of licensed products as specified in the Company’s agreements with its licensors. Sublicense fees are based on a percentage of license fees received by the Company from NAV

Marketable Securities

Marketable Securities	12 Months Ended
	Dec. 31, 2020
Investments Debt And Equity Securities [Abstract]
Marketable Securities	3. Marketable Securities The following tables present a summary of the Company’s marketable securities, which consist of available-for-sale debt securities and equity securities (in thousands): Amortized Cost Unrealized Gains Unrealized Losses Fair Value December 31, 2020 U.S. government and federal agency securities $ 12,782 $ 22 $ — $ 12,804 Certificates of deposit 1,956 34 — 1,990 Corporate bonds 165,850 497 (55 ) 166,292 Municipal securities 3,035 2 — 3,037 $ 183,623 $ 555 $ (55 ) $ 184,123 Amortized Cost / Cost Unrealized Gains Unrealized Losses Fair Value December 31, 2019 U.S. government and federal agency securities $ 62,637 $ 215 $ (5 ) $ 62,847 Certificates of deposit 8,506 77 — 8,583 Corporate bonds 226,137 808 (29 ) 226,916 Equity securities 351 31,784 — 32,135 $ 297,631 $ 32,884 $ (34 ) $ 330,481 As of December 31, 2020 and 2019, no available-for-sale debt securities had remaining maturities greater than three years. The amortized cost of marketable debt securities is adjusted for amortization of premiums and accretion of discounts to maturity, or to the earliest call date for callable debt securities purchased at a premium. As of December 31, 2020 and 2019, the balance in the Company’s accumulated other comprehensive income (loss) consisted solely of unrealized gains and losses on available-for-sale debt securities, net of reclassification adjustments for realized gains and losses and income tax effects. Unrealized gain (loss) on available-for-sale securities, net, presented in the statements of operations and comprehensive income (loss) consisted of the following (in thousands): Years Ended December 31, 2020 2019 2018 Unrealized gain (loss) before reclassifications $ (426 ) $ 1,392 $ (44 ) Realized losses (gains) reclassified to investment income (139 ) (40 ) 39 Income tax expense — (467 ) — Unrealized gain (loss) on available-for-sale securities, net $ (565 ) $ 885 $ (5 ) The following tables present the fair values and unrealized losses of available-for-sale debt securities held by the Company in an unrealized loss position for less than 12 months and 12 months or greater (in thousands): Less than 12 Months 12 Months or Greater Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses December 31, 2020 Corporate bonds $ 55,507 $ (55 ) $ — $ — $ 55,507 $ (55 ) $ 55,507 $ (55 ) $ — $ — $ 55,507 $ (55 ) Less than 12 Months 12 Months or Greater Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses December 31, 2019 U.S. government and federal agency securities $ 12,562 $ (5 ) $ — $ — $ 12,562 $ (5 ) Corporate bonds 48,556 (29 ) — — 48,556 (29 ) $ 61,118 $ (34 ) $ — $ — $ 61,118 $ (34 ) As of December 31, 2020, available-for-sale debt securities held by the Company which were in an unrealized loss position consisted of 17 investment grade security positions. The Company has the intent and ability to hold such securities until recovery, and due to the credit quality of the issuers and low severity of each unrealized loss position relative to its amortized cost basis, the Company did not identify any credit losses associated with its available-for-sale debt securities. The Company did not recognize any impairment or credit losses on available-for-sale debt securities during the years ended December 31, 2020, 2019 and 2018. Marketable equity securities as of December 31, 2019 consisted solely of common stock of Prevail Therapeutics Inc. (Prevail). The Company acquired the securities as consideration for a license to the NAV Technology Platform granted to Prevail in August 2017. Prevail completed its initial public offering (IPO) in June 2019, prior to which the securities were accounted for as non-marketable equity securities without a readily determinable fair value and had a carrying value of $0.4 million. Upon Prevail’s IPO in June 2019, the securities were reclassified to marketable securities and subsequently measured at fair value at each reporting date. During the year ended December 31, 2019, the Company recognized total net realized and unrealized gains of $37.8 million related to the Prevail securities. During the year ended December 31, 2020, the Company sold all of its remaining Prevail securities and recognized total net realized and unrealized gains of $4.8 million during the period.

Fair Value of Financial Instrum

Fair Value of Financial Instruments	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Fair Value of Financial Instruments	4. Fair Value of Financial Instruments Financial instruments reported at fair value on a recurring basis include cash equivalents and marketable securities. The following tables present the fair value of cash equivalents and marketable securities in accordance with the hierarchy discussed in Note 2 (in thousands): Quoted Significant prices other Significant in active observable unobservable markets inputs inputs (Level 1) (Level 2) (Level 3) Total December 31, 2020 Cash equivalents: Money market mutual funds $ — $ 96,307 $ — $ 96,307 Total cash equivalents — 96,307 — 96,307 Marketable securities: U.S. government and federal agency securities — 12,804 — 12,804 Certificates of deposit — 1,990 — 1,990 Corporate bonds — 166,292 — 166,292 Municipal securities — 3,037 — 3,037 Total marketable securities — 184,123 — 184,123 Total cash equivalents and marketable securities $ — $ 280,430 $ — $ 280,430 Quoted Significant prices other Significant in active observable unobservable markets inputs inputs (Level 1) (Level 2) (Level 3) Total December 31, 2019 Cash equivalents: Money market mutual funds $ — $ 56,058 $ — $ 56,058 Total cash equivalents — 56,058 — 56,058 Marketable securities: U.S. government and federal agency securities — 62,847 — 62,847 Certificates of deposit — 8,583 — 8,583 Corporate bonds — 226,916 — 226,916 Equity securities 32,135 — — 32,135 Total marketable securities 32,135 298,346 — 330,481 Total cash equivalents and marketable securities $ 32,135 $ 354,404 $ — $ 386,539 There were no transfers of financial instruments between levels of the fair value hierarchy during the years ended December 31, 2020 and 2019. Management estimates that the carrying amounts of its current accounts receivable, accounts payable and accrued expenses and other current liabilities approximate fair value due to the short-term nature of those instruments. Accounts receivable which contain non-current portions are recorded at their present values using a discount rate that is based on prevailing market rates and the credit profile of the licensee on the date the amounts are initially recorded. Management does not believe there have been any significant changes in market conditions or credit quality that would cause the discount rates initially used to be materially different from those that would be used as of December 31, 2020 to determine the present value of the receivables. Accordingly, management estimates that the carrying value of its non-current accounts receivable approximates the fair value of those instruments. Non-marketable equity securities are measured at cost less impairment, adjusted for observable price changes for identical or similar investments of the same issuer. As of December 31, 2020, non-marketable equity securities had a carrying value of $1.1 million and were included in other assets on the consolidated balance sheet. The Company did not identify any observable price changes or changes in circumstances that would have had an adverse effect on the fair value of the securities as of December 31, 2020. The Company did not hold any non-marketable equity securities as of December 31, 2019. No remeasurements or impairment losses were recorded on non-marketable equity securities during the years ended December 31, 2020, 2019 and 2018.

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2020
Property Plant And Equipment [Abstract]
Property and Equipment, Net	5. Property and Equipment, Net Property and equipment, net consists of the following (in thousands): December 31, 2020 December 31, 2019 Laboratory and manufacturing equipment $ 26,306 $ 19,663 Computer equipment and software 3,764 2,545 Furniture and fixtures 4,114 2,188 Leasehold improvements 44,957 18,915 Total property and equipment 79,141 43,311 Accumulated depreciation and amortization (22,674 ) (14,338 ) Property and equipment, net $ 56,467 $ 28,973 During the years ended December 31, 2020, 2019 and 2018, the Company recorded depreciation and amortization expense of $8.4 million ,

Leases

Leases	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Leases	6. Leases 9804 Medical Center Drive In November 2018, the Company entered into an operating lease, as amended from time to time, for approximately 177,000 square feet of office, laboratory and manufacturing facilities at a new building to be constructed at 9804 Medical Center Drive in Rockville, Maryland (the 9804 Medical Center Drive Lease). The new facility will serve as the Company’s future corporate, research and manufacturing headquarters. The initial construction of the building was performed by the landlord, and the lease commenced in September 2020 upon the delivery of leased premises to the Company to make additional improvements to the building. Monthly payments under the lease begin in September 2021 and escalate annually in accordance with the lease agreement. The lease expires in September 2036 Pursuant to the 9804 Medical Center Drive Lease, the Company received a $19.5 million tenant improvement allowance from the landlord to perform improvements to the leased premises. The tenant improvement allowance has been recorded as a reduction of the right-of-use assets for the lease and is amortized on a straight-line basis as a reduction of lease expense over the term of the lease. As of December 31, 2020, the Company had unreimbursed amounts remaining under the tenant improvement allowance of $12.9 million, which were deemed in-substance lease payments and recorded as a reduction of the lease liability. As of December 31, 2020, the Company had recorded property and equipment of $36.3 million related to the buildout of the facility at 9804 Medical Center Drive, which have not yet been placed in service. The Company recorded the right-of-use assets and lease liabilities related to the 9804 Medical Center Drive Lease upon its commencement in September 2020. As of December 31, 2020, the Company had recorded right-of-use assets of $50.1 million and lease liabilities of $57.8 million related to the 9804 Medical Center Drive Lease. 9712 Medical Center Drive In March 2015, the Company entered into an operating lease for office space at 9712 Medical Center Drive in Rockville, Maryland (the 9712 Medical Center Drive Lease). The lease term commenced in April 2015, and monthly payments under the lease began in October 2015 and escalate annually in accordance with the lease agreement. The 9712 Medical Center Drive Lease has been amended from time to time to include additional office and laboratory space at 9714 Medical Center Drive and extend the term of the lease. In October 2020, the 9712 Medical Center Drive Lease was amended to extend the lease term from September 2021 to February 2027, subject to extension options held by the Company. The October 2020 amendment resulted in a $7.2 million increase in the right-of-use assets and lease liabilities under the 9712 Medical Center Drive Lease. The Company has an option to extend the term of the lease for three additional years, as well as an option to extend the lease term to be coterminous with the 9804 Medical Center Drive Lease, which expires in September 2036 9600 Blackwell Road In January 2016, the Company entered into an operating lease for its corporate headquarters at 9600 Blackwell Road in Rockville, Maryland (the 9600 Blackwell Road Lease). The lease term commenced in February 2016, and monthly payments under the lease began in September 2016 and escalate annually in accordance with the lease agreement. In November 2017, the 9600 Blackwell Road Lease was amended to include additional office space for the remainder of the lease term. The Company received a $0.8 million tenant improvement allowance from the landlord which has been recorded as a reduction of the right-of-use assets for the lease and is amortized on a straight-line basis as a reduction of lease expense over the term of the lease. In November 2020, the Company exercised its termination option under the 9600 Blackwell Road Lease. Upon exercise of the termination option, the Company was obligated to pay an early termination fee of $0.4 million and the lease term was reduced from September 2023 to September 2021 400 Madison Avenue In May 2016, the Company entered into an operating lease for office space at 400 Madison Avenue in New York, New York (the 400 Madison Lease). The lease term commenced in July 2016, and monthly payments under the lease began in October 2016 and escalate annually in accordance with the lease agreement. In May 2019, the 400 Madison Lease was amended to include additional office space and extend the lease term from October 2020 to April 2027. The May 2019 amendment resulted in a $5.2 million increase in the right-of-use assets and lease liabilities under the 400 Madison Lease. The Company received a $0.7 million tenant improvement allowance from the landlord which has been recorded as a reduction of the right-of-use assets for the lease and is amortized on a straight-line basis as a reduction of lease expense over the term of the lease. As required by the lease agreement, the Company has provided the landlord with an irrevocable letter of credit of $0.2 million which the landlord may draw upon in the event of any uncured default by the Company under the terms of the lease. Other Leases The Company leases additional office and laboratory facilities, laboratory equipment and other equipment under operating leases with various expiration dates through 2028, including leases which have been executed but have not yet commenced. Operating Lease Information All of the Company’s leases are classified as operating leases. The following table summarizes the Company’s lease costs and supplemental cash flow information related to its operating leases (in thousands): Years Ended December 31, 2020 2019 Operating lease cost $ 5,246 $ 3,040 Variable lease cost 1,104 666 Total lease cost $ 6,350 $ 3,706 Cash paid (received) for amounts included in operating lease liabilities $ (678 ) $ 2,724 Right-of-use assets acquired through operating lease liabilities $ 56,956 $ 5,114 Cash received for amounts included in operating lease liabilities for the year ended December 31, 2020 includes $5.0 million received by the Company during the period under its tenant improvement allowances, which were deemed in-substance lease payments and included in the calculation of the lease liability. Right-of-use assets acquired through operating lease liabilities for the years ended December 31, 2020 and 2019 include additions and reductions to right-of-use assets resulting from lease modifications and changes in lease term. Short-term lease expense for the years ended December 31, 2020 and 2019 was not material and is included in operating lease cost in the table above. Variable lease cost under the Company’s operating leases includes items such as common area maintenance, utilities, taxes and other charges. The weighted-average remaining lease term and weighted-average discount rate of the Company’s operating leases were as follows: As of December 31, 2020 Weighted-average remaining lease term (years) 13.7 Weighted-average discount rate 5.6 % The following table presents a reconciliation of the undiscounted future minimum lease payments remaining under the Company’s operating leases to the amounts reported as operating lease liabilities on the consolidated balance sheet as of December 31, 2020 (in thousands): As of December 31, 2020 Undiscounted future minimum lease payments: 2021 $ 4,155 2022 6,021 2023 8,671 2024 9,745 2025 9,999 Thereafter 93,148 Total undiscounted future minimum lease payments $ 131,739 Amount representing imputed interest (46,212 ) Tenant improvement allowance not yet received (12,874 ) Total operating lease liabilities 72,653 Current portion of operating lease liabilities (2,500 ) Operating lease liabilities, non-current $ 70,153 The table above excludes future minimum lease payments for leases which were executed but had not yet commenced as of December 31, 2020, the total of which were not material. Rent expense under all leases, including additional rent charges for utilities, parking, property management, operating expenses and real estate taxes, was $2.8 million for the year ended December 31, 2018, which was recognized in accordance with Topic 840.

Liability Related to Sale of Fu

Liability Related to Sale of Future Royalties	12 Months Ended
	Dec. 31, 2020
Liability Related To Sale Of Future Royalties [Abstract]
Liability Related to Sale of Future Royalties	7. Liability Related to Sale of Future Royalties In December 2020, the Company entered into a royalty purchase agreement (the Royalty Purchase Agreement) with HCR. Under the agreement, HCR purchased the Company’s rights to a capped amount of Zolgensma royalty payments under the Company’s license agreement with Novartis Gene Therapies, including $4.0 million of royalty payments received by the Company in the fourth quarter of 2020 (the Pledged Royalties). In consideration for these rights, HCR paid the Company $200.0 million (the Purchase Price), less $4.0 million representing the payment of the Pledged Royalties to HCR. Beginning upon the effective date of the agreement, Zolgenmsa royalty payments will be paid to HCR, net of upstream royalties payable by the Company to certain licensors in accordance with existing license agreements. Pursuant to the Royalty Purchase Agreement, the total amount of royalty payments to be received by HCR under the agreement is subject to an increasing cap (the Cap Amount) equal to (i) $260.0 million applicable for the period from the effective date of the agreement through November 7, 2024, and (ii) $300.0 million applicable for the period from November 8, 2024 through the effective date of termination of the license agreement with Novartis Gene Therapies. If, on or prior to the defined dates for each Cap Amount, the total amount of royalty payments received by HCR equals or exceeds the Cap Amount applicable to such date, the Royalty Purchase Agreement will automatically terminate and all rights to the Zolgensma royalty payments will revert back to the Company. The Company has a call option to repurchase its rights to the purchased royalties from HCR for a repurchase price equal to, as of the option exercise date, $300.0 million minus the total amount of royalty payments received by HCR; provided, however, that with respect to a call option exercised on or before November 7, 2024, in the event that the then applicable Cap Amount minus the total amount of royalty payments received by HCR is less than $ 1.0 million, the repurchase price shall equal such difference. The proceeds received from HCR of $196.0 million were recorded as a liability, net of transaction costs of $3.5 million, which will be amortized over the estimated life of the arrangement using the effective interest method. In order to determine the amortization of the liability, the Company is required to estimate the total amount of future royalty payments to be received by HCR, subject to the Cap Amount, over the life of the arrangement. The total amount of royalty payments received by HCR under the agreement, less the net proceeds received by the Company of $192.5 million, is recorded as non-cash interest expense over the life of the arrangement using the effective interest method. Due to its continuing involvement in the underlying license agreement with Novartis Gene Therapies, the Company continues to recognize royalty revenue on net sales of Zolgensma and records the royalty payments to HCR as a reduction of the liability when paid. As such payments are made to HCR, the balance of the liability will be effectively repaid over the life of the Royalty Purchase Agreement. The Company estimates the effective interest rate used to record non-cash interest expense under the Royalty Purchase Agreement based on its estimate of future royalty payments to be received by HCR. As of December 31, 2020, the estimated effective interest rate under the agreement was 13.6%. Over the life of the arrangement, the actual effective interest rate will be affected by the amount and timing of the royalty payments received by HCR and changes in the Company’s forecasted royalties. At each reporting date, the Company reassesses its estimate of total future royalty payments to be received by HCR at the applicable Cap Amount, and prospectively adjusts the effective interest rate and amortization of the liability as necessary. The following table presents the changes in the liability related to the sale of future royalties under the Royalty Purchase Agreement with HCR (in thousands): Year Ended December 31, 2020 Liability related to sale of future royalties, beginning balance $ — Proceeds from sale of future royalties 196,000 Deferred transaction costs (3,473 ) Non-cash interest expense 771 Liability related to sale of future royalties, ending balance 193,298 Current portion of liability related to sale of future royalties (18,794 ) Liability related to sale of future royalties, non-current $ 174,504

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2020
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	8. Commitments and Contingencies Licenses and Collaborations The Company in-licenses intellectual property from third parties for technology and know-how used in its product candidates and development programs, some of which is further sublicensed to NAV Technology Licensees. In-licenses may require the Company to make future payments relating to sublicense fees, milestone fees and royalties on future sales of licensed products. Additionally, the Company may be responsible for the cost of the maintenance of the intellectual property as incurred by its licensors. Up-front fees to obtain licensed technology are recorded as research and development expenses if the technology has no alternative future use. Sublicense fees are based on a specified percentage of license fees earned by the Company as a result of sublicensing the technology to NAV Technology Licensees and are recorded as cost of revenues. Milestone fees are recorded as cost of revenues if the underlying milestone is achieved by a licensee, or as research and development expense if the underlying milestone is achieved by the Company as a result of the development of its product candidates and the technology has no alternative future use. Royalties due to licensors on sales of licensed products, including sales by NAV Technology Licensees, are recorded as cost of revenues. Patent maintenance costs are recorded as general and administrative expenses. The Trustees of the University of Pennsylvania In February 2009, the Company entered into a license agreement, which has been amended from time to time, with The Trustees of the University of Pennsylvania (together with the University of Pennsylvania, Penn) for exclusive, worldwide rights to certain patents owned by Penn underlying the Company’s NAV Technology Platform, as well as exclusive rights to certain data, results and other information. Pursuant to the license agreement, the Company is obligated to pay Penn royalties on net sales of licensed products and sublicense fees. Additionally, the Company is obligated to reimburse Penn for certain costs incurred related to the maintenance of the licensed patents. In April 2019, the Company amended its license from Penn to include exclusive license rights to certain patent rights and know-how, including research data and other information, relating to the treatment of late-infantile neuronal ceroid lipofuscinosis type 2 (CLN2) disease. In consideration for the additional licensed rights, and in addition to any consideration owed under the license prior to the amendment, the Company paid Penn an up-front fee and is obligated to pay milestone fees of up to $20.5 million upon the achievement of various development and sales-based milestones and additional royalties on net sales of licensed products for the treatment of CLN2 disease. Additionally, the amendment modified the percentage of sublicense fees the Company is obligated to pay Penn on amounts received by the Company from third parties for the sublicensing of the licensed rights for the treatment of CLN2 disease. Expenses incurred by the Company related to its license from Penn were recorded as follows (in thousands): Years Ended December 31, 2020 2019 2018 Cost of revenues $ 39 $ — $ (18 ) Research and development — 200 — General and administrative 821 905 130 $ 860 $ 1,105 $ 112 As of December 31, 2020 and 2019, the Company had recorded $0.1 million and $0.1 million, respectively, in expenses payable to Penn under the license agreement, which are included in accounts payable, accrued expenses and other current liabilities and other liabilities on the consolidated balance sheets. GlaxoSmithKline LLC In March 2009, the Company entered into a license agreement, which was amended in April 2009, with GlaxoSmithKline LLC (GSK) for exclusive, worldwide rights to certain patents underlying the Company’s NAV Technology Platform which are owned by Penn and exclusively licensed to GSK. Pursuant to the license agreement, the Company is obligated to pay GSK royalties on net sales of licensed products and sublicense fees. Additionally, the Company is obligated to reimburse GSK for certain costs incurred related to the maintenance of the licensed patents. The Company was also obligated to pay $1.5 million to GSK upon the achievement of various milestones, all of which have been achieved and paid as of December 31, 2020. Expenses incurred by the Company related to its license from GSK were recorded as follows (in thousands): Years Ended December 31, 2020 2019 2018 Cost of revenues: Royalties on net sales of Zolgensma $ 26,278 $ 5,822 $ — Other cost of revenues 9,398 2,419 9,407 Total cost of revenues 35,676 8,241 9,407 General and administrative 1,046 928 548 $ 36,722 $ 9,169 $ 9,955 As of December 31, 2020 and 2019, the Company had recorded $13.1 million and $6.7 million, respectively, in expenses payable to GSK under the license agreement, which are included in accounts payable, accrued expenses and other current liabilities and other liabilities on the consolidated balance sheets. Neurimmune AG In July 2019, the Company entered into a collaboration and license agreement with Neurimmune AG (Neurimmune) pursuant to which the Company and Neurimmune will jointly develop and commercialize novel gene therapies using AAV vectors from the NAV Technology Platform to deliver human antibodies for chronic neurodegenerative diseases. The Company and Neurimmune will share all research and development costs for the first two years of the agreement, after which each party will have the option, on a target-by-target basis, to: (i) continue as a 50% partner in the collaboration; (ii) receive a phase-based worldwide royalty in lieu of continued development investment; or (iii) negotiate with the other party to lead the development and commercialization of the respective program. Unless the parties agree otherwise, upon the commercialization of any product candidates, if any, it is anticipated that profits and losses will be shared equally on a worldwide basis. The Company determined that the collaboration and license agreement with Neurimmune is a collaborative arrangement within the scope of Topic 808, and that no unit of account under the arrangement should be accounted for as a transaction with a customer within the scope of Topic 606. In accordance with the Company’s accounting policies for collaborative arrangements, if Neurimmune’s development costs incurred under the collaboration exceed those incurred by the Company during a reporting period, the Company will recognize research and development expense and record a liability for the amount due to Neurimmune at the end of the period. Alternatively, if the Company’s development costs incurred under the collaboration exceed those incurred by Neurimmune during a reporting period, the Company will recognize a reduction of research and development expenses and record an amount due from Neurimmune at the end of the period. During the years ended December 31, 2020 and 2019, the Company recognized net research and development expenses of $0.5 million and less than $0.1 million, respectively, under the collaboration and license agreement with Neurimmune. Clearside Biomedical, Inc. In August 2019, the Company entered into an option and license agreement with Clearside Biomedical, Inc. (Clearside) pursuant to which the Company was granted an option to exclusively license the worldwide rights to certain patents related to Clearside’s proprietary, in-office SCS Microinjector™ for the delivery of RGX-314 to the suprachoroidal space to treat wet age-related macular degeneration (wet AMD), diabetic neuropathy (DR) and other diseases. The Company exercised its license option in October 2019, resulting in a payment of $1.6 million to Clearside which was recognized as research and development expense upon exercise. Additionally, the Company is obligated to pay milestone fees of up to $136.0 million upon the achievement of various development and sales-based milestones, as well as royalties on net sales of licensed products using the SCS Microinjector. Clearside is responsible for supplying the SCS Microinjector to the Company to support all preclinical, clinical and commercial needs. From the inception of the agreement through December 31, 2020, the Company had incurred $3.0 million for development milestones achieved, or deemed probable of achievement, under the agreement. Other Licenses In November 2014, the Company entered into a license agreement, which was amended in November 2016, with Regents of the University of Minnesota (Minnesota), for an exclusive license under Minnesota’s interest in certain patent rights which are co-owned by Minnesota and the Company to commercialize products covered by the licensed patent rights in any country or territory in which a licensed patent has been issued and is unexpired, or a licensed patent application is pending. Pursuant to the license agreement, the Company is obligated to pay Minnesota annual maintenance fees, royalties on net sales, sublicense fees and fees upon the achievement of various milestones. Additionally, the Company is obligated to pay for certain costs incurred related to the maintenance of the licensed patents. In August 2018, the Company entered into a license agreement with Emory University (Emory) for an exclusive license under Emory’s interest in certain patent rights which are co-owned by Emory and the Company to commercialize products covered by the licensed patent rights in any country or territory. Pursuant to the license agreement, the Company is obligated to reimburse Emory for patent prosecution and maintenance expenses and pay Emory annual maintenance fees under certain circumstances, royalties on net sales, sublicense fees and fees upon the achievement of various milestones for the first licensed product. Other Funding Commitments In the normal course of business, the Company enters into agreements with contract research organizations, contract manufacturing organizations and other third-parties for services to be provided to the Company. Generally, these agreements provide for termination upon notice, with specified amounts due upon termination based on the timing of termination and the terms of the agreement. The actual amounts and timing of payments under these agreements are uncertain and contingent upon the initiation and completion of services to be provided to the Company. Guarantees and Indemnifications In the normal course of business, the Company enters into agreements that contain a variety of representations and provide for general indemnification. The Company’s potential exposure under these agreements is unknown because it involves claims that may be made against the Company in the future. To date, the Company has not paid any claims or been required to defend any action related to its indemnification obligations. As of December 31, 2020 and 2019, the Company did not have any material indemnification claims that were probable or reasonably possible and consequently had not recorded any related liabilities.

Capitalization

Capitalization	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Capitalization	9. Capitalization As of December 31, 2020 and 2019, the authorized capital stock of the Company included 100,000,000 shares of common stock, par value $0.0001 per share, and 10,000,000 shares of preferred stock, par value $0.0001 per share. The Company’s restated certificate of incorporation and bylaws contain the rights, preferences and privileges of the Company’s stockholders and their respective shares. In August 2018, the Company completed a public offering of 3,105,000 shares of its common stock (inclusive of 405,000 shares pursuant to the full exercise by the underwriters of their option to purchase additional shares) at a price of $65.00 per share. The aggregate net proceeds received by the Company from the offering, inclusive of the underwriters’ option exercise, were $189.1 million, net of underwriting discounts and commissions and offering expenses payable by the Company. The Company’s reserved shares of common stock for future issuance were as follows (in thousands): December 31, 2020 December 31, 2019 Reserved for issuance under equity incentive plans 8,659 7,607 Reserved for issuance under employee stock purchase plan 448 134 9,107 7,741 In January 2021, the Company completed a public offering of 4,899,000 shares of its common stock (inclusive of 639,000 shares pursuant to the full exercise by the underwriters of their option to purchase additional shares) at a price of $47.00 per share. The aggregate net proceeds received by the Company from the offering, inclusive of the underwriters’ option exercise, were $216.1 million, net of underwriting discounts and commissions and offering expenses payable by the Company.

License and Royalty Revenue

License and Royalty Revenue	12 Months Ended
	Dec. 31, 2020
License Agreement Revenue Recognition [Abstract]
License and Royalty Revenue	10. License and Royalty Revenue As of December 31, 2020, the Company’s NAV Technology Platform was being applied by NAV Technology Licensees in one commercial product, Zolgensma, and in the development of more than 20 product candidates. Development milestone payments are evaluated each reporting period and are only included in the transaction price of each license and recognized as license revenue to the extent the milestones are considered probable of achievement. Sales-based milestones are excluded from the transaction price of each license agreement and recognized as royalty revenue in the period of achievement. As of December 31, 2020, the Company’s license agreements, excluding additional licenses that could be granted upon the exercise of options by licensees, contained unachieved milestones which could result in aggregate milestone payments to the Company of up to $213.1 million, including (i) $26.6 million upon the commencement of various stages of clinical trials, (iii) $26.0 million upon the submission of regulatory approval filings, (iii) $103.5 million upon the approval of commercial products by regulatory agencies and (iv) $57.0 million upon the achievement of specified sales targets for licensed products. To the extent the milestone payments are realized by the Company, the Company will be obligated to pay sublicense fees to licensors based on a specified percentage of the fees earned by the Company. The achievement of milestones by licensees is highly dependent on the successful development and commercialization of licensed products and it is at least reasonably possible that some or all of the milestone fees will not be realized by the Company. Changes in Accounts Receivable, Contract Assets and Deferred Revenue The following table presents changes in the balances of the Company’s net accounts receivable, contract assets and deferred revenue, as well as other information regarding revenue recognized, during the periods presented (in thousands): Years Ended December 31, 2020 2019 2018 Accounts receivable, current and non-current: Balance, beginning of period $ 42,303 $ 31,599 $ 5,850 Additions 158,682 39,203 231,154 Deductions (154,719 ) (28,499 ) (205,405 ) Balance, end of period $ 46,266 $ 42,303 $ 31,599 Contract assets: Balance, beginning of period $ — $ 750 $ 350 Additions 350 1,000 3,000 Deductions — (1,750 ) (2,600 ) Balance, end of period $ 350 $ — $ 750 Deferred revenue, current and non-current: Balance, beginning of period $ 3,333 $ 3,933 $ — Additions 1,124 — 3,933 Deductions (225 ) (600 ) — Balance, end of period $ 4,232 $ 3,333 $ 3,933 Revenue recognized during the period from: Amounts included in deferred revenue at beginning of period $ — $ 600 $ — Performance obligations satisfied in previous periods $ 146,772 $ 26,689 $ 5,348 Additions to accounts receivable during the periods presented consisted primarily of receivables recorded related to royalties on net sales of Zolgenmsa, new licenses granted by the Company and the achievement of milestones by licensees during the period, as well as interest income from licensing recognized during the period. Deductions to accounts receivable during the periods presented consisted primarily of amounts collected from licensees and increases in the allowance for credit losses, as discussed further below. Additions to contract assets during the periods presented consisted of development milestones deemed probable of achievement by licensees during the periods. Deductions to contract assets during the periods presented consisted of the achievement of such milestones and billing of the associated milestone payments by the Company. As of December 31, 2020, the Company had recorded deferred revenue of $4.2 million which represents consideration received from licensees for performance obligations that have not yet been satisfied by the Company. Unsatisfied performance obligations consisted of (i) options granted to licensees that provide material rights to the licensee to acquire additional licenses from the Company, which will be satisfied upon the exercise or expiration of the options and (ii) research and development services to be performed by the Company related to licensed products, which will be satisfied as the research and development services are performed. Revenue recognized from performance obligations satisfied in previous periods was primarily attributable to royalty and sublicense revenues as well as changes in the transaction prices of the Company’s license agreements. Changes in transaction prices were primarily attributable to development milestones achieved or deemed probable of achievement during the periods, which were previously not considered probable of achievement. Accounts Receivable, Contract Assets and the Allowance for Credit Losses Accounts receivable, net consisted of the following (in thousands): December 31, 2020 December 31, 2019 Current accounts receivable: Billed to customers $ 30,573 $ 376 Unbilled 20,104 37,772 Allowance for credit losses (7,678 ) — Current accounts receivable, net 42,999 38,148 Non-current accounts receivable: Unbilled 3,267 4,155 Allowance for credit losses — — Non-current accounts receivable, net 3,267 4,155 Total accounts receivable, net $ 46,266 $ 42,303 The following table presents the changes in the allowance for credit losses related to accounts receivable and contract assets for the year ended December 31, 2020 (in thousands): Accounts Receivable Contract Assets Balance at December 31, 2019 $ — $ — Provision for credit losses 7,678 — Write-offs — — Balance at December 31, 2020 $ 7,678 $ — The Company’s allowance for credit losses as of December 31, 2020 was related solely to accounts receivable from Abeona. Please refer to the section below, “Abeona Therapeutics Inc.”, for further information regarding amounts due from Abeona and the associated allowance for credit losses. The Company’s provision for credit losses for the year ended December 31, 2020 was $7.7 million and was related solely to changes in estimates regarding the allowance for credit losses associated with the accounts receivable from Abeona. No allowance for credit losses was recorded as of December 31, 2019, and no provision for credit losses was recorded for the years ended December 31, 2019 and 2018. Novartis Gene Therapies In March 2014, the Company entered into an exclusive license agreement, as amended, (the March 2014 License) with Novartis Gene Therapies (formerly AveXis, Inc.). Under the March 2014 License, the Company granted Novartis Gene Therapies an exclusive, worldwide commercial license, with rights to sublicense, to the NAV AAV9 vector for the treatment of SMA in humans by in vivo In January 2018, the Company and Novartis Gene Therapies amended the March 2014 License (the January 2018 Amendment). Under the January 2018 Amendment, the licensed intellectual property was expanded to include, in addition to the NAV AAV9 vector previously licensed, sublicenses to other third-party patents exclusively licensed by the Company and any other recombinant AAV vector in the Company’s intellectual property portfolio during a period of 14 years from the effective date of the January 2018 Amendment, for the treatment of SMA in humans by in vivo The January 2018 Amendment also modified the assignment provision of the March 2014 License. Under the amended assignment provision, Novartis Gene Therapies was permitted to transfer the March 2014 License without the Company’s written consent in connection with a change of control of Novartis Gene Therapies, subject to certain conditions. Prior to the January 2018 Amendment, any assignment by Novartis Gene Therapies without the Company’s prior written consent had been prohibited under the March 2014 License. In consideration for the additional rights granted under the January 2018 Amendment, and in addition to any consideration owed under the original March 2014 License, Novartis Gene Therapies paid to the Company a fee of $80.0 million upon entry into the January 2018 Amendment. In addition, Novartis Gene Therapies was obligated to pay the Company (i) $30.0 million on the first anniversary of the effective date of the January 2018 Amendment, (ii) $ 30.0 million on the second anniversary of the effective date of the January 2018 Amendment and (iii) potential sales-based milestone payments of up to $ 120.0 million. In the event of a change of control of Novartis Gene Therapies , to the extent that any fee described in (i) or (ii) above, or the first $ 40.0 million of sales-based milestone payments described in (iii) above, had not yet been paid to the Company, Novartis Gene Therapies was required to pay any such unpaid fee to the Company upon the change of control. For any product developed for the treatment of SMA using the NAV AAV9 vector, Novartis Gene Therapies will continue to be obligated to pay to the Company mid-single to low double-digit royalties on net sales as required by the March 2014 License, and for any product developed for the treatment of SMA using a licensed vector other than NAV AAV9, the Company will receive a low double-digit royalty on net sales. In May 2018, AveXis, Inc. (now Novartis Gene Therapies) was acquired by Novartis, which Novartis Gene Therapies launched commercial sales of Zolgensma, a licensed product under the March 2014 License, in the second quarter of 2019, upon which the Company began recognizing royalty revenue on net sales of the licensed product. In accordance with the license agreement, Novartis Gene Therapies was obligated to pay a sales-based milestone fee of $80.0 million to the Company upon the achievement of $1.0 billion in cumulative net sales of licensed products. Novartis Gene Therapies achieved cumulative net sales of Zolgensma of $1.0 billion in third quarter of 2020, upon which the Company recognized revenue of $80.0 million related to the sales-based milestone, and the Company received payment of the $80.0 million milestone fee in October 2020. Accounting Analysis The January 2018 Amendment was accounted for under Topic 606 as a modification of the license agreement resulting in a new and separate contract from the original March 2014 License for revenue recognition purposes. The Company determined that a substantive termination penalty is associated with Novartis Gene Therapies’ termination rights under both the original March 2014 License and the January 2018 Amendment, and therefore the contract term for revenue recognition purposes is equal to the stated term of the amended license agreement. The only material performance obligation of the Company under the January 2018 Amendment was for the delivery of the modified license, which occurred upon the execution of the amendment in January 2018. As of December 31, 2020, the transaction price of the original March 2014 License was $14.5 million. The transaction price included: (i) the up-front payment in March 2014 of $2.0 million, (ii) the present value of aggregate annual fees payable to the Company over the term of the license and (iii) $12.3 million of payments for development milestones achieved to date. The discounted portion of the annual fees represents the financing benefit provided to Novartis Gene Therapies and is recognized as interest income from licensing over the term of the license. Variable consideration under the original March 2014 License, which has been excluded from the transaction price, includes royalties on net sales of licensed products and any potential sublicense fees, if any, which will be recognized in the period of the underlying sales or sublicenses. The transaction price of the original March 2014 License increased by $3.5 million during the year ended December 31, 2020 as a result of development milestones achieved during the period which were previously excluded from the transaction price. As of December 31, 2020, all development milestones under the March 2014 License had been achieved and associated milestone payments had been received from Novartis Gene Therapies. Upon its execution, the transaction price of the January 2018 Amendment was $132.1 million, which was fully recognized as license revenue upon the delivery of the modified license in January 2018. In May 2018, as a result of the acquisition of AveXis, Inc. (now Novartis Gene Therapies) by Novartis, the transaction price was increased by $40.0 million to account for the acceleration of the sale-based milestone which was previously excluded from the transaction price. The $40.0 million increase in the transaction price was recognized as license revenue upon the completion of the change of control in May 2018 since the amended license had been fully delivered by the Company. Additionally, due to the acceleration of the two $30.0 million payments originally due in January 2019 and January 2020, the Company recognized $6.1 million of interest income from licensing upon the completion of the change of control of AveXis, Inc., which represented the remaining present value discount on such payments as of the date of the change of control. As of December 31, 2020, the transaction price of the January 2018 Amendment was $172.1 million, which included: (i) the $80.0 million payment in January 2018, (ii) the present value, as of the date of the January 2018 Amendment, of the two $30.0 million payments originally due in January 2019 and January 2020 and (iii) the $40.0 million sales-based milestone which was accelerated upon the change of control in May 2018. Variable consideration under the January 2018 Amendment, which has been excluded from the transaction price, includes the $80.0 million sales-based milestone received in 2020 upon Novartis Gene Therapies’ achievement of $1.0 billion in cumulative net sales of Zolgensma, royalties on net sales of licensed products and any potential sublicense fees, if any, which will be recognized in the period of the underlying sales or sublicenses. There were no increases in the transaction price of the January 2018 Amendment during the year ended December 31, 2020. As of December 31, 2020, all sales-based milestones under the January 2018 Amendment had been achieved and associated milestone payments had been received from Novartis Gene Therapies. The Company recognized the following amounts under the March 2014 License with Novartis Gene Therapies (in thousands): Years Ended December 31, 2020 2019 2018 License revenue $ 3,500 $ 3,500 $ 176,066 Royalties on net sales of Zolgensma 61,631 20,829 — Achievement of sales-based milestone for Zolgensma 80,000 — — Total license and royalty revenue $ 145,131 $ 24,329 $ 176,066 Interest income from licensing $ 26 $ 29 $ 7,966 As of December 31, 2020, the Company had recorded total accounts receivable of $19.6 million from Novartis Gene Therapies under the March 2014 License, of which $19.4 million were included in current assets and $0.2 million were included in non-current assets. As of December 31, 2019, the Company had recorded total accounts receivable of $11.0 million from Novartis Gene Therapies under the March 2014 License, of which $10.8 million were included in current assets and $0.2 million were included in non-current assets. Current accounts receivable as of December 31, 2020 included unbilled Zolgensma royalties of $19.4 million, of which $9.2 million is expected to be paid to HCR in accordance with the Royalty Purchase Agreement discussed in Note 7. Abeona Therapeutics Inc. In November 2018, the Company entered into an exclusive license agreement, as amended, (the November 2018 License) with Abeona. Under the November 2018 License, the Company granted Abeona an exclusive, worldwide commercial license (subject to certain non-exclusive rights previously granted by the Company), with rights to sublicense, to the NAV AAV9 vector for the treatment of Mucopolysaccharidosis Type IIIA (MPS IIIA), also known as Sanfilippo Syndrome Type A, Mucopolysaccharidosis Type IIIB (MPS IIIB), also known as Sanfilippo Syndrome Type B, Neuronal Ceroid Lipfuscinosis-1 (CLN1 disease), also known as infantile Batten Disease, and Neuronal Ceroid Lipfuscinosis-3 (CLN3 disease), also known as juvenile Batten Disease, by in vivo In November 2019, the November 2018 License was amended to increase the total amount of license fees payable to the Company by $1.0 million and modify the timing of the first required annual payment under the license agreement. In consideration for the license, as amended, Abeona was obligated to pay up-front and annual fees to the Company totaling up to $121.0 million, payable as follows: (i) $10.0 million upon the execution of the license agreement, (ii) $3.0 million within 12 months of the effective date of the license agreement, (iii) $8.0 million by April 1, 2020 and (iv) $100.0 million payable in five annual installments of $20.0 million beginning on the second anniversary of the effective date of the license agreement, which were subject to reduction should Abeona terminate some but not all of the licensed indications. Pursuant to the license agreement, any unpaid portion of the first $41.0 million of up-front and annual fees described above shall become payable upon termination of the license agreement. In the event of a change of control of Abeona, any remaining unpaid portion of the $121.0 million of up-front and annual fees described above shall become payable upon the change of control. Additionally, Abeona was obligated to pay the Company up to $60.0 million upon the achievement of specified sales-based milestones, low double-digit royalties on net sales of licensed products, subject to reduction in specified circumstances, and a lower mid-double-digit percentage of any sublicense fees Abeona receives from sublicensees for the licensed intellectual property rights. Termination and Arbitration Proceedings Pursuant to the November 2018 License, Abeona was required to pay a license fee of $8.0 million to the Company no later than April 1, 2020. Abeona failed to make this payment, and in April 2020, the Company delivered to Abeona a notice of its breach of the license agreement and written demand for payment. Upon expiration of the applicable cure period in May 2020, the license agreement was terminated. As a result of the termination, Abeona was required to pay a $20.0 million license fee to the Company within 15 days of the termination date, which otherwise would have been due to the Company in November 2020. As of February 25, 2021, the Company had not received any portion of the $28.0 million in license fees due from Abeona under the license agreement. Unpaid balances due under the November 2018 License accrue interest at 1.5% per month. In May 2020, subsequent to the termination of the November 2018 License, Abeona filed a claim in arbitration alleging that the Company had breached certain responsibilities to communicate with Abeona regarding the Company’s prosecution of licensed patents under the November 2018 License. The Company disputes Abeona’s claim and filed a counterclaim in arbitration demanding payment of the $ 28.0 million of unpaid fees from Abeona, plus accrued interest . Based on its evaluation of the merits of Abeona’s claims, the Company had not recorded any liabilities related these claims as of December 31, 2020, and the Company currently expects that its demand for payment in full will be upheld in arbitration. The Company intends to enforce the full collection of all amounts due from Abeona upon completion of arbitration, which is currently scheduled to occur in March 2021. However, the duration and outcome of arbitration and timing of payment from Abeona are unpredictable and uncertain at this time. Accounting Analysis The Company determined that the November 2018 License had an initial contract term of three years for revenue recognition purposes due to the lack of a substantive termination penalty for license periods beyond three years from the date of the license agreement. The annual payments which were due under the agreement beginning in November 2021 represented contract renewal options granted to Abeona for revenue recognition purposes, and therefore were not accounted for under the initial license agreement. The Company determined that the contract renewal options did not represent material rights granted to Abeona, and the only material performance obligations of the Company under the license agreement were for the delivery of the initial intellectual property licenses, which occurred upon the execution of the license agreement in November 2018. Upon its execution, the transaction price of the November 2018 License was $35.6 million, which was fully recognized as license revenue upon the delivery of the licenses in November 2018. As a result of the November 2019 amendment to the license agreement, the transaction price was increased by $0.6 million to account for the modifications to the amount and timing of annual fees under the license. Upon its termination in May 2020, the transaction price of the November 2018 License, as amended, was $36.3 million, which included the following fixed consideration payable to the Company over contract term: (i) the $10.0 million payment in November 2018 and (ii) the present values, as of the date of the license agreement, of the $3.0 million payment due in November 2019, the $8.0 million payment due in April 2020 and the $20.0 million payment due in November 2020. The discounted portion of the annual payments represents the financing benefit provided to Abeona and was recognized as interest income from licensing over the financing term of the license, which ended upon the termination of the agreement in May 2020. Variable consideration under the license agreement, which was excluded from the transaction price, included the sales-based milestone payments of $60.0 million, as well as any potential royalties on sales of licensed products or sublicense fees, which would be recognized in the period of the underlying sales or sublicenses. The annual payments due under the agreement beginning in November 2021 represented contract renewal options granted to Abeona for revenue recognition purposes, and therefore were excluded from the transaction price. During the years ended December 31, 2020, 2019 and 2018, the Company recognized license revenue of zero, $0.6 million and $35.6 million, respectively, and interest income from licensing of $3.8 million, $2.6 million and $0.4 million, respectively under the November 2018 License. Interest income from licensing recognized during the year ended December 31, 2020 includes $2.1 million of interest accrued on unpaid license fees recognized subsequent to the termination of the license agreement in May 2020, as discussed further below. As of December 31, 2020, the Company had recorded gross accounts receivable of $30.1 million from Abeona under the November 2018 License, which consisted of the $8.0 million fee due April 1, 2020, the $20.0 million fee due within 15 days of the termination of the license agreement in May 2020 and accrued interest on the outstanding balances. While the Company currently expects its demand for payment in full will be upheld in arbitration, the Company assessed the collectability of the $30.1 million due from Abeona as of December 31, 2020 as it relates to credit risk. In performing this assessment, the Company evaluated Abeona’s credit profile and financial condition, as well its expectations regarding Abeona’s future cash flows and ability to satisfy this obligation upon the completion of arbitration in 2021. Additionally, the Company considered Abeona’s continued failure to remit payment to the Company, as well as events which occurred during the year ended December 31, 2020 impacting Abeona’s business and credit profile, specifically the departure of key members of Abeona’s management and board of directors and subsequent decline in market capitalization. As a result of its evaluation, the Company recorded an allowance for credit losses of $7.7 million as of December 31, 2020 related to the accounts receivable due from Abeona. However, management intends to enforce the full collection of all amounts due from Abeona upon the completion of arbitration. From the termination of the agreement in May 2020 to December 31, 2020, the Company recognized interest income from licensing of $2.1 million related to unpaid license fees from Abeona under the November 2018 License. In accordance with its interest accrual policy, the Company ceased the recognition of interest income accrued under the license agreement subsequent to the recognition of the allowance for credit losses in the third quarter of 2020, and will continue to maintain the accounts receivable from Abeona on non-accrual status unless and until such amounts are deemed to be collectable. However, management intends to enforce the full collection of all accrued interest contractually due from Abeona upon the completion of arbitration.

Stock-based Compensation

Stock-based Compensation	12 Months Ended
	Dec. 31, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-based Compensation	11. Stock-based Compensation In September 2014, the Board of Directors adopted the 2014 Stock Plan (2014 Plan). In June 2015, the Board of Directors adopted the 2015 Equity Incentive Plan (2015 Plan), which became effective upon the Company’s initial public offering in September 2015. The 2015 Plan replaced the 2014 Plan, and as of the effective date of the 2015 Plan, no further awards may be issued under the 2014 Plan. Any options or awards outstanding under the 2014 Plan as of the effective date of the 2015 Plan remained outstanding and effective. The number of authorized shares under the 2015 Plan automatically increases annually on the first business day of each fiscal year, by the lesser of (i) 4% of the total number of shares of common stock outstanding on December 31 of the prior year, or (ii) a number of common shares determined by the Board of Directors. As of December 31, 2020, the total number of shares of common stock authorized for issuance under the 2015 Plan and 2014 Plan was 12,412,917, of which 2,291,626 remained available for future grants under the 2015 Plan. In January 2021, the Board of Directors authorized an additional 1,499,037 shares to be issued under the 2015 Plan. The 2014 Plan and 2015 Plan provide for the issuance of stock options, stock appreciation rights, restricted and unrestricted stock and unit awards, and performance cash awards to employees, members of the Board of Directors and consultants of the Company. Since the inception of the plans, the Company has issued only stock options and restricted stock units under the plans. Stock options under the 2014 Plan and 2015 Plan generally expire 10 years following the date of grant. Options typically vest over a four-year Shares of common stock underlying awards previously issued under the 2014 Plan and 2015 Plan which are reacquired by the Company, withheld by the Company in payment of the purchase price, exercise price or withholding taxes, expired, cancelled due to forfeiture or otherwise terminated other than by exercise or settlement, are added to the number of shares of common stock available for issuance under the 2015 Plan. Shares available for issuance under the 2015 Plan may be either authorized but unissued shares of the Company’s common stock or common stock reacquired by the Company and held in treasury. The 2015 Plan expires in June 2025, 10 years from the date it was adopted by the Board of Directors, unless earlier terminated. Stock-based Compensation Expense The Company’s stock-based compensation expense by award type was as follows (in thousands): Years Ended December 31, 2020 2019 2018 Stock options $ 31,178 $ 25,964 $ 15,960 Restricted stock units — 257 275 Employee stock purchase plan 771 633 406 $ 31,949 $ 26,854 $ 16,641 As of December 31, 2020, the Company had $62.1 million of unrecognized stock-based compensation expense related to stock options and the 2015 Employee Stock Purchase Plan (the 2015 ESPP), which is expected to be recognized over a weighted-average period of 2.4 years. The Company recorded aggregate stock-based compensation expense in the consolidated statements of operations and comprehensive income (loss) as follows (in thousands): Years Ended December 31, 2020 2019 2018 Research and development $ 16,280 $ 13,031 $ 7,612 General and administrative 15,669 13,823 9,029 $ 31,949 $ 26,854 $ 16,641 Stock Options The following table summarizes stock option activity under the 2014 Plan and 2015 Plan (in thousands, except per share data): Weighted- average Weighted- Remaining average Contractual Aggregate Exercise Life Intrinsic Shares Price (Years) Value (a) Outstanding at December 31, 2019 5,544 $ 28.79 7.5 $ 86,509 Granted 1,633 $ 37.84 Exercised (435 ) $ 12.95 Cancelled or forfeited (381 ) $ 45.04 Outstanding at December 31, 2020 6,361 $ 31.21 7.2 $ 101,356 Exercisable at December 31, 2020 3,703 $ 24.27 6.2 $ 84,115 Vested and expected to vest at December 31, 2020 6,361 $ 31.21 7.2 $ 101,356 (a) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of the common stock for the options that were in the money at the dates reported. The weighted-average grant date fair value per share of options granted during the years ended December 31, 2020, 2019 and 2018 was $23.82, $31.19 and $27.49, respectively. During the years ended December 31, 2020, 2019 and 2018, the total number of stock options exercised was 434,534, 796,847 and 1,684,522, respectively, resulting in total proceeds of $5.6 million, $7.1 million and $14.5 million, respectively. The total intrinsic value of options exercised during the years ended December 31, 2020, 2019 and 2018 was $11.5 million, $30.8 million and $68.2 million, respectively. The fair values of options granted were estimated at each grant date using the Black-Scholes valuation model with the following weighted-average assumptions: Years Ended December 31, 2020 2019 2018 Expected volatility 71 % 74 % 75 % Expected term (in years) 6.0 6.1 6.0 Risk-free interest rate 1.4 % 2.3 % 2.6 % Expected dividend yield 0.0 % 0.0 % 0.0 % Restricted Stock Units The Company had no unvested restricted stock units outstanding as of December 31, 2020 and 2019, nor at any point during the year ended December 31, 2020. No restricted stock units vested during the year ended December 31, 2020. The total intrinsic value of restricted stock units vested during the year ended December 31, 2019 was $1.8 million. No restricted stock units vested during the year ended December 31, 2018. Employee Stock Purchase Plan In June 2015, the Company’s Board of Directors adopted the 2015 ESPP, which became effective upon the Company’s initial public offering in September 2015. The number of authorized shares reserved for issuance under the 2015 ESPP automatically increases on the first business day of each fiscal year by the lesser of (i) 1% of the shares of common stock outstanding on the last business day of the prior fiscal year; or (ii) the number of shares determined by the Company’s Board of Directors. Unless otherwise determined by the administrator of the 2015 ESPP, two offering periods of six months’ duration will begin each year on January 1 and July 1. As of December 31, 2020, the total number of shares of common stock authorized for issuance under the 2015 ESPP was 623,924, of which 448,011 remained available for future issuance. During the years ended December 31, 2020, 2019 and 2018, 55,499, 35,994 and 36,700 shares of common stock, respectively, were issued under the 2015 ESPP. In January 2021, the Board of Directors authorized an additional 374,759 shares to be issued under the 2015 ESPP.

Retirement Plan

Retirement Plan	12 Months Ended
	Dec. 31, 2020
Postemployment Benefits [Abstract]
Retirement Plan	12. Retirement Plan The Company sponsors a defined-contribution retirement plan under Section 401(k) of the Internal Revenue Code (the 401(k) Plan). The 401(k) Plan covers all employees who meet defined minimum age and service requirements, and allows participants to defer a portion of their annual compensation. The Company matches employee deferrals up to a specified percentage of eligible compensation. For the years ended December 31, 2020, 2019 and 2018, the Company incurred expenses of $2.2 million, $1.8 million and $1.3 million, respectively, for matching contributions to the 401(k) Plan.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Income Taxes	13. Income Taxes The components of income (loss) before income taxes were as follows (in thousands): Years Ended December 31, 2020 2019 2018 United States $ (105,960 ) $ (97,435 ) $ 104,181 Foreign (50 ) (53 ) (65 ) Total income (loss) before income taxes $ (106,010 ) $ (97,488 ) $ 104,116 The components of the provision for income tax expense (benefit) were as follows (in thousands): Years Ended December 31, 2020 2019 2018 Current: Federal $ — $ — $ — State 5,240 (2,288 ) 4,179 Foreign — — — Total current 5,240 (2,288 ) 4,179 Deferred: Federal — (284 ) — State — (183 ) — Foreign — — — Total deferred — (467 ) — Total income tax expense (benefit) $ 5,240 $ (2,755 ) $ 4,179 In response to the COVID-19 pandemic, the Coronavirus Aid, Relief and Economic Security Act (the CARES Act) was signed into law in March 2020. The CARES Act (i) lifts certain deduction limitations originally imposed by the TCJA, (ii) allows corporate taxpayers to carryback net operating losses (NOLs) originating during 2018 through 2020 for up to five years, which was not previously allowed under the TCJA, (iii) eliminates the 80% of taxable income limitations on NOL utilization imposed by the TCJA, allowing corporate entities to fully utilize NOL carryforwards to offset taxable income in 2018, 2019 or 2020, and (iv) enacts various other changes to corporate taxation. Also included in the CARES Act was a change to the TCJA related to qualified improvement property, retroactively allowing for a 15-year recovery period and bonus depreciation. As a result of this change, the Company recorded current income tax benefit of $0.5 million for the year ended December 31, 2020 related to a reduction of state taxes associated with additional depreciation deductions allowed for the 2018 tax year. Overall, the enactment of the CARES Act, including the change for qualified improvement property, did not result in any material adjustments to the Company’s income tax provision for the year ended December 31, 2020, or to the Company’s net deferred tax assets as of December 31, 2020. A reconciliation of income tax expense (benefit) computed at the statutory federal income tax rate of 21% to income tax expense (benefit) as reflected in the consolidated statements of operations and comprehensive income (loss) is as follows (in thousands): Years Ended December 31, 2020 2019 2018 Federal income tax expense (benefit) at statutory rate $ (22,263 ) $ (20,473 ) $ 21,862 State income tax expense (benefit), net of federal tax effect (11,495 ) (15,323 ) 9,691 Research and development credits (7,793 ) (11,075 ) (7,847 ) Stock-based compensation 587 (2,134 ) (6,493 ) Other non-deductible expenses and reconciling items 886 144 139 Change in corporate tax rates 109 130 (729 ) Change in valuation allowance 45,209 45,976 (12,444 ) Total income tax expense (benefit) $ 5,240 $ (2,755 ) $ 4,179 The significant components of the Company’s net deferred tax assets were as follows (in thousands): December 31, 2020 December 31, 2019 Deferred tax assets: Net operating loss carryforwards $ 14,683 $ 53,841 Research and development tax credits 47,970 40,365 Stock-based compensation 15,644 10,224 Lease liabilities 21,961 3,916 Liability related sale of future royalties 62,089 — Depreciation and amortization — 26 Accruals and other 7,370 4,779 Total deferred tax assets before valuation allowance 169,717 113,151 Valuation allowance (142,907 ) (97,511 ) Total deferred tax assets 26,810 15,640 Deferred tax liabilities: Unrealized gains on marketable securities (163 ) (11,344 ) Right-of-use assets (21,094 ) (3,467 ) Depreciation and amortization (5,155 ) — Other (398 ) (829 ) Total deferred tax liabilities (26,810 ) (15,640 ) Net deferred tax assets $ — $ — The Company evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets. Based on the Company’s history of operating losses, including a three-year As of December 31, 2020 and 2019, the Company had U.S. federal NOL carryforwards of approximately $44.7 million and $170.4 million, respectively, and U.S. state NOL carryforwards of approximately $79.7 million and $269.3 million, respectively, which may be available to offset future income tax liabilities. The Company’s U.S. federal NOL carryforwards as of December 31, 2020 may be carried forward indefinitely. A portion of the Company’s U.S. state NOL carryforwards as of December 31, 2020 may be carried forward indefinitely, with the remaining portion expiring at various dates between 2034 and 2040. As of December 31, 2020 and 2019, the Company had U.S. federal and state research and development tax credit carryforwards of approximately $48.0 million and $40.4 million, respectively, net of unrecognized tax benefits of $0.1 million and $0.1 million, respectively, which may be available to reduce future income tax liabilities and expire at various dates between 2035 and 2040. The calculation of these credits requires assumptions to be made by the Company to estimate qualified research expenses. The Company conducts formal studies to document the qualified activities and expenses used to calculate these credits, however a portion of these credits may be subject to future studies which have not yet occurred, the results of which may result in an adjustment to the Company’s credit carryforwards. The Company accounts for uncertain tax positions in accordance with the requirements of ASC 740, and recognizes the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. As of December 31, 2020 and 2019, the Company had total unrecognized tax benefits of $0.1 million and $0.1 million, respectively, which were reserved against its research and development tax credit carryforwards as uncertain tax positions. No reserve for uncertain tax positions has been placed against qualified expenses for which a study has not been conducted. However, a full valuation allowance has been provided against the net credit carryforwards and, if an adjustment is required upon the completion of the study, this adjustment would be offset by an adjustment to the deferred tax asset established for the research and development credit carryforwards and the valuation allowance. If these unrecognized tax benefits were to be recognized, the impact would be offset by an adjustment to the valuation allowance, resulting in no impact on the Company’s effective tax rate. The Company does not expect that a significant portion of its unrecognized tax benefits will increase or decrease in the next 12 months as of December 31, 2020. Under the provisions of the Internal Revenue Code, the Company’s NOL and tax credit carryforwards are subject to review and possible adjustment by the Internal Revenue Service and state tax authorities. NOL and tax credit carryforwards may be subject to an annual limitation in the event of certain cumulative changes in the ownership interest of significant stockholders over a three-year The Company and its subsidiaries file income tax returns in the United States, at the federal level and in various states, and in foreign jurisdictions. The U.S. federal and state income tax returns are generally subject to tax examinations for the tax years ended December 31, 2016 onward. To the extent the Company has tax attribute carryforwards, the tax years in which the attribute was generated may still be adjusted upon examination by the Internal Revenue Service or state tax authorities to the extent utilized in a future period.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2020
Related Party Transactions [Abstract]
Related Party Transactions	14. Related Party Transactions FOXKISER LLP Since 2016, the Company has been party to professional services agreements with FOXKISER LLP (FOXKISER), an affiliate of certain stockholders of the Company and an affiliate of a member of the Company’s Board of Directors, pursuant to which the Company pays a fixed monthly fee in consideration for certain strategic services provided by FOXKISER. Effective January 2019, the Company entered into a new professional services agreement with FOXKISER with similar terms and conditions as the previous agreements. The agreement was amended effective June 2019 to expand the scope of the services provided and increase the monthly fee. Effective August 2020, the agreement was further amended to extend the term of the agreement by two years through December 2022. The agreement may be terminated by either party with six months’ advanced written notice. Expenses incurred under the agreements with FOXKISER for the years ended December 31, 2020, 2019 and 2018 were $4.8 million, $4.1 million and $2.1 million, respectively, and were recorded as research and development expenses in the consolidated statements of operations and comprehensive income (loss).

Net Income (Loss) Per Share

Net Income (Loss) Per Share	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
Net Income (Loss) Per Share	15. Net Income (Loss) Per Share The computations of basic and diluted net income (loss) per share were as follows (in thousands, except per share data): Years Ended December 31, 2020 2019 2018 Basic net income (loss) per share: Net income (loss) $ (111,250 ) $ (94,733 ) $ 99,937 Shares used in computation: Weighted-average common shares outstanding 37,281 36,690 33,427 Basic net income (loss) per share $ (2.98 ) $ (2.58 ) $ 2.99 Diluted net income (loss) per share: Net income (loss) $ (111,250 ) $ (94,733 ) $ 99,937 Shares used in computation: Weighted-average common shares outstanding 37,281 36,690 33,427 Stock options — — 3,186 Restricted stock units — — 32 Employee stock purchase plan — — 3 Weighted-average diluted common shares 37,281 36,690 36,648 Diluted net income (loss) per share $ (2.98 ) $ (2.58 ) $ 2.73 For periods in which the Company incurred net losses, common stock equivalents were excluded from the calculation of diluted net loss per share as their effect would be anti-dilutive. Accordingly, basic and diluted net loss per share were the same for such periods. The following potentially dilutive common stock equivalents outstanding at the end of the period were excluded from the computations of weighted-average diluted common shares for the periods indicated as their effects would be anti-dilutive (in thousands): Years Ended December 31, 2020 2019 2018 Stock options issued and outstanding 6,361 5,544 928 Employee stock purchase plan 19 17 — 6,380 5,561 928

Supplemental Disclosures

Supplemental Disclosures	12 Months Ended
	Dec. 31, 2020
Payables And Accruals [Abstract]
Supplemental Disclosures	16. Supplemental Disclosures Accrued expenses and other current liabilities consisted of the following (in thousands): December 31, 2020 December 31, 2019 Accrued personnel costs $ 13,155 $ 10,903 Accrued sublicense fees and royalties 12,160 4,542 Accrued external research and development expenses 9,738 5,791 Accrued purchases of property and equipment 7,853 1,328 Accrued income taxes payable 3,135 — Accrued external general and administrative expenses 2,865 2,053 Other accrued expenses and current liabilities 176 229 $ 49,082 $ 24,846

Selected Quarterly Financial In

Selected Quarterly Financial Information (Unaudited)	12 Months Ended
	Dec. 31, 2020
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Information (Unaudited)	17. Selected Quarterly Financial Information (Unaudited) The following tables contain quarterly financial information for the years ended December 31, 2020 and 2019. Management believes that the following information reflects all normal recurring adjustments necessary for a fair statement of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period. Amounts in the following table are in thousands, except per share data. Quarters Ended March 31, 2020 June 30, 2020 September 30, 2020 December 31, 2020 Total revenues $ 17,644 $ 16,566 $ 98,912 $ 21,445 Cost of revenues 3,409 4,684 17,364 10,257 Research and development expense 37,035 38,111 43,968 47,180 General and administrative expense 14,833 15,554 15,859 17,571 Total operating expenses 55,344 58,399 84,961 75,096 Net income (loss) (40,038 ) (33,762 ) 8,791 (46,241 ) Net income (loss) per share: Basic $ (1.08 ) $ (0.91 ) $ 0.24 $ (1.24 ) Diluted (1.08 ) (0.91 ) 0.23 (1.24 ) Quarters Ended March 31, 2019 June 30, 2019 September 30, 2019 December 31, 2019 Total revenues $ 884 $ 7,881 $ 14,700 $ 11,768 Cost of revenues 29 1,927 2,494 3,791 Research and development expense 25,203 29,483 35,692 33,807 General and administrative expense 11,558 13,405 12,402 14,450 Total operating expenses 36,790 44,753 50,596 52,092 Net income (loss) (32,228 ) (1,457 ) (34,584 ) (26,464 ) Net income (loss) per share: Basic $ (0.89 ) $ (0.04 ) $ (0.94 ) $ (0.72 ) Diluted (0.89 ) (0.04 ) (0.94 ) (0.72 )

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Basis of Presentation and Principles of Consolidation	Basis of Presentation and Principles of Consolidation The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (GAAP) and include the accounts of the Company and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation.
Foreign Currency Transactions	Foreign Currency Transactions The functional currency of the Company and its consolidated subsidiaries is the U.S. dollar. Transaction gains and losses that arise from exchange rate fluctuations on transactions denominated in currencies other than the U.S. dollar are included in results of operations as incurred.
Use of Estimates	Use of Estimates The preparation of the consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses and related disclosure of contingent assets and liabilities for the periods presented. Management bases its estimates on historical experience and on various other factors that it believes are reasonable under the circumstances, the results of which form the basis for making judgments about the carrying value of assets and liabilities, and other reported amounts, that are not readily apparent from other sources. Actual results may differ materially from these estimates. Significant estimates are used in the following areas, among others: license and royalty revenue, the allowance for credit losses, accrued research and development expenses and other accrued liabilities, stock-based compensation expense, income taxes and the fair value of financial instruments. The Company is actively monitoring the impact of the COVID-19 pandemic on its business, results of operations and financial condition. The full extent to which the COVID-19 pandemic will directly or indirectly impact the Company’s business, results of operations and financial condition in the future is unknown at this time and will depend on future developments that are highly unpredictable. The most significant estimates affecting the Company’s consolidated financial statements that may be impacted by the COVID-19 pandemic are related to the Company’s assessment of credit losses on accounts receivable, contract assets and available-for-sale debt securities.
Reclassifications	Reclassifications Certain amounts reported in prior periods have been reclassified to conform to current period financial statement presentation. These reclassifications are not material and have no effect on previously reported financial position, results of operations and cash flows.
Segment and Geographical Information	Segment and Geographical Information Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker (CODM), or decision-making group, in making decisions on how to allocate resources and assess performance. The Company’s CODM, its Chief Executive Officer, views its operations and manages its business as one operating segment. The Company’s revenues consist of license and royalty revenue. For the year ended December 31, 2020, 80% of the Company’s revenues were attributed to the U.S. and no other countries were attributed 10% or more of the Company’s revenues. For the year ended December 31, 2019, 90% of the Company’s revenues were attributed to the U.S. and no other countries were attributed 10% or more of the Company’s revenues. For the year ended December 31, 2018, 99% of the Company’s revenue were attributed to the U.S. The country of origin for license revenue is determined based on the country of domicile of the licensee. The country of origin for royalty revenue is determined based on the location of the underlying net sales of licensed products. The substantial majority of the Company’s assets reside in the U.S.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments purchased with original maturities of 90 days or less at acquisition to be cash equivalents.
Restricted Cash	Restricted Cash Restricted cash includes money market mutual funds used to collateralize irrevocable letters of credit as required by the Company’s lease agreements. The following table provides a reconciliation of cash and cash equivalents and restricted cash as reported on the consolidated balance sheets to the total of these amounts as reported at the end of the period in the consolidated statements of cash flows (in thousands): December 31, 2020 December 31, 2019 December 31, 2018 Cash and cash equivalents $ 338,426 $ 69,514 $ 75,561 Restricted cash 1,330 1,330 1,053 Total cash and cash equivalents and restricted cash $ 339,756 $ 70,844 $ 76,614
Marketable Securities	Marketable Securities Marketable securities consist of available-for-sale debt securities and equity securities and are carried at fair value. Marketable debt securities with remaining maturity dates exceeding 12 months which are not intended to be sold prior to maturity for use in current operations are classified as non-current assets. Marketable equity securities are classified as current assets. Unrealized gains and losses on available-for-sale debt securities, net of any related tax effects, are excluded from results of operations and are included in other comprehensive income (loss) and reported as a separate component of stockholders’ equity until realized. The Company uses the aggregate portfolio approach to release the tax effects of unrealized gains and losses on available-for-sale debt securities in accumulated other comprehensive income (loss). Purchase premiums and discounts on marketable debt securities are amortized or accreted into the cost basis over the life of the related security as adjustments to the yield using the effective-interest method. Interest income is recognized when earned. Unrealized gains and losses on marketable equity securities are included in results of operations as investment income. Realized gains and losses from the sale or maturity of marketable securities are based on the specific identification method and are included in results of operations as investment income. At each reporting date, the Company evaluates available-for-sale debt securities which have an amortized cost basis in excess of the fair value of the security to determine if the unrealized loss or any potential credit losses should be recognized in results of operations . If the Company does not have the intent and ability to hold the security until recovery of the unrealized loss, the difference between the fair value and amortized cost basis of the security is charged to results of operations resulting in a new amortized cost basis of the security. If the Company has the intent and ability to hold the security until recovery of the unrealized loss, the security is evaluated for potential credit losses. If a credit loss is deemed to exist, the credit loss is recognized in results of operations and an allowance for credit losses is recorded against the amortized cost basis of the security. In determining whether a credit loss exists related to impaired available-for-sale debt securities, the Company considers, among other factors, the extent of the unrealized loss relative to the amortized cost basis, the credit rating of the issuer and any recent changes thereto, current and expected future economic conditions, and any adverse events or other changes in circumstances that have occurred which may indicate a potential credit loss. The Company did not record an allowance for credit losses on its available-for-sale debt securities as of December 31 , 2020 or 2019 .
Accounts Receivable	Accounts Receivable Accounts receivable primarily consist of consideration due to the Company resulting from its license agreements with NAV Technology Licensees. Accounts receivable include amounts invoiced to licensees as well as rights to consideration which have not yet been invoiced, including unbilled royalties, and for which payment is conditional solely upon the passage of time. If a licensee elects to terminate a license prior to the end of the license term, the licensed intellectual property is returned to the Company and any accounts receivable from the licensee which are not contractually payable to the Company are charged off as a reduction of license revenue in the period of the termination. Accounts receivable which are not expected to be received by the Company within 12 months from the reporting date are stated net of a discount to present value and recorded as non-current assets on the consolidated balance sheets. The present value discount is recognized as a reduction of revenue in the period in which the accounts receivable are initially recorded and is accreted as interest income from licensing over the term of the receivables. Accounts receivable are stated net of an allowance for credit losses, if deemed necessary based on the Company’s evaluation of collectability and potential credit losses. Management assesses the collectability of its accounts receivable using the specific identification of account balances, and considers the credit quality and financial condition of its significant customers, historical information regarding credit losses and the Company’s evaluation of current and expected future economic conditions. If necessary, an allowance for credit losses is recorded against accounts receivable such that the carrying value of accounts receivable reflects the net amount expected to be collected. Accounts receivable balances are written off against the allowance for credit losses when the potential for collectability is considered remote. Please refer to Note 10 for further information regarding the allowance for credit losses related to accounts receivable.
Concentrations of Credit Risk and Off-balance Sheet Risk	Concentrations of Credit Risk and Off-balance Sheet Risk Cash and cash equivalents, marketable debt securities and accounts receivable are financial instruments that are potentially subject to concentrations of credit risk. The Company’s cash and cash equivalents are deposited in accounts at multiple financial institutions, and amounts may exceed federally insured limits. The Company believes it is not exposed to significant credit risk due to the financial strength of the depository institutions in which the cash and cash equivalents are held. The Company’s marketable debt securities consist of investment grade securities and may be subject to concentrations of credit risk. The Company has adopted an investment policy which limits potential concentrations of investments and establishes minimum acceptable credit ratings, thereby reducing credit risk exposure. With the exception of accounts receivable from Abeona Therapeutics Inc. (Abeona), as discussed further in Note 10, the Company believes that it is not exposed to significant credit risk related to accounts receivable due to the credit quality and history of collections from its significant customers, and the Company is unaware of any concentrations of credit risk related to accounts receivable from significant customers with deteriorated credit quality. The Company has no financial instruments with off-balance sheet risk of loss. The following table summarizes those customers who represented at least 10% of revenues or total net accounts receivable for the periods presented: Revenues Accounts Receivable, Net Years Ended December 31, December 31, 2020 2019 2018 2020 2019 Customer A 94 % 69 % 81 % 44 % 28 % Customer B * * 16 % 48 % 62 % Customer C * 13 % * * * Customer D * 10 % * * * * Represented less than 10%
Leases	Leases Effective January 1, 2019, the Company adopted Accounting Standards Update (ASU) 2016-02, Leases Leases Under Topic 842, the Company classifies its leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the Company. Lease classification is evaluated at the inception of the lease agreement. Regardless of classification, the Company records a right-of-use asset and a lease liability for all leases with a term greater than 12 months. All of the Company’s leases as of December 31, 2020 and 2019 have been classified as operating leases. Operating lease expense is recognized on a straight-line basis over the term of the lease, with the exception of variable lease expenses which are recognized as incurred. The Company identifies leases in its contracts if the contract conveys the right to control the use of identified property, plant or equipment for a period of time in exchange for consideration. The Company does not allocate lease consideration between lease and nonlease components and records a lease liability equal to the present value of the remaining fixed consideration under the lease. The interest rates implicit in the Company’s leases are generally not readily determinable. Accordingly, the Company uses its estimated incremental borrowing rate at the commencement date of the lease to determine the present value discount of the lease liability. The Company estimates its incremental borrowing rate for each lease based on an evaluation of its expected credit rating and the prevailing market rates for collateralized debt in a similar economic environment with similar payment terms and maturity dates commensurate with the term of the lease. The right-of-use asset for each lease is equal to the lease liability, adjusted for unamortized initial direct costs and lease incentives and prepaid or accrued rent. Initial direct costs of entering into a lease are included in the right-of-use asset and amortized as lease expense over the term of the lease. Lease incentives, such as tenant improvements allowances, are recorded as a reduction of the right-of-use asset and amortized as a reduction of lease expense over the term of the lease. The Company excludes options to extend or terminate leases from the calculation of the lease liability unless it is reasonably certain the option will be exercised.
Property and Equipment	Property and Equipment Property and equipment is stated at cost less accumulated depreciation and amortization. Maintenance and repairs that do not improve or extend the lives of the respective assets are expensed to operations as incurred. Upon disposal, the related cost and accumulated depreciation is removed from the accounts and any resulting gain or loss is included in the results of operations. Depreciation and amortization is calculated using the straight-line method over the estimated useful lives of the assets, which are as follows: Estimated Useful Life Computer equipment and software 3 years Furniture and fixtures 5 years Laboratory and manufacturing equipment 5 to 15 years Leasehold improvements Shorter of lease term or estimated useful life
Impairment of Long-lived Assets	Impairment of Long-lived Assets The Company evaluates its long-lived assets for impairment when events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Recoverability is measured by comparison of the book values of the assets to estimated future net undiscounted cash flows that the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the book value of the assets exceed their fair value, which is measured based on the projected discounted future net cash flows arising from the assets. No impairment losses on long-lived assets were recorded during the years ended December 31, 2020, 2019 and 2018.
Non-marketable Equity Securities	Non-marketable Equity Securities The Company’s non-marketable equity securities consist of equity investments in other entities in which the Company’s ownership interest is below 20% and the Company does not have significant influence over the operations of the entity, or for which the equity securities are not common stock or in-substance common stock. The Company’s non-marketable equity securities do not have readily determinable fair values and are measured at cost less impairment, adjusted for observable price changes for identical or similar investments of the same issuer. Please refer to Note 4 for further information on non-marketable equity securities.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The Company is required to disclose information on all assets and liabilities reported at fair value that enables an assessment of the inputs used in determining the reported fair values. ASC 820, Fair Value Measurements and Disclosures • Level 1—Valuations based on unadjusted quoted prices in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date. • Level 2—Valuations based on quoted prices for similar assets or liabilities in markets that are not active or for which all significant inputs are observable, either directly or indirectly. • Level 3—Valuations that require inputs that reflect the Company’s own assumptions that are both significant to the fair value measurement and unobservable. To the extent that valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. The fair values of the Company’s Level 2 instruments are based on quoted market prices or broker or dealer quotations for similar assets. These investments are initially valued at the transaction price and subsequently valued utilizing third party pricing providers or other market observable data. Please refer to Note 4 for further information on the fair value measurement of the Company’s financial instruments.
Liability Related to Sale of Future Royalties	Liability Related to Sale of Future Royalties As discussed in Note 7, the Company recorded a liability for the net proceeds received from the sale of its Zolgensma royalty payments to entities managed by Healthcare Royalty Management, LLC (collectively, HCR). The liability is accounted for as debt since the return to HCR is explicitly capped under the royalty purchase agreement, and is amortized over the estimated life of the arrangement using the effective interest method. The total amount of royalty payments received by HCR under the agreement, less the net proceeds received by the Company, is recorded as non-cash interest expense over the life of the arrangement. The Company estimates the effective interest rate based on its estimate of total royalty payments to be received by HCR under the agreement. The Company reassesses these estimates at each reporting date and adjusts the effective interest rate and amortization of the liability on a prospective basis as necessary. Due to its continuing involvement in the underlying license agreement with Novartis Gene Therapies, Inc. (formerly AveXis, Inc.), the Company continues to recognize royalty revenue on net sales of Zolgensma and records the royalty payments to HCR as a reduction of the liability when paid. As such payments are made to HCR, the balance of the liability will be effectively repaid over the life of the royalty purchase agreement. The portion of the liability related to the sale of future royalties which is expected to be amortized within 12 months of the reporting date is recorded as a current liability, with the remaining portion of the liability recorded as a non-current liability.
Revenue Recognition	Revenue Recognition The Company recognizes revenue in accordance with ASC 606, Revenue from Contracts with Customers The Company applies the five-step model to contracts that are within the scope of Topic 606 only when it is probable that the Company will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, for contracts within the scope of Topic 606, the Company assesses the goods or services promised within each contract and determine those that are performance obligations and whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to respective performance obligations when (or as) the respective performance obligations are satisfied. The Company evaluates its contracts with customers for the presence of significant financing components. If a significant financing component is identified in a contract and provides a financing benefit to the customer, the transaction price for the contract is adjusted to account for the financing portion of the arrangement, which is recognized as interest income over the financing term using the effective interest method. In determining the appropriate interest rates for significant financing components, the Company evaluates the credit profile of the customer and prevailing market interest rates and selects an interest rate in which it believes would be charged to the customer in a separate financing arrangement over a similar financing term. License and Royalty Revenue The Company licenses its NAV Technology Platform to other biotechnology and pharmaceutical companies. The terms of the licenses vary, and licenses may be exclusive or non-exclusive and may be sublicensable by the licensee. Licenses may grant intellectual property rights for purposes of internal and preclinical research and development only, or may include the rights, or options to obtain future rights, to commercialize drug therapies for specific diseases using the Company’s NAV Technology Platform. License agreements generally have a term at least equal to the life of the underlying patents, but are terminable at the option of the licensee. Consideration payable to the Company under its license agreements may include: (i) up-front and annual fees, (ii) option fees to acquire additional licenses, (iii) milestone payments based on the achievement of certain development and sales-based milestones by licensees, (iv) sublicense fees and (v) royalties on sales of licensed products. The Company’s license agreements are accounted for as contracts with customers within the scope of Topic 606. At the inception of each license agreement, the Company determines the contract term for purposes of applying the requirements of Topic 606. Licenses are generally terminable at the option of the licensee with advance notice to the Company. For each license granted, including licenses granted upon the exercise of license options, the Company evaluates these termination rights to determine whether a substantive termination penalty would be incurred by the licensee upon termination. If the licensee incurs a substantive termination penalty upon termination, the contract term for revenue recognition purposes is generally equal to the stated term of the license, which is the life of the underlying licensed patents. Alternatively, if the licensee does not incur a substantive termination penalty upon termination, the contract term for revenue recognition purposes may be shorter than the stated term of the license, in which case the termination rights may be accounted for as contract renewal options. The determination of whether a substantive termination penalty is associated with the termination rights requires significant judgment. In making this determination, the Company considers, among other things, the nature of the intellectual property rights that would be returned to the Company upon termination, including the exclusivity of the licensed rights and the stage of development of the licensed products, the payment terms, including the amount and timing of non-refundable or guaranteed payments, and the business purpose of the termination rights granted to the licensee. Generally, the most significant judgment in determining whether a substantive termination penalty exists relates to the amount of any up-front or guaranteed non-refundable payments relative to the amount of annual payments that may be avoided by the licensee upon termination of the license. The Company considers all of the facts and circumstances relevant to each license when making this determination. Performance obligations under the Company’s license agreements may include (i) the delivery of intellectual property licenses, (ii) options granted to licensees to acquire additional licenses, to the extent the options represent material rights to the licensee, and (iii) research and development services to be performed by the Company related to licensed products. At the inception of each license agreement which contains options for the licensee to acquire additional licenses, or contract renewal options, the Company evaluates the options to determine whether they provide material rights to the licensee. In making this determination, the Company considers whether the options are priced at a discount to the standalone selling price for the underlying licenses. If an option is priced at a discount to the standalone selling price for the underlying license, the option is considered to be a material right to the licensee and is accounted for as a separate performance obligation under the current license agreement. At the inception of each license agreement which contains performance obligations for research and development services, the Company evaluates whether the license is distinct from the research and development services, which requires judgment. In making this determination, the Company considers, among other things, the stage of development of the licensed products and whether the research and development services will significantly impact further development of the licensed products. If it is determined that the license if not distinct from the research and development services, the license is combined with the research and development services into a single performance obligation. The Company evaluates the transaction price of its license agreements at the inception of each agreement and at each reporting date. The transaction price includes the fixed consideration payable to the Company during the contract term, as well as any variable consideration to the extent that it is probable that a significant reversal of revenue will not occur in the future. Fixed consideration under the license agreements includes up-front and annual fees payable during the contract term. Variable consideration under the license agreements includes development and sales-based milestone payments, sublicense fees and royalties on sales of licensed products. Consideration contingent upon the exercise of options by a licensee is excluded from the transaction price and not accounted for as part of the license agreement until the option is exercised. The transaction price for each license agreement is allocated to the underlying performance obligations based on their relative standalone selling prices and recognized as revenue when (or as) the performance obligations are satisfied. Consideration allocated to performance obligations for the delivery of an intellectual property license is recognized as revenue in full upon the delivery of the license to the licensee. Consideration allocated to performance obligations for license options is recognized as revenue in full upon the earlier of the option exercise or expiration. The exercise of a license option by a licensee is accounted for as a new license for revenue recognition purposes. Consideration allocated to performance obligations for research and development services is recognized as revenue as the services are performed by the Company. Up-front and annual licenses fees payable to the Company over the contract term of each license are included in the transaction price, and the portion of this consideration that is allocated to the performance obligation for the delivery of the intellectual property license is recognized as revenue in full upon the delivery of the license to the licensee. If annual license fees are payable to the Company in periods beyond 12 months from the delivery of the license, a significant financing component is deemed to exist which provides a financing benefit to the licensee. If a significant financing component is identified, the Company adjusts the transaction price for the license to include only the present value of the annual license fees payable to the Company over the contract term. The discounted portion of the license fees is recognized as interest income from licensing over the financing period of the license. Development milestone payments are payable to the Company upon the achievement of specified development milestones by licensees. At the inception of each license agreement that contains development milestone payments, the Company evaluates whether the milestones are considered probable of achievement and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal will not occur in the future, milestone payments are included in the transaction price and recognized as revenue upon the delivery of the license. Milestone payments contingent on the achievement of development milestones that are not within the control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved and are excluded from the transaction price until the milestone is achieved. At each reporting date, the Company re-evaluates the probability of achievement of each outstanding development milestone and, if necessary, adjusts the transaction price for any milestones for which the probability of achievement has changed due to current facts and circumstances. Any such adjustments are recorded on a cumulative catch-up basis and recognized as revenue in the period of the adjustment. Royalties on sales of licensed products, sales-based milestone payments and sublicense fees based on the receipt of certain fees by licensees from any sublicensees are excluded from the transaction price of each license and recognized as revenue in the period that the related sales or sublicenses occur, provided that the associated license has been delivered to the licensee. Royalty revenue to date consists primarily of royalties on net sales of Zolgensma, which is a licensed product under the Company’s license agreement with Novartis Gene Therapies, Inc. (formerly AveXis, Inc.) (Novartis Gene Therapies), a wholly owned subsidiary of Novartis AG (Novartis), for the development and commercialization of treatments for spinal muscular atrophy (SMA). The Company recognizes royalty revenue from net sales of Zolgensma in the period in which the underlying products are sold by Novartis Gene Therapies, which in certain cases may require the Company to estimate royalty revenue for periods of net sales which have not yet been reported to the Company. Sales-based milestone payments related to net sales of Zolgensma are recognized as royalty revenue in the period in which the milestone is achieved. The Company receives payments from licensees based on the billing schedules established in each license agreement. Amounts recognized as revenue which have not yet been received from licensees, including unbilled royalties, are recorded as accounts receivable when the Company’s rights to the consideration are conditional solely upon the passage of time. Amounts recognized as revenue which have not yet been received from licensees are recorded as contract assets when the Company’s rights to the consideration are not unconditional. Contract assets are recorded as other current assets on the consolidated balance sheets. If a licensee elects to terminate a license prior to the end of the license term, the licensed intellectual property is returned to the Company and any consideration recorded as accounts receivable or contract assets which is not contractually payable by the licensee is charged off as a reduction of license revenue in the period of the termination. Amounts received by the Company prior to the delivery of underlying performance obligations are deferred and recognized as revenue upon the satisfaction of the performance obligations by the Company. Deferred revenue which is not expected to be recognized within 12 months from the reporting date is recorded as non-current on the consolidated balance sheets.
Cost of Revenues	Cost of Revenues Cost of revenues consists primarily of sublicense fees, milestone payments and royalties on net sales of licensed products as specified in the Company’s agreements with its licensors. Sublicense fees are based on a percentage of license fees received by the Company from NAV Technology Licensees and are recognized in the period that the underlying revenue is recognized. Milestone payments are payable to licensors upon the achievement of specified milestones by NAV Technology Licensees and are recognized in the period the milestone is achieved or deemed probable of achievement. Royalties are based on a percentage of net sales of licensed products by NAV Technology Licensees and are recognized in the period that the underlying sales occur. Amounts which are payable to licensors in periods beyond 12 months from the reporting date are recorded as non-current liabilities on the consolidated balance sheets.
Research and Development Expenses	Research and Development Expenses Research and development costs are expensed as incurred in performing research and development activities. Advance payments for goods or services related to research and development activities are deferred and expensed as the goods are delivered or the services are performed. Research and development costs include salaries, benefits and other personnel costs, laboratory and facilities costs, allocated overhead costs, license and milestone fees, and costs of goods and services associated with preclinical research and clinical trial activities, associated manufacturing-related activities, regulatory activities and other related services performed by third-parties. At the end of each reporting period, the Company compares payments made to third-party service providers to the estimated expenses incurred based on the services provided and progress toward completion of the research or development objectives. Such estimates are subject to change as additional information becomes available. Depending on the timing of payments to the service providers and the estimated expenses incurred, the Company may record net prepaid or accrued research and development expenses relating to these costs. Up-front fees incurred in obtaining technology licenses, as well as milestone payments to licensors, are charged to research and development expense as incurred if the technology licensed has no alternative future use.
Collaborative Arrangements	Collaborative Arrangements The Company evaluates its collaboration arrangements to determine whether they are within the scope of ASC 808, Collaborative Arrangements
Stock-based Compensation	Stock-based Compensation The Company accounts for its stock-based compensation awards in accordance with ASC 718, Compensation—Stock Compensation The Company’s stock-based awards may be subject to either service or performance-based vesting conditions. Compensation expense related to awards to employees and nonemployees with service-based vesting conditions is recognized on a straight-line basis based on the estimated grant date fair value over the requisite service period of the award, which is generally the vesting term. Compensation expense related to awards to employees and nonemployees with performance-based vesting conditions is recognized based on the estimated grant date fair value over the requisite service period using the accelerated attribution method to the extent achievement of the performance condition is probable. The Company has elected to not estimate forfeitures of stock-based awards and to account for forfeitures as they occur. The Company estimates the fair value of its stock option awards using the Black-Scholes option-pricing model, which requires the input of subjective assumptions, including (i) the fair value of the underlying common stock, (ii) the expected stock price volatility, (iii) the expected term of the award, (iv) the risk-free interest rate and (v) expected dividends. The Company does not have sufficient historical and implied volatility data for its common stock necessary to estimate the expected the volatility of its common stock over a period of time commensurate with the expected term of its stock option awards. As a result, the Company estimates expected volatility based on the historical volatility of both its common stock and the common stock of a selected peer group of similar publicly traded companies for which sufficient historical volatility data is available. Due to the lack of historical volatility data for its common stock, the Company has historically place d a higher weight on the historical volatility of the selected peer group in estimating expected volatility and has increased the weight placed on the historical volatility of its common stock as more historical trading data has become available . The Company computes the historical volatility data using the daily closing prices for the selected companies’ shares during a period equivalent to the expected term of the stock option awards. For the purpose of identifying the selected peer group companies, the Company considers characteristics such as enterprise value, risk profiles, position within the industry and length of historical share price information. The Company plans to continue using historical peer group volatility data as an input to estimate expected volatility until a sufficient amount of historical volatility data for its common stock becomes available. The Company estimates the expected term of its employee stock options using the “simplified” method, whereby the expected term equals the arithmetic average of the vesting term and the original contractual term of the option due to its lack of sufficient historical data. For stock options granted to nonemployees, the Company uses the contractual term of the award rather than expected term to estimate the fair value of the award. The Company estimates the risk-free interest rates for periods within the expected term of its options based on the rates of U.S. Treasury securities with maturity dates commensurate with the expected term of the associated awards. The Company assumes a dividend yield of zero for its common stock as it has never paid dividends and do es not expect to pay dividends for the foreseeable future. The Company estimates the fair value of restricted stock units based on the fair value of the Company’s common stock on the date of the grant.
Income Taxes	Income Taxes Income taxes are recorded in accordance with ASC 740, Income Taxes The Company accounts for uncertain tax positions in accordance with the provisions of ASC 740. When uncertain tax positions exist, the Company recognizes the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. The determination as to whether the tax benefit will more likely than not be realized is based upon the technical merits of the tax position as well as consideration of the available facts and circumstances. The Company recognizes interest and penalties related to uncertain tax positions in income tax expense. As of December 31, 2020 and 2019, the Company had no accrued interest or penalties related to uncertain tax positions and no amounts have been recognized in the Company’s consolidated statements of operations and comprehensive income (loss).
Net Income (Loss) Per Share	Net Income (Loss) Per Share Basic net income (loss) per share is calculated by dividing net income (loss) applicable to common stockholders by the weighted-average common shares outstanding during the period, without consideration for common stock equivalents. Diluted net income (loss) per share is calculated by adjusting the weighted-average common shares outstanding for the dilutive effect of common stock equivalents outstanding for the period, determined using the treasury-stock method. Contingently convertible shares in which conversion is based on non-market-priced contingencies are excluded from the calculations of both basic and diluted net income (loss) per share until the contingency has been fully met. For purposes of the diluted net income (loss) per share calculation, common stock equivalents are excluded from the calculation of diluted net income (loss) per share if their effect would be anti-dilutive.
Comprehensive Income (Loss)	Comprehensive Income (Loss) Comprehensive income (loss) includes net income (loss) as well as unrealized gains and losses on available-for-sale debt securities, net of income tax effects and reclassification adjustments for realized gains and losses.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Recently Adopted Accounting Pronouncements In June 2016, the Financial Accounting Standards Board (FASB) issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement In August 2018, the FASB issued ASU 2018-15, Intangibles—Goodwill and Other—Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740)—Simplifying the Accounting for Income Taxes In February 2016, the FASB issued ASU 2016-02, Leases Leases In February 2018, the FASB issued ASU 2018-02, Income Statement—Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income comprehensive income to provide an option for an entity to reclassify the stranded tax effects of the Tax Cuts and Jobs Act of 2017 (the TCJA) that was signed into law in December 2017 from accumulated other comprehensive income directly to retained earnings. The stranded tax effects result from the remeasurement of deferred tax assets and liabilities which were originally recorded in comprehensive income but whose remeasurement is reflected in the income statement. The Company adopted this standard effective January 1, 2019, and upon adoption recorded a cumulative adjustment of less than $ 0.1 million to reclassify the stranded tax effects of unrealized gains and losses on available-for-sale securities from accumulated other comprehensive income (loss) to accumulated deficit. The adoption of this standard did not have a material impact on the Company’s financial position or results of operations. In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers Revenue Recognition

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Summary of Reconciliation of Cash and Cash Equivalents and Restricted Cash	The following table provides a reconciliation of cash and cash equivalents and restricted cash as reported on the consolidated balance sheets to the total of these amounts as reported at the end of the period in the consolidated statements of cash flows (in thousands): December 31, 2020 December 31, 2019 December 31, 2018 Cash and cash equivalents $ 338,426 $ 69,514 $ 75,561 Restricted cash 1,330 1,330 1,053 Total cash and cash equivalents and restricted cash $ 339,756 $ 70,844 $ 76,614
Schedule of Revenue or Accounts Receivable by Major Customers	The following table summarizes those customers who represented at least 10% of revenues or total net accounts receivable for the periods presented: Revenues Accounts Receivable, Net Years Ended December 31, December 31, 2020 2019 2018 2020 2019 Customer A 94 % 69 % 81 % 44 % 28 % Customer B * * 16 % 48 % 62 % Customer C * 13 % * * * Customer D * 10 % * * * * Represented less than 10%
Summary of Estimated Useful Lives of Assets	Depreciation and amortization is calculated using the straight-line method over the estimated useful lives of the assets, which are as follows: Estimated Useful Life Computer equipment and software 3 years Furniture and fixtures 5 years Laboratory and manufacturing equipment 5 to 15 years Leasehold improvements Shorter of lease term or estimated useful life

Marketable Securities (Tables)

Marketable Securities (Tables)	12 Months Ended
	Dec. 31, 2020
Investments Debt And Equity Securities [Abstract]
Summary of Company Marketable Securities	The following tables present a summary of the Company’s marketable securities, which consist of available-for-sale debt securities and equity securities (in thousands): Amortized Cost Unrealized Gains Unrealized Losses Fair Value December 31, 2020 U.S. government and federal agency securities $ 12,782 $ 22 $ — $ 12,804 Certificates of deposit 1,956 34 — 1,990 Corporate bonds 165,850 497 (55 ) 166,292 Municipal securities 3,035 2 — 3,037 $ 183,623 $ 555 $ (55 ) $ 184,123 Amortized Cost / Cost Unrealized Gains Unrealized Losses Fair Value December 31, 2019 U.S. government and federal agency securities $ 62,637 $ 215 $ (5 ) $ 62,847 Certificates of deposit 8,506 77 — 8,583 Corporate bonds 226,137 808 (29 ) 226,916 Equity securities 351 31,784 — 32,135 $ 297,631 $ 32,884 $ (34 ) $ 330,481
Summary of unrealized gain (loss) on available-for-sale securities, net, presented in the statements of operations and comprehensive income (loss)	Unrealized gain (loss) on available-for-sale securities, net, presented in the statements of operations and comprehensive income (loss) consisted of the following (in thousands): Years Ended December 31, 2020 2019 2018 Unrealized gain (loss) before reclassifications $ (426 ) $ 1,392 $ (44 ) Realized losses (gains) reclassified to investment income (139 ) (40 ) 39 Income tax expense — (467 ) — Unrealized gain (loss) on available-for-sale securities, net $ (565 ) $ 885 $ (5 )
Summary of Fair Values and Unrealized Losses of Marketable Securities Held by the Company in an Unrealized Loss Position for Less Than 12 months and 12 Months or Greater	The following tables present the fair values and unrealized losses of available-for-sale debt securities held by the Company in an unrealized loss position for less than 12 months and 12 months or greater (in thousands): Less than 12 Months 12 Months or Greater Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses December 31, 2020 Corporate bonds $ 55,507 $ (55 ) $ — $ — $ 55,507 $ (55 ) $ 55,507 $ (55 ) $ — $ — $ 55,507 $ (55 ) Less than 12 Months 12 Months or Greater Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses December 31, 2019 U.S. government and federal agency securities $ 12,562 $ (5 ) $ — $ — $ 12,562 $ (5 ) Corporate bonds 48,556 (29 ) — — 48,556 (29 ) $ 61,118 $ (34 ) $ — $ — $ 61,118 $ (34 )

Fair Value of Financial Instr_2

Fair Value of Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Schedule of Fair Value of Cash Equivalents and Marketable Securities	The following tables present the fair value of cash equivalents and marketable securities in accordance with the hierarchy discussed in Note 2 (in thousands): Quoted Significant prices other Significant in active observable unobservable markets inputs inputs (Level 1) (Level 2) (Level 3) Total December 31, 2020 Cash equivalents: Money market mutual funds $ — $ 96,307 $ — $ 96,307 Total cash equivalents — 96,307 — 96,307 Marketable securities: U.S. government and federal agency securities — 12,804 — 12,804 Certificates of deposit — 1,990 — 1,990 Corporate bonds — 166,292 — 166,292 Municipal securities — 3,037 — 3,037 Total marketable securities — 184,123 — 184,123 Total cash equivalents and marketable securities $ — $ 280,430 $ — $ 280,430 Quoted Significant prices other Significant in active observable unobservable markets inputs inputs (Level 1) (Level 2) (Level 3) Total December 31, 2019 Cash equivalents: Money market mutual funds $ — $ 56,058 $ — $ 56,058 Total cash equivalents — 56,058 — 56,058 Marketable securities: U.S. government and federal agency securities — 62,847 — 62,847 Certificates of deposit — 8,583 — 8,583 Corporate bonds — 226,916 — 226,916 Equity securities 32,135 — — 32,135 Total marketable securities 32,135 298,346 — 330,481 Total cash equivalents and marketable securities $ 32,135 $ 354,404 $ — $ 386,539

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2020
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment, net consists of the following (in thousands): December 31, 2020 December 31, 2019 Laboratory and manufacturing equipment $ 26,306 $ 19,663 Computer equipment and software 3,764 2,545 Furniture and fixtures 4,114 2,188 Leasehold improvements 44,957 18,915 Total property and equipment 79,141 43,311 Accumulated depreciation and amortization (22,674 ) (14,338 ) Property and equipment, net $ 56,467 $ 28,973

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Summary of Lease Costs and Supplemental Cash Flow Information of Operating Leases	The following table summarizes the Company’s lease costs and supplemental cash flow information related to its operating leases (in thousands): Years Ended December 31, 2020 2019 Operating lease cost $ 5,246 $ 3,040 Variable lease cost 1,104 666 Total lease cost $ 6,350 $ 3,706 Cash paid (received) for amounts included in operating lease liabilities $ (678 ) $ 2,724 Right-of-use assets acquired through operating lease liabilities $ 56,956 $ 5,114
Weighted-Average Remaining Lease Term and Weighted-Average Discount Rate of Operating Leases	The weighted-average remaining lease term and weighted-average discount rate of the Company’s operating leases were as follows: As of December 31, 2020 Weighted-average remaining lease term (years) 13.7 Weighted-average discount rate 5.6 %
Reconciliation of Undiscounted Future Minimum Lease Payments Remaining Operating Leases	The following table presents a reconciliation of the undiscounted future minimum lease payments remaining under the Company’s operating leases to the amounts reported as operating lease liabilities on the consolidated balance sheet as of December 31, 2020 (in thousands): As of December 31, 2020 Undiscounted future minimum lease payments: 2021 $ 4,155 2022 6,021 2023 8,671 2024 9,745 2025 9,999 Thereafter 93,148 Total undiscounted future minimum lease payments $ 131,739 Amount representing imputed interest (46,212 ) Tenant improvement allowance not yet received (12,874 ) Total operating lease liabilities 72,653 Current portion of operating lease liabilities (2,500 ) Operating lease liabilities, non-current $ 70,153

Liability Related to Sale of _2

Liability Related to Sale of Future Royalties (Tables)	12 Months Ended
	Dec. 31, 2020
Liability Related To Sale Of Future Royalties [Abstract]
Schedule of Activity Within Liability Related to Sale of Future Royalties	The following table presents the changes in the liability related to the sale of future royalties under the Royalty Purchase Agreement with HCR (in thousands): Year Ended December 31, 2020 Liability related to sale of future royalties, beginning balance $ — Proceeds from sale of future royalties 196,000 Deferred transaction costs (3,473 ) Non-cash interest expense 771 Liability related to sale of future royalties, ending balance 193,298 Current portion of liability related to sale of future royalties (18,794 ) Liability related to sale of future royalties, non-current $ 174,504

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2020
The Trustees of the University of Pennsylvania [Member]
Summary of Expenses Incurred by Company	Expenses incurred by the Company related to its license from Penn were recorded as follows (in thousands): Years Ended December 31, 2020 2019 2018 Cost of revenues $ 39 $ — $ (18 ) Research and development — 200 — General and administrative 821 905 130 $ 860 $ 1,105 $ 112
GlaxoSmithKline LLC [Member]
Summary of Expenses Incurred by Company	Expenses incurred by the Company related to its license from GSK were recorded as follows (in thousands): Years Ended December 31, 2020 2019 2018 Cost of revenues: Royalties on net sales of Zolgensma $ 26,278 $ 5,822 $ — Other cost of revenues 9,398 2,419 9,407 Total cost of revenues 35,676 8,241 9,407 General and administrative 1,046 928 548 $ 36,722 $ 9,169 $ 9,955

Capitalization (Tables)

Capitalization (Tables)	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Schedule of Reserved Shares of Common Stock for Future Issuance	The Company’s reserved shares of common stock for future issuance were as follows (in thousands): December 31, 2020 December 31, 2019 Reserved for issuance under equity incentive plans 8,659 7,607 Reserved for issuance under employee stock purchase plan 448 134 9,107 7,741

License and Royalty Revenue (Ta

License and Royalty Revenue (Tables)	12 Months Ended
	Dec. 31, 2020
License Agreement Revenue Recognition [Abstract]
Summary of Changes in Balances of Receivables, Contract Assets and Deferred Revenue	The following table presents changes in the balances of the Company’s net accounts receivable, contract assets and deferred revenue, as well as other information regarding revenue recognized, during the periods presented (in thousands): Years Ended December 31, 2020 2019 2018 Accounts receivable, current and non-current: Balance, beginning of period $ 42,303 $ 31,599 $ 5,850 Additions 158,682 39,203 231,154 Deductions (154,719 ) (28,499 ) (205,405 ) Balance, end of period $ 46,266 $ 42,303 $ 31,599 Contract assets: Balance, beginning of period $ — $ 750 $ 350 Additions 350 1,000 3,000 Deductions — (1,750 ) (2,600 ) Balance, end of period $ 350 $ — $ 750 Deferred revenue, current and non-current: Balance, beginning of period $ 3,333 $ 3,933 $ — Additions 1,124 — 3,933 Deductions (225 ) (600 ) — Balance, end of period $ 4,232 $ 3,333 $ 3,933 Revenue recognized during the period from: Amounts included in deferred revenue at beginning of period $ — $ 600 $ — Performance obligations satisfied in previous periods $ 146,772 $ 26,689 $ 5,348
Summary of Accounts Receivables, net	Accounts receivable, net consisted of the following (in thousands): December 31, 2020 December 31, 2019 Current accounts receivable: Billed to customers $ 30,573 $ 376 Unbilled 20,104 37,772 Allowance for credit losses (7,678 ) — Current accounts receivable, net 42,999 38,148 Non-current accounts receivable: Unbilled 3,267 4,155 Allowance for credit losses — — Non-current accounts receivable, net 3,267 4,155 Total accounts receivable, net $ 46,266 $ 42,303
Summary of Changes in Allowance For Credit Losses	The following table presents the changes in the allowance for credit losses related to accounts receivable and contract assets for the year ended December 31, 2020 (in thousands): Accounts Receivable Contract Assets Balance at December 31, 2019 $ — $ — Provision for credit losses 7,678 — Write-offs — — Balance at December 31, 2020 $ 7,678 $ —
Schedule of License Revenue	The Company recognized the following amounts under the March 2014 License with Novartis Gene Therapies (in thousands): Years Ended December 31, 2020 2019 2018 License revenue $ 3,500 $ 3,500 $ 176,066 Royalties on net sales of Zolgensma 61,631 20,829 — Achievement of sales-based milestone for Zolgensma 80,000 — — Total license and royalty revenue $ 145,131 $ 24,329 $ 176,066 Interest income from licensing $ 26 $ 29 $ 7,966

Stock-based Compensation (Table

Stock-based Compensation (Tables)	12 Months Ended
	Dec. 31, 2020
Stock-Based Compensation Expense by Award Type	The Company’s stock-based compensation expense by award type was as follows (in thousands): Years Ended December 31, 2020 2019 2018 Stock options $ 31,178 $ 25,964 $ 15,960 Restricted stock units — 257 275 Employee stock purchase plan 771 633 406 $ 31,949 $ 26,854 $ 16,641
Stock-Based Compensation Expense	The Company recorded aggregate stock-based compensation expense in the consolidated statements of operations and comprehensive income (loss) as follows (in thousands): Years Ended December 31, 2020 2019 2018 Research and development $ 16,280 $ 13,031 $ 7,612 General and administrative 15,669 13,823 9,029 $ 31,949 $ 26,854 $ 16,641
Schedule of Fair Value of Options Granted on Weighted-Average Assumptions	The fair values of options granted were estimated at each grant date using the Black-Scholes valuation model with the following weighted-average assumptions: Years Ended December 31, 2020 2019 2018 Expected volatility 71 % 74 % 75 % Expected term (in years) 6.0 6.1 6.0 Risk-free interest rate 1.4 % 2.3 % 2.6 % Expected dividend yield 0.0 % 0.0 % 0.0 %
2014 and 2015 Equity Incentive Plan [Member]
Summary of Stock Option Activity	The following table summarizes stock option activity under the 2014 Plan and 2015 Plan (in thousands, except per share data): Weighted- average Weighted- Remaining average Contractual Aggregate Exercise Life Intrinsic Shares Price (Years) Value (a) Outstanding at December 31, 2019 5,544 $ 28.79 7.5 $ 86,509 Granted 1,633 $ 37.84 Exercised (435 ) $ 12.95 Cancelled or forfeited (381 ) $ 45.04 Outstanding at December 31, 2020 6,361 $ 31.21 7.2 $ 101,356 Exercisable at December 31, 2020 3,703 $ 24.27 6.2 $ 84,115 Vested and expected to vest at December 31, 2020 6,361 $ 31.21 7.2 $ 101,356 (a) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of the common stock for the options that were in the money at the dates reported.

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Components of Income (Loss) Before Income Taxes	The components of income (loss) before income taxes were as follows (in thousands): Years Ended December 31, 2020 2019 2018 United States $ (105,960 ) $ (97,435 ) $ 104,181 Foreign (50 ) (53 ) (65 ) Total income (loss) before income taxes $ (106,010 ) $ (97,488 ) $ 104,116
Components of the Provision For Income Tax Expense (Benefit)	The components of the provision for income tax expense (benefit) were as follows (in thousands): Years Ended December 31, 2020 2019 2018 Current: Federal $ — $ — $ — State 5,240 (2,288 ) 4,179 Foreign — — — Total current 5,240 (2,288 ) 4,179 Deferred: Federal — (284 ) — State — (183 ) — Foreign — — — Total deferred — (467 ) — Total income tax expense (benefit) $ 5,240 $ (2,755 ) $ 4,179
Schedule of Reconciliation of Effective Income Tax Expense (Benefit)	A reconciliation of income tax expense (benefit) computed at the statutory federal income tax rate of 21% to income tax expense (benefit) as reflected in the consolidated statements of operations and comprehensive income (loss) is as follows (in thousands): Years Ended December 31, 2020 2019 2018 Federal income tax expense (benefit) at statutory rate $ (22,263 ) $ (20,473 ) $ 21,862 State income tax expense (benefit), net of federal tax effect (11,495 ) (15,323 ) 9,691 Research and development credits (7,793 ) (11,075 ) (7,847 ) Stock-based compensation 587 (2,134 ) (6,493 ) Other non-deductible expenses and reconciling items 886 144 139 Change in corporate tax rates 109 130 (729 ) Change in valuation allowance 45,209 45,976 (12,444 ) Total income tax expense (benefit) $ 5,240 $ (2,755 ) $ 4,179
Components of Net Deferred Tax Assets	The significant components of the Company’s net deferred tax assets were as follows (in thousands): December 31, 2020 December 31, 2019 Deferred tax assets: Net operating loss carryforwards $ 14,683 $ 53,841 Research and development tax credits 47,970 40,365 Stock-based compensation 15,644 10,224 Lease liabilities 21,961 3,916 Liability related sale of future royalties 62,089 — Depreciation and amortization — 26 Accruals and other 7,370 4,779 Total deferred tax assets before valuation allowance 169,717 113,151 Valuation allowance (142,907 ) (97,511 ) Total deferred tax assets 26,810 15,640 Deferred tax liabilities: Unrealized gains on marketable securities (163 ) (11,344 ) Right-of-use assets (21,094 ) (3,467 ) Depreciation and amortization (5,155 ) — Other (398 ) (829 ) Total deferred tax liabilities (26,810 ) (15,640 ) Net deferred tax assets $ — $ —

Net Income (Loss) Per Share (Ta

Net Income (Loss) Per Share (Tables)	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
Computations of Basic and Diluted Net Income (Loss) Per Share	The computations of basic and diluted net income (loss) per share were as follows (in thousands, except per share data): Years Ended December 31, 2020 2019 2018 Basic net income (loss) per share: Net income (loss) $ (111,250 ) $ (94,733 ) $ 99,937 Shares used in computation: Weighted-average common shares outstanding 37,281 36,690 33,427 Basic net income (loss) per share $ (2.98 ) $ (2.58 ) $ 2.99 Diluted net income (loss) per share: Net income (loss) $ (111,250 ) $ (94,733 ) $ 99,937 Shares used in computation: Weighted-average common shares outstanding 37,281 36,690 33,427 Stock options — — 3,186 Restricted stock units — — 32 Employee stock purchase plan — — 3 Weighted-average diluted common shares 37,281 36,690 36,648 Diluted net income (loss) per share $ (2.98 ) $ (2.58 ) $ 2.73
Schedules for Computation of Diluted Weighted-Average Shares Outstanding	The following potentially dilutive common stock equivalents outstanding at the end of the period were excluded from the computations of weighted-average diluted common shares for the periods indicated as their effects would be anti-dilutive (in thousands): Years Ended December 31, 2020 2019 2018 Stock options issued and outstanding 6,361 5,544 928 Employee stock purchase plan 19 17 — 6,380 5,561 928

Supplemental Disclosures (Table

Supplemental Disclosures (Tables)	12 Months Ended
	Dec. 31, 2020
Payables And Accruals [Abstract]
Schedules of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consisted of the following (in thousands): December 31, 2020 December 31, 2019 Accrued personnel costs $ 13,155 $ 10,903 Accrued sublicense fees and royalties 12,160 4,542 Accrued external research and development expenses 9,738 5,791 Accrued purchases of property and equipment 7,853 1,328 Accrued income taxes payable 3,135 — Accrued external general and administrative expenses 2,865 2,053 Other accrued expenses and current liabilities 176 229 $ 49,082 $ 24,846

Selected Quarterly Financial _2

Selected Quarterly Financial Information (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2020
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Information	Quarters Ended March 31, 2020 June 30, 2020 September 30, 2020 December 31, 2020 Total revenues $ 17,644 $ 16,566 $ 98,912 $ 21,445 Cost of revenues 3,409 4,684 17,364 10,257 Research and development expense 37,035 38,111 43,968 47,180 General and administrative expense 14,833 15,554 15,859 17,571 Total operating expenses 55,344 58,399 84,961 75,096 Net income (loss) (40,038 ) (33,762 ) 8,791 (46,241 ) Net income (loss) per share: Basic $ (1.08 ) $ (0.91 ) $ 0.24 $ (1.24 ) Diluted (1.08 ) (0.91 ) 0.23 (1.24 ) Quarters Ended March 31, 2019 June 30, 2019 September 30, 2019 December 31, 2019 Total revenues $ 884 $ 7,881 $ 14,700 $ 11,768 Cost of revenues 29 1,927 2,494 3,791 Research and development expense 25,203 29,483 35,692 33,807 General and administrative expense 11,558 13,405 12,402 14,450 Total operating expenses 36,790 44,753 50,596 52,092 Net income (loss) (32,228 ) (1,457 ) (34,584 ) (26,464 ) Net income (loss) per share: Basic $ (0.89 ) $ (0.04 ) $ (0.94 ) $ (0.72 ) Diluted (0.89 ) (0.04 ) (0.94 ) (0.72 )

Nature of Business - Additional

Nature of Business - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Accumulated deficit	$ (289,072)	$ (177,822)
Cash, cash equivalents and marketable securities	$ 522,500

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Detail)	Jan. 01, 2019USD ($)	Dec. 31, 2020USD ($)Segment	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Significant Accounting Policies [Line Items]
Number of operating segments | Segment		1
Impairment losses		$ 0	$ 0	$ 0
Percentage of ownership interest		20.00%
Accrued interest or penalties related to uncertain tax positions		$ 0	0
Unrecognized tax benefits		0	0
Accumulated deficit		$ (289,072,000)	$ (177,822,000)
Maximum [Member] | ASU 2016-02 [Member]
Significant Accounting Policies [Line Items]
Accumulated deficit	$ 100,000
Maximum [Member] | ASU 2018-02 [Member]
Significant Accounting Policies [Line Items]
Accumulated deficit	100,000
Reclassification of stranded tax effects of unrealized gains and losses on available-for-sale securities from accumulated other comprehensive income (loss) to accumulated deficit	$ 100,000
Customer Concentration Risk [Member] | Revenue [Member] | UNITED STATES
Significant Accounting Policies [Line Items]
Concentration risk percentage		80.00%	90.00%	99.00%
Customer Concentration Risk [Member] | Revenue [Member] | Other Countries | Maximum [Member]
Significant Accounting Policies [Line Items]
Concentration risk percentage		10.00%	10.00%

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Summary of Reconciliation of Cash and Cash Equivalents and Restricted Cash (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Cash and cash equivalents	$ 338,426	$ 69,514	$ 75,561
Restricted cash	1,330	1,330	1,053
Total cash and cash equivalents and restricted cash	$ 339,756	$ 70,844	$ 76,614

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Schedule of Revenue or Accounts Receivable by Major Customers (Detail) - Customer Concentration Risk [Member]	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue [Member] | Customer A [Member]
Entity Wide Revenue Major Customer [Line Items]
Concentration risk percentage	94.00%	69.00%	81.00%
Revenue [Member] | Customer B [Member]
Entity Wide Revenue Major Customer [Line Items]
Concentration risk percentage			16.00%
Revenue [Member] | Customer C [Member]
Entity Wide Revenue Major Customer [Line Items]
Concentration risk percentage		13.00%
Revenue [Member] | Customer D [Member]
Entity Wide Revenue Major Customer [Line Items]
Concentration risk percentage		10.00%
Accounts Receivable | Customer A [Member]
Entity Wide Revenue Major Customer [Line Items]
Concentration risk percentage	44.00%	28.00%
Accounts Receivable | Customer B [Member]
Entity Wide Revenue Major Customer [Line Items]
Concentration risk percentage	48.00%	62.00%

Summary of Significant Accoun_7

Summary of Significant Accounting Policies - Summary of Estimated Useful Lives of Assets (Detail)	12 Months Ended
	Dec. 31, 2020
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Property plant and equipment estimated useful lives	3 years
Furniture and Fixtures [Member]
Property Plant And Equipment [Line Items]
Property plant and equipment estimated useful lives	5 years
Laboratory and Manufacturing Equipment [Member] | Minimum [Member]
Property Plant And Equipment [Line Items]
Property plant and equipment estimated useful lives	5 years
Laboratory and Manufacturing Equipment [Member] | Maximum [Member]
Property Plant And Equipment [Line Items]
Property plant and equipment estimated useful lives	15 years
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Property plant and equipment estimated useful lives	Shorter of lease term or estimated useful life

Marketable Securities - Summary

Marketable Securities - Summary of Company Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost / Cost	$ 183,623	$ 297,631
Unrealized Gains	555	32,884
Unrealized Losses	(55)	(34)
Fair Value	184,123	330,481
U.S. Government and Federal Agency Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost / Cost	12,782	62,637
Unrealized Gains	22	215
Unrealized Losses		(5)
Fair Value	12,804	62,847
Certificates of Deposit [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost / Cost	1,956	8,506
Unrealized Gains	34	77
Fair Value	1,990	8,583
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost / Cost	165,850	226,137
Unrealized Gains	497	808
Unrealized Losses	(55)	(29)
Fair Value	166,292	226,916
Municipal Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost / Cost	3,035
Unrealized Gains	2
Fair Value	$ 3,037
Equity Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost / Cost		351
Unrealized Gains		31,784
Fair Value		$ 32,135

Marketable Securities - Additio

Marketable Securities - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2020USD ($)Security	Dec. 31, 2019USD ($)
Schedule Of Available For Sale Securities [Line Items]
Available for sale debt securities remaining maturities greater than three years	$ 0	$ 0
Number of investment grade fixed income debt security | Security	17
Non-marketable equity securities, carrying value	$ 400,000
Marketable securities, realized gain	$ 4,800,000
Equity Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Marketable securities, unrealized gain		$ 37,800,000

Marketable Securities - Summa_2

Marketable Securities - Summary of unrealized gain (loss) on available-for-sale securities, net, presented in the statements of operations and comprehensive income (loss) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Unrealized gain (loss) before reclassifications	$ (426)	$ 1,392	$ (44)
Realized losses (gains) reclassified to investment income	(139)	(40)	39
Income tax expense		(467)
Unrealized gain (loss) on available-for-sale securities, net	$ (565)	$ 885	$ (5)

Marketable Securities - Summa_3

Marketable Securities - Summary of Fair Values and Unrealized Losses of Marketable Securities Held by the Company in an Unrealized Loss Position for Less Than 12 months and 12 Months or Greater (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 55,507
Less than 12 Months, Unrealized Losses	(55)	$ (34)
Total, Fair Value	55,507
Total, Unrealized Losses	(55)	(34)
Less than 12 Months, Fair Value		61,118
Total, Fair Value		61,118
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Less than 12 Months, Fair Value	55,507
Less than 12 Months, Unrealized Losses	(55)	(29)
Total, Fair Value	55,507
Total, Unrealized Losses	$ (55)	(29)
Less than 12 Months, Fair Value		48,556
Total, Fair Value		48,556
U.S. Government and Federal Agency Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Less than 12 Months, Unrealized Losses		(5)
Total, Unrealized Losses		(5)
Less than 12 Months, Fair Value		12,562
Total, Fair Value		$ 12,562

Fair Value of Financial Instr_3

Fair Value of Financial Instruments - Schedule of Fair Value of Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Marketable securities:
Fair Value	$ 184,123	$ 330,481
Cash Equivalents and Marketable Securities [Member]
Marketable securities:
Total cash equivalents and marketable securities	280,430	386,539
Corporate Bonds [Member]
Marketable securities:
Fair Value	166,292	226,916
Municipal Securities [Member]
Marketable securities:
Fair Value	3,037
Cash Equivalents [Member]
Cash equivalents:
Total cash equivalents	96,307	56,058
U.S. Government and Federal Agency Securities [Member]
Marketable securities:
Fair Value	12,804	62,847
Certificates of Deposit [Member]
Marketable securities:
Fair Value	1,990	8,583
Equity Securities [Member]
Marketable securities:
Fair Value		32,135
Money Market Mutual Funds [Member] | Cash Equivalents [Member]
Cash equivalents:
Total cash equivalents	96,307	56,058
Quoted Prices in Active Markets (Level 1) [Member]
Marketable securities:
Fair Value		32,135
Quoted Prices in Active Markets (Level 1) [Member] | Cash Equivalents and Marketable Securities [Member]
Marketable securities:
Total cash equivalents and marketable securities		32,135
Quoted Prices in Active Markets (Level 1) [Member] | Equity Securities [Member]
Marketable securities:
Fair Value		32,135
Significant Other Observable Inputs (Level 2) [Member]
Marketable securities:
Fair Value	184,123	298,346
Significant Other Observable Inputs (Level 2) [Member] | Cash Equivalents and Marketable Securities [Member]
Marketable securities:
Total cash equivalents and marketable securities	280,430	354,404
Significant Other Observable Inputs (Level 2) [Member] | Corporate Bonds [Member]
Marketable securities:
Fair Value	166,292	226,916
Significant Other Observable Inputs (Level 2) [Member] | Municipal Securities [Member]
Marketable securities:
Fair Value	3,037
Significant Other Observable Inputs (Level 2) [Member] | Cash Equivalents [Member]
Cash equivalents:
Total cash equivalents	96,307	56,058
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government and Federal Agency Securities [Member]
Marketable securities:
Fair Value	12,804	62,847
Significant Other Observable Inputs (Level 2) [Member] | Certificates of Deposit [Member]
Marketable securities:
Fair Value	1,990	8,583
Significant Other Observable Inputs (Level 2) [Member] | Money Market Mutual Funds [Member] | Cash Equivalents [Member]
Cash equivalents:
Total cash equivalents	$ 96,307	$ 56,058

Fair Value of Financial Instr_4

Fair Value of Financial Instruments - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Fair value hierarchy level 1 to level 2 transfers amount	$ 0	$ 0
Fair value hierarchy level 2 to level 1 transfers amount	0	0
Non-marketable equity securities	1,100,000
Remeasurements or impairment losses on non-marketable equity securities	$ 0	$ 0	$ 0

Property and Equipment Net - Sc

Property and Equipment Net - Schedule of Property and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property Plant And Equipment [Line Items]
Total property and equipment	$ 79,141	$ 43,311
Accumulated depreciation and amortization	(22,674)	(14,338)
Property and equipment, net	56,467	28,973
Laboratory and Manufacturing Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	26,306	19,663
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	3,764	2,545
Furniture and Fixtures [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	4,114	2,188
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	$ 44,957	$ 18,915

Property and Equipment Net - Ad

Property and Equipment Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Abstract]
Depreciation and amortization expense	$ 8,407	$ 7,152	$ 3,982

Leases - Additional Information

Leases - Additional Information (Detail) $ in Thousands	1 Months Ended						12 Months Ended
	Nov. 30, 2020USD ($)	May 31, 2019USD ($)	Aug. 31, 2018USD ($)ft²	May 31, 2016USD ($)	Jan. 31, 2016USD ($)	Mar. 31, 2015	Dec. 31, 2020USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2019USD ($)
Other Commitments [Line Items]
Tenant improvement allowance received							$ 5,000
Property and equipment, net							56,467		$ 28,973
Operating lease right-of-use assets							63,815		$ 10,078
Lease liabilities							$ 72,653
Rent expense								$ 2,800
Rockville, Maryland [Member]
Other Commitments [Line Items]
Operating lease expiration year							2028
9804 Medical Center Drive [Member]
Other Commitments [Line Items]
Tenant improvement allowance received							$ 19,500
Tenant improvement allowance receivable							12,900
Property and equipment, net							36,300
Operating lease right-of-use assets							50,100
Lease liabilities							$ 57,800
9804 Medical Center Drive [Member] | Rockville, Maryland [Member]
Other Commitments [Line Items]
Number of square feet to be constructed | ft²			177,000
Construction completion, description							The initial construction of the building was performed by the landlord, and the lease commenced in September 2020 upon the delivery of leased premises to the Company to make additional improvements to the building.
Lease commenced date			2020-09
Lease expiration date			Sep. 30, 2036
Lease not yet commenced, option to terminate the lease							true
Lease not yet commenced, option to terminate the lease, description							The Company has the option to extend the term of the lease for up to 10 additional years and the option to terminate the lease, with payment of an early termination fee, after 12 years from the delivery of the leased premises to the Company.
Lease not yet commenced, additional lease term			10 years
Minimum release term for option to terminate			12 years
Letters of credit			$ 1,100
9712 Medical Center Drive [Member] | Rockville, Maryland [Member]
Other Commitments [Line Items]
Lease commenced date						2015-04
Lease expiration date							Sep. 30, 2036
Tenant improvement allowance received							$ 400
Lease term							In October 2020, the 9712 Medical Center Drive Lease was amended to extend the lease term from September 2021 to February 2027, subject to extension options held by the Company.
Operating lease, existence of option to extend the lease							true
Increase decrease in right-of-use assets and lease liabilities							$ 7,200
Operating lease, option to extend description							The Company has an option to extend the term of the lease for three additional years, as well as an option to extend the lease term to be coterminous with the 9804 Medical Center Drive Lease, which expires in September 2036.
Additional lease term under option to extend							3 years
9600 Blackwell Road [Member] | Rockville, Maryland [Member]
Other Commitments [Line Items]
Lease commenced date					2016-02
Lease expiration date	Sep. 30, 2021
Tenant improvement allowance received					$ 800
Increase decrease in right-of-use assets and lease liabilities	$ (700)
Operating lease, existence of option to terminate	true
Early termination fee	$ 400
400 Madison Avenue [Member] | 400 Madison Lease [Member]
Other Commitments [Line Items]
Letters of credit				$ 200
Increase decrease in right-of-use assets and lease liabilities		$ 5,200
Operating Lease Commencement Year				2016-07
Tenant improvement allowance entitled to received				$ 700

Leases - Summary of Lease Costs

Leases - Summary of Lease Costs and Supplemental Cash Flow Information of Operating Leases (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating lease cost	$ 5,246	$ 3,040
Variable lease cost	1,104	666
Total lease cost	6,350	3,706
Cash paid (received) for amounts included in operating lease liabilities	(678)	2,724
Right-of-use assets acquired through operating lease liabilities	$ 56,956	$ 5,114

Leases - Weighted-Average Remai

Leases - Weighted-Average Remaining Lease Term and Weighted-Average Discount Rate of Operating Leases (Detail)	Dec. 31, 2020
Leases [Abstract]
Weighted-average remaining lease term (years)	13 years 8 months 12 days
Weighted-average discount rate	5.60%

Leases - Reconciliation of Undi

Leases - Reconciliation of Undiscounted Future Minimum Lease Payments Remaining Operating Leases (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Undiscounted future minimum lease payments:
2021	$ 4,155
2022	6,021
2023	8,671
2024	9,745
2025	9,999
Thereafter	93,148
Total undiscounted future minimum lease payments	131,739
Amount representing imputed interest	(46,212)
Tenant improvement allowance not yet received	(12,874)
Total operating lease liabilities	72,653
Current portion of operating lease liabilities	(2,500)	$ (2,421)
Operating lease liabilities, non-current	$ 70,153	$ 8,874

Liability Related to Sale of _3

Liability Related to Sale of Future Royalties - Additional Information (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)
Liability Related To Sale Of Future Royalties [Line Items]
Repurchase of call option amount	$ 300,000
Proceeds from sale of future royalties	196,000
November 7, 2024
Liability Related To Sale Of Future Royalties [Line Items]
Repurchase of call option amount, befor exercised	1,000
HCR [Member]
Liability Related To Sale Of Future Royalties [Line Items]
Proceeds from Royalties Received	4,000
Deferred Transaction Cost	3,500
Non-cash interest expense	$ 192,500
Interest Rate, Effective Percentage	13.60%
HCR [Member] | Other Noncurrent Liabilities
Liability Related To Sale Of Future Royalties [Line Items]
Proceeds from sale of future royalties	$ 196,000
HCR [Member] | November 7, 2024
Liability Related To Sale Of Future Royalties [Line Items]
Royalty Payment Cap Amount	260,000
HCR [Member] | November 8, 2024
Liability Related To Sale Of Future Royalties [Line Items]
Royalty Payment Cap Amount	300,000
Royalty Purchase Agreement [Member]
Liability Related To Sale Of Future Royalties [Line Items]
Proceeds from sale of future royalties	196,000
Deferred Transaction Cost	(3,473)
Non-cash interest expense	771
Royalty Purchase Agreement [Member] | HCR [Member]
Liability Related To Sale Of Future Royalties [Line Items]
Purchase Agreement	$ 200,000

Liability Related to Sale of _4

Liability Related to Sale of Future Royalties -Schedule of Activity Within Liability Related to Sale of Future Royalties (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)
Liability Related To Sale Of Future Royalties [Line Items]
Proceeds from sale of future royalties	$ 196,000
Current portion of liability related to sale of future royalties	(18,794)
Liability related to sale of future royalties	174,504
Royalty Purchase Agreement [Member]
Liability Related To Sale Of Future Royalties [Line Items]
Proceeds from sale of future royalties	196,000
Deferred Transaction Cost	(3,473)
Non-cash interest expense	771
Liability related to sale of future royalties, ending balance	193,298
Current portion of liability related to sale of future royalties	(18,794)
Liability related to sale of future royalties	$ 174,504

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($)

1 Months Ended

3 Months Ended

12 Months Ended

Aug. 31, 2019

Jul. 31, 2019

Dec. 31, 2020

Sep. 30, 2020

Jun. 30, 2020

Mar. 31, 2020

Dec. 31, 2019

Sep. 30, 2019

Jun. 30, 2019

Mar. 31, 2019

Dec. 31, 2020

Dec. 31, 2019

Dec. 31, 2018

Apr. 30, 2019

Mar. 31, 2009

Other Commitments [Line Items]

Research and development

$ 47,180,000

$ 43,968,000

$ 38,111,000

$ 37,035,000

$ 33,807,000

$ 35,692,000

$ 29,483,000

$ 25,203,000

$ 166,294,000

$ 124,185,000

$ 83,873,000

Claims paid to date related to indemnification issues

0

Accruals or expenses related to indemnification issues

0

0

University of Pennsylvania [Member]

Other Commitments [Line Items]

Accrued expenses

100,000

100,000

100,000

100,000

University of Pennsylvania [Member] | Maximum [Member]

Other Commitments [Line Items]

Upfront fee, milestone fees

$ 20,500,000

GlaxoSmithKline LLC [Member]

Other Commitments [Line Items]

Accrued expenses

13,100,000

$ 6,700,000

13,100,000

6,700,000

Milestone payment obligation

$ 1,500,000

Neurimmune AG [Member]

Other Commitments [Line Items]

Period for sharing of research and development cost

2 years

Percentage of partnership in collaboration

50.00%

Research and development

500,000

Neurimmune AG [Member] | Maximum [Member]

Other Commitments [Line Items]

Research and development

$ 100,000

Clearside Biomedical Inc. [Member]

Other Commitments [Line Items]

Milestone payment obligation

$ 3,000,000

$ 3,000,000

Payments against exercise of license option

$ 1,600,000

Clearside Biomedical Inc. [Member] | Maximum [Member]

Other Commitments [Line Items]

Milestone payment obligation

$ 136,000,000

Commitments and Contingencies_2

Commitments and Contingencies - Summary of Expenses Incurred by Company (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cost Of Revenue [Abstract]
Royalties on net sales of Zolgensma	$ 145,131	$ 24,329	$ 176,066
Zolgensma Royalties [Member]
Cost Of Revenue [Abstract]
Royalties on net sales of Zolgensma	61,631	20,829
GlaxoSmithKline LLC [Member]
Other Commitments [Line Items]
Total related party transaction expense	36,722	9,169	9,955
GlaxoSmithKline LLC [Member] | Zolgensma Royalties [Member]
Cost Of Revenue [Abstract]
Royalties on net sales of Zolgensma	26,278	5,822
GlaxoSmithKline LLC [Member] | Other Costs Of Revenue [Member]
Other Commitments [Line Items]
Total related party transaction expense	9,398	2,419	9,407
GlaxoSmithKline LLC [Member] | Cost of Revenues [Member]
Other Commitments [Line Items]
Total related party transaction expense	35,676	8,241	9,407
GlaxoSmithKline LLC [Member] | General and Administrative [Member]
Other Commitments [Line Items]
Total related party transaction expense	1,046	928	548
License Fees [Member] | The Trustees of the University of Pennsylvania [Member]
Other Commitments [Line Items]
Total related party transaction expense	860	1,105	112
License Fees [Member] | The Trustees of the University of Pennsylvania [Member] | Cost of Revenues [Member]
Other Commitments [Line Items]
Total related party transaction expense	39		(18)
License Fees [Member] | The Trustees of the University of Pennsylvania [Member] | Research and Development [Member]
Other Commitments [Line Items]
Total related party transaction expense		200
License Fees [Member] | The Trustees of the University of Pennsylvania [Member] | General and Administrative [Member]
Other Commitments [Line Items]
Total related party transaction expense	$ 821	$ 905	$ 130

Capitalization - Additional Inf

Capitalization - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended		12 Months Ended
	Jan. 31, 2021	Aug. 31, 2018	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2019
Schedule of Capitalization [Line Items]
Common stock, shares authorized				100,000,000	100,000,000
Common stock, par value				$ 0.0001	$ 0.0001
Preferred stock, shares authorized				10,000,000	10,000,000
Preferred stock, par value				$ 0.0001	$ 0.0001
Common Stock [Member]
Schedule of Capitalization [Line Items]
Issuance of stock, net of transaction costs			3,105,000
Public Offering [Member]
Schedule of Capitalization [Line Items]
Issuance of stock, net of transaction costs	4,899,000	3,105,000
Stock issuance price per share	$ 47	$ 65
Net proceeds from issuance of common stock	$ 216.1	$ 189.1
Overallotment Option [Member] | Common Stock [Member]
Schedule of Capitalization [Line Items]
Issuance of stock, net of transaction costs	639,000	405,000

Capitalization - Schedule of Re

Capitalization - Schedule of Reserved Shares of Common Stock for Future Issuance (Detail) - shares shares in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule of Capitalization [Line Items]
Reserved common shares	9,107	7,741
Equity Incentive Plans [Member]
Schedule of Capitalization [Line Items]
Reserved common shares	8,659	7,607
Employee Stock Purchase Plan [Member]
Schedule of Capitalization [Line Items]
Reserved common shares	448	134

License and Royalty Revenue - A

License and Royalty Revenue - Additional Information (Detail)	Apr. 01, 2021USD ($)	Feb. 25, 2021USD ($)	May 31, 2020USD ($)	May 30, 2020USD ($)	Oct. 30, 2020USD ($)	Nov. 30, 2018USD ($)	May 31, 2018USD ($)	Jan. 31, 2018USD ($)	Mar. 31, 2014USD ($)	Dec. 31, 2020USD ($)ProductCandidate	Sep. 30, 2020USD ($)	Jun. 30, 2020USD ($)	Mar. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Sep. 30, 2019USD ($)	Jun. 30, 2019USD ($)	Mar. 31, 2019USD ($)	Dec. 31, 2020USD ($)ProductCandidate	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
License Revenue [Line Items]
Milestone payment upon commencement of clinical trials in humans																		$ 26,600,000
Milestone payment upon submission of regulatory approval filings																		26,000,000
Milestone payment upon approval of commercial products by regulatory agencies																		103,500,000
Milestone payment upon achievement of specified sales targets for licensed products																		57,000,000
Deferred revenue, current and non-current										$ 4,232,000				$ 3,333,000				4,232,000	$ 3,333,000	$ 3,933,000
Provision for credit losses																		7,678,000	0	0
Allowance for credit losses														0					0
Accounts receivable										46,266,000				42,303,000				$ 46,266,000	42,303,000	31,599,000	$ 5,850,000
Transaction price description																		Upon its execution, the transaction price of the January 2018 Amendment was $132.1 million, which was fully recognized as license revenue upon the delivery of the modified license in January 2018. In May 2018, as a result of the acquisition of AveXis, Inc. (now Novartis Gene Therapies) by Novartis, the transaction price was increased by $40.0 million to account for the acceleration of the sale-based milestone which was previously excluded from the transaction price. The $40.0 million increase in the transaction price was recognized as license revenue upon the completion of the change of control in May 2018 since the amended license had been fully delivered by the Company. Additionally, due to the acceleration of the two $30.0 million payments originally due in January 2019 and January 2020, the Company recognized $6.1 million of interest income from licensing upon the completion of the change of control of AveXis, Inc., which represented the remaining present value discount on such payments as of the date of the change of control. As of December 31, 2020, the transaction price of the January 2018 Amendment was $172.1 million, which included: (i) the $80.0 million payment in January 2018, (ii) the present value, as of the date of the January 2018 Amendment, of the two $30.0 million payments originally due in January 2019 and January 2020 and (iii) the $40.0 million sales-based milestone which was accelerated upon the change of control in May 2018. Variable consideration under the January 2018 Amendment, which has been excluded from the transaction price, includes the $80.0 million sales-based milestone received in 2020 upon Novartis Gene Therapies’ achievement of $1.0 billion in cumulative net sales of Zolgensma, royalties on net sales of licensed products and any potential sublicense fees, if any, which will be recognized in the period of the underlying sales or sublicenses.
Interest income from licensing																		$ 4,271,000	2,951,000	8,946,000
Revenues										21,445,000	$ 98,912,000	$ 16,566,000	$ 17,644,000	11,768,000	$ 14,700,000	$ 7,881,000	$ 884,000	154,567,000	35,233,000	218,505,000
Novartis Gene Therapies [Member]
License Revenue [Line Items]
Interest income from licensing																		26,000	29,000	7,966,000
Abeona Therapeutics Inc. [Member]
License Revenue [Line Items]
Interest income from licensing																		3,800,000	2,600,000	400,000
Increase in upfront and annual fees						$ 1,000,000
Upfront and annual fees upon termination of license agreement						41,000,000
Remaining unpaid portion of upfront and annual fees						121,000,000
Sales-based milestone payments						60,000,000
March 2014 License Agreement [Member] | Novartis Gene Therapies [Member]
License Revenue [Line Items]
Aggregate milestone payment for all the targets																		80,000,000
Milestone payment upon achievement of specified sales targets for licensed products																		1,000,000,000
Up-front fee paid									$ 2,000,000									2,000,000
Milestone fee payments upon achievement of various development and commercialization									$ 12,300,000
License agreement amendment date									2018-01
Milestone payment					$ 80,000,000
Transaction price of license										14,500,000								14,500,000
Milestone payments for development																		12,300,000
Increase in transaction price of license																		3,500,000
Accounts receivable										19,600,000				11,000,000				19,600,000	11,000,000
Accounts receivable, current										19,400,000				10,800,000				19,400,000	10,800,000
Accounts receivable, non -current										200,000				$ 200,000				200,000	$ 200,000
January 2018 Amendment Agreement [Member] | Novartis Gene Therapies [Member]
License Revenue [Line Items]
License agreement period								14 years
License fee								$ 80,000,000
Sales-based milestone payment unpaid in the event of change of control								40,000,000
License fees received							$ 100,000,000
Transaction price of license								132,100,000		172,100,000								172,100,000
Increase in transaction price of license																		0
Interest income from licensing							6,100,000
Sales-based milestone payment exclude transaction price																		$ 80,000,000
Payments due from related party																	$ 30,000,000
Consideration payment due period one																		2019-01
Consideration payment due period two																		2020-01
January 2018 Amendment Agreement [Member] | Novartis Gene Therapies [Member] | Share-based Milestone [Member]
License Revenue [Line Items]
Increase in transaction price of license							40,000,000
January 2018 Amendment Agreement [Member] | Novartis Gene Therapies [Member] | First Anniversary [Member]
License Revenue [Line Items]
License fee								30,000,000
January 2018 Amendment Agreement [Member] | Novartis Gene Therapies [Member] | Second Anniversary [Member]
License Revenue [Line Items]
License fee								30,000,000
November 2018 [Member] | Abeona Therapeutics Inc. [Member]
License Revenue [Line Items]
License fee	$ 8,000,000			$ 20,000,000
Up-front and anuual fees						10,000,000
November 2019 [Member] | Abeona Therapeutics Inc. [Member]
License Revenue [Line Items]
Up-front and anuual fees						3,000,000
November 2020 [Member] | Abeona Therapeutics Inc. [Member]
License Revenue [Line Items]
Up-front and anuual fees						20,000,000
November 2021 [Member] | Abeona Therapeutics Inc. [Member]
License Revenue [Line Items]
Up-front and anuual fees						20,000,000
November 2022 [Member] | Abeona Therapeutics Inc. [Member]
License Revenue [Line Items]
Up-front and anuual fees						20,000,000
November 2023 [Member] | Abeona Therapeutics Inc. [Member]
License Revenue [Line Items]
Up-front and anuual fees						20,000,000
November 2024 [Member] | Abeona Therapeutics Inc. [Member]
License Revenue [Line Items]
Up-front and anuual fees						20,000,000
April 2020 [Member] | Abeona Therapeutics Inc. [Member]
License Revenue [Line Items]
Up-front and anuual fees						$ 8,000,000
November 2018 License Agreement [Member]
License Revenue [Line Items]
Interest percentage on unpaid balances under license agreement						1.50%
November 2018 License Agreement [Member] | Abeona Therapeutics Inc. [Member]
License Revenue [Line Items]
Allowance for credit losses										7,700,000								$ 7,700,000
License fee	$ 8,000,000
Transaction price of license						$ 35,600,000				36,300,000								36,300,000
Increase in transaction price of license																		$ 600,000
Transaction price description																		Upon its execution, the transaction price of the November 2018 License was $35.6 million, which was fully recognized as license revenue upon the delivery of the licenses in November 2018. As a result of the November 2019 amendment to the license agreement, the transaction price was increased by $0.6 million to account for the modifications to the amount and timing of annual fees under the license. Upon its termination in May 2020, the transaction price of the November 2018 License, as amended, was $36.3 million, which included the following fixed consideration payable to the Company over contract term: (i) the $10.0 million payment in November 2018 and (ii) the present values, as of the date of the license agreement, of the $3.0 million payment due in November 2019, the $8.0 million payment due in April 2020 and the $20.0 million payment due in November 2020. The discounted portion of the annual payments represents the financing benefit provided to Abeona and was recognized as interest income from licensing over the financing term of the license, which ended upon the termination of the agreement in May 2020. Variable consideration under the license agreement, which was excluded from the transaction price, included the sales-based milestone payments of $60.0 million, as well as any potential royalties on sales of licensed products or sublicense fees, which would be recognized in the period of the underlying sales or sublicenses. The annual payments due under the agreement beginning in November 2021 represented contract renewal options granted to Abeona for revenue recognition purposes, and therefore were excluded from the transaction price.
Sales-based milestone payment exclude transaction price																		$ 60,000,000
Consideration payment due period one																			2019-11
Consideration payment due period two																			2020-11
Accounts receivable										30,100,000								$ 30,100,000
License agreement initial contract term																		3 years
Upfront payment received						$ 10,000,000
November 2018 License Agreement [Member] | Abeona Therapeutics Inc. [Member] | November 2018 License Agreement Termination [Member]
License Revenue [Line Items]
Accounts receivable			$ 28,000,000							30,100,000								$ 30,100,000
License agreement termination claims, description			subsequent to the termination of the November 2018 License, Abeona filed a claim in arbitration alleging that the Company had breached certain responsibilities to communicate with Abeona regarding the Company’s prosecution of licensed patents under the November 2018 License. The Company disputes Abeona’s claim and filed a counterclaim in arbitration demanding payment of the $28.0 million of unpaid fees from Abeona, plus accrued interest			As a result of the termination, Abeona was required to pay a $20.0 million license fee to the Company within 15 days of the termination date, which otherwise would have been due to the Company in November 2020.
Interest income from license on unpaid license fees																		2,100,000
November 2018 License Agreement [Member] | Abeona Therapeutics Inc. [Member] | Subsequent Event [Member] | November 2018 License Agreement Termination [Member]
License Revenue [Line Items]
License fee		$ 28,000,000
November 2018 License Agreement [Member] | Abeona Therapeutics Inc. [Member] | First Anniversary [Member]
License Revenue [Line Items]
Payments due from related party										3,000,000								3,000,000
November 2018 License Agreement [Member] | Abeona Therapeutics Inc. [Member] | Second Anniversary [Member]
License Revenue [Line Items]
Payments due from related party										8,000,000								8,000,000
November 2018 License Agreement [Member] | Abeona Therapeutics Inc. [Member] | Third Anniversary
License Revenue [Line Items]
Payments due from related party										$ 20,000,000								20,000,000
Minimum [Member] | January 2018 Amendment Agreement [Member] | Novartis Gene Therapies [Member]
License Revenue [Line Items]
Milestone payment								$ 120,000,000
Maximum [Member]
License Revenue [Line Items]
Aggregate milestone payment for all the targets																		$ 213,100,000
Maximum [Member] | Abeona Therapeutics Inc. [Member]
License Revenue [Line Items]
Up-front and anuual fees						$ 121,000,000
N A V Technology Platform [Member]
License Revenue [Line Items]
Number of commercial product candidates | ProductCandidate										1								1
N A V Technology Platform [Member] | Minimum [Member]
License Revenue [Line Items]
Number of development partnered product candidates | ProductCandidate										20								20
Zolgensma [Member] | March 2014 License Agreement [Member]
License Revenue [Line Items]
Accounts receivable											$ 1,000,000,000
Zolgensma [Member] | January 2018 Amendment Agreement [Member] | Novartis Gene Therapies [Member]
License Revenue [Line Items]
Accounts receivable										$ 1,000,000,000								$ 1,000,000,000
License [Member] | Abeona Therapeutics Inc. [Member]
License Revenue [Line Items]
Revenues																		0	$ 600,000	$ 35,600,000
Interest accrued on unpaid license fees										2,100,000								2,100,000
License [Member] | January 2018 Amendment Agreement [Member] | Novartis Gene Therapies [Member]
License Revenue [Line Items]
Increase in transaction price of license							$ 40,000,000
Zolgensma Royalties [Member] | March 2014 License Agreement [Member] | Novartis Gene Therapies [Member]
License Revenue [Line Items]
Accounts receivable, current										19,400,000								19,400,000
HCR [Member] | March 2014 License Agreement [Member] | Novartis Gene Therapies [Member]
License Revenue [Line Items]
Accounts receivable, current										$ 9,200,000								$ 9,200,000

License and Royalty Revenue - S

License and Royalty Revenue - Summary of Changes in Balances of Receivables, Contract Assets and Deferred Revenue (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounts receivable, current and non-current:
Balance, beginning of period	$ 42,303	$ 31,599	$ 5,850
Additions	158,682	39,203	231,154
Deductions	(154,719)	(28,499)	(205,405)
Balance, end of period	46,266	42,303	31,599
Contract assets:
Balance, beginning of period		750	350
Additions	350	1,000	3,000
Deductions		(1,750)	(2,600)
Balance, end of period	350		750
Deferred revenue, current and non-current:
Balance, beginning of period	3,333	3,933
Additions	1,124		3,933
Deductions	(225)	(600)
Balance, end of period	4,232	3,333	3,933
Revenue recognized during the period from:
Amounts included in deferred revenue at beginning of period		600
Performance obligations satisfied in previous periods	$ 146,772	$ 26,689	$ 5,348

License and Royalty Revenue -_2

License and Royalty Revenue - Summary of Accounts Recerivable, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current accounts receivable:
Billed to customers	$ 30,573	$ 376
Unbilled	20,104	37,772
Allowance for credit losses	(7,678)
Current accounts receivable, net	42,999	38,148
Non-current accounts receivable:
Unbilled	3,267	4,155
Non-current accounts receivable, net	3,267	4,155
Total accounts receivable, net	$ 46,266	$ 42,303

License and Royalty Revenue -_3

License and Royalty Revenue - Summary of Changes in Allowance For Credit Losses (Detail) - Accounts Receivable $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)
Financing Receivable Allowance For Credit Losses [Line Items]
Provision for credit losses	$ 7,678
Ending Balance	$ 7,678

License and Royalty Revenue -_4

License and Royalty Revenue - Schedule Of License Revenue (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
License Revenue [Line Items]
Total license and royalty revenue	$ 145,131	$ 24,329	$ 176,066
Interest income from licensing	4,271	2,951	8,946
Novartis Gene Therapies [Member]
License Revenue [Line Items]
Interest income from licensing	26	29	7,966
License [Member]
License Revenue [Line Items]
Total license and royalty revenue	3,500	3,500	$ 176,066
Zolgensma Royalties [Member]
License Revenue [Line Items]
Total license and royalty revenue	61,631	$ 20,829
Achievement of Sales Based Milestone for Zolgensma [Member]
License Revenue [Line Items]
Total license and royalty revenue	$ 80,000

Stock-based Compensation - Addi

Stock-based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Sep. 16, 2015	Jan. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation expense			$ 62,100
Unrecognized stock-based compensation, weighted-average period			2 years 4 months 24 days
Proceeds from exercise of stock options			$ 5,623	$ 7,062	$ 14,499
Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unvested stock options outstanding			0	0
Total intrinsic values vested				$ 1,800
Restricted stock units vested			0		0
2015 Equity Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of annual increase in number of shares available for future issuance			4.00%
Common stock shares authorized for issuance			12,412,917
Shares available for future grants			2,291,626
Stock options expiration date			10 years
Stock options vesting period			4 years
Stock plan expiration date			2025-06
Weighted-average fair values of options granted			$ 23.82	$ 31.19	$ 27.49
Exercise of stock options, Shares			434,534	796,847	1,684,522
Total intrinsic value of options exercised			$ 11,500	$ 30,800	$ 68,200
2015 Equity Incentive Plan [Member] | Subsequent Event [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Additional shares to be issued		1,499,037
2015 Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock shares authorized for issuance			623,924
Shares available for future grants			448,011
Share-based compensation arrangement by share-based payment award, description			The number of authorized shares reserved for issuance under the 2015 ESPP automatically increases on the first business day of each fiscal year
Share-based compensation arrangement by share-based payment award, percentage of outstanding stock minimum	1.00%
Common stock shares issued to participants			55,499	35,994	36,700
2015 Employee Stock Purchase Plan [Member] | Subsequent Event [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Additional shares to be issued		374,759

Stock-based Compensation - Stoc

Stock-based Compensation - Stock-Based Compensation Expense by Award Type (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 31,949	$ 26,854	$ 16,641
Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	31,178	25,964	15,960
Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense		257	275
Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 771	$ 633	$ 406

Stock-based Compensation - St_2

Stock-based Compensation - Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 31,949	$ 26,854	$ 16,641
Research and Development [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	16,280	13,031	7,612
General and Administrative [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 15,669	$ 13,823	$ 9,029

Stock-based Compensation - Summ

Stock-based Compensation - Summary of Stock Option Activity (Detail) - 2014 and 2015 Equity Incentive Plan [Member] - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares [Roll Forward]
Shares Outstanding, Beginning Balance	5,544
Shares, Granted	1,633
Shares, Exercised	(435)
Shares, Cancelled or forfeited	(381)
Shares Outstanding, Ending Balance	6,361	5,544
Shares, Exercisable	3,703
Shares, Vested and expected to vest	6,361
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted-average Exercise Price Outstanding, Beginning Balance	$ 28.79
Weighted-average Exercise Price, Granted	37.84
Weighted-average Exercise Price, Exercised	12.95
Weighted-average Exercise Price, Cancelled or forfeited	45.04
Weighted-average Exercise Price, Outstanding, Ending Balance	31.21	$ 28.79
Weighted-average Exercise Price, Exercisable	24.27
Weighted-average Exercise Price, Vested and expected to vest	$ 31.21
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted-average Remaining Contractual Life (Years) Outstanding	7 years 2 months 12 days	7 years 6 months
Weighted-average Remaining Contractual Life (Years), Exercisable	6 years 2 months 12 days
Weighted-average Remaining Contractual Life (Years), Vested and expected to vest	7 years 2 months 12 days
Aggregate Intrinsic Value Outstanding	$ 101,356	$ 86,509
Aggregate Intrinsic Value, Exercisable	84,115
Aggregate Intrinsic Value, Vested and expected to vest	$ 101,356

Stock-based Compensation - Sche

Stock-based Compensation - Schedule of Fair Value of Options Granted on Weighted-Average Assumptions (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions And Methodology [Abstract]
Expected volatility	71.00%	74.00%	75.00%
Expected term (in years)	6 years	6 years 1 month 6 days	6 years
Risk-free interest rate	1.40%	2.30%	2.60%
Expected dividend yield	0.00%	0.00%	0.00%

Retirement Plan - Additional In

Retirement Plan - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Compensation And Retirement Disclosure [Abstract]
Employer matching contribution expense	$ 2.2	$ 1.8	$ 1.3

Income Taxes - Components of In

Income Taxes - Components of Income (Loss) Before Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
United States	$ (105,960)	$ (97,435)	$ 104,181
Foreign	(50)	(53)	(65)
Income (loss) before income taxes	$ (106,010)	$ (97,488)	$ 104,116

Income Taxes - Components of th

Income Taxes - Components of the Provision For Income Tax Expense (Benefit) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Current:
State	$ 5,240	$ (2,288)	$ 4,179
Total current	5,240	(2,288)	4,179
Deferred:
Federal		(284)
State		(183)
Total deferred		(467)
Total income tax expense (benefit)	$ 5,240	$ (2,755)	$ 4,179

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Line Items]
Elimination of limitation in net operating losses deductions as percentage of current year taxable income, percentage	80.00%
Recovery period on qualified improvement of property	15 years
Current income tax benefit	$ 5,240,000	$ (2,288,000)	$ 4,179,000
Federal statutory income tax rate, percent	21.00%
Cumulative loss position period	3 years
Increase (decrease) in valuation allowance	$ 45,400,000	46,000,000
Research and development and orphan drug tax credit carryforwards	47,970,000	40,365,000
Net of unrecognized tax benefits	$ 100,000	$ 100,000
Tax credit carryforwards, expiration year	2035	2040
Reserve for uncertain tax positions placed against credit carryforwards	$ 100,000	$ 100,000
Reserve for uncertain tax positions placed against qualified expenses	$ 0	0
Unrecognized tax benefits period	12 months
Cumulative changes in ownership Interest percentage threshold, subject to annual limitation of NOL and tax credit carryforwards	50.00%
Ownership interest change period	3 years
Domestic Country [Member]
Income Tax Disclosure [Line Items]
Operating loss carryforwards	$ 44,700,000	170,400,000
Operating Loss Carryforwards Expiration Year	2034
Research and development and orphan drug tax credit carryforwards	$ 48,000,000	40,400,000
U. S State Tax Authority [Member]
Income Tax Disclosure [Line Items]
Operating loss carryforwards	$ 79,700,000	$ 269,300,000
Operating Loss Carryforwards Expiration Year	2040
2018 Tax Year [Member]
Income Tax Disclosure [Line Items]
Current income tax benefit	$ (500,000)

Income Taxes - Schedule of Reco

Income Taxes - Schedule of Reconciliation of Effective Income Tax Expense (Benefit) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Federal income tax expense (benefit) at statutory rate	$ (22,263)	$ (20,473)	$ 21,862
State income tax expense (benefit), net of federal tax effect	(11,495)	(15,323)	9,691
Research and development credits	(7,793)	(11,075)	(7,847)
Stock-based compensation	587	(2,134)	(6,493)
Other non-deductible expenses and reconciling items	886	144	139
Change in corporate tax rates	109	130	(729)
Change in valuation allowance	45,209	45,976	(12,444)
Total income tax expense (benefit)	$ 5,240	$ (2,755)	$ 4,179

Income Taxes - Components of Ne

Income Taxes - Components of Net Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Net operating loss carryforwards	$ 14,683	$ 53,841
Research and development tax credits	47,970	40,365
Stock-based compensation	15,644	10,224
Lease liabilities	21,961	3,916
Liability related sale of future royalties	62,089
Depreciation and amortization		26
Accruals and other	7,370	4,779
Total deferred tax assets before valuation allowance	169,717	113,151
Valuation allowance	(142,907)	(97,511)
Total deferred tax assets	26,810	15,640
Deferred tax liabilities:
Unrealized gains on marketable securities	(163)	(11,344)
Right-of-use assets	(21,094)	(3,467)
Depreciation and amortization	(5,155)
Other	(398)	(829)
Total deferred tax liabilities	$ (26,810)	$ (15,640)

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
FOXKISER LLP [Member] | Service Agreements [Member] | Research and Development [Member]
Related Party Transaction [Line Items]
Related party transaction expense	$ 4.8	$ 4.1	$ 2.1

Net Income (Loss) Per Share - C

Net Income (Loss) Per Share - Computations of Basic and Diluted Net Income (Loss) Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Basic net income (loss) per share:
Net income (loss)									$ (111,250)	$ (94,733)	$ 99,937
Shares used in computation:
Weighted-average common shares outstanding									37,281	36,690	33,427
Basic net income (loss) per share	$ (1.24)	$ 0.24	$ (0.91)	$ (1.08)	$ (0.72)	$ (0.94)	$ (0.04)	$ (0.89)	$ (2.98)	$ (2.58)	$ 2.99
Diluted net income (loss) per share:
Net income (loss)									$ (111,250)	$ (94,733)	$ 99,937
Shares used in computation:
Weighted-average common shares outstanding									37,281	36,690	33,427
Weighted-average diluted common shares									37,281	36,690	36,648
Diluted net income (loss) per share	$ (1.24)	$ 0.23	$ (0.91)	$ (1.08)	$ (0.72)	$ (0.94)	$ (0.04)	$ (0.89)	$ (2.98)	$ (2.58)	$ 2.73
Stock Option [Member]
Shares used in computation:
Weighted-average diluted common shares											3,186
Restricted Stock Units (RSUs)
Shares used in computation:
Weighted-average diluted common shares											32
Employee Stock Purchase Plan [Member]
Shares used in computation:
Weighted-average diluted common shares											3

Net Income (Loss) Per Share - S

Net Income (Loss) Per Share - Schedules for Computation of Diluted Weighted-Average Shares Outstanding (Detail) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted weighted average shares outstanding	6,380	5,561	928
Stock Options Issued and Outstanding [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted weighted average shares outstanding	6,361	5,544	928
Employee Stock Purchase Plan [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted weighted average shares outstanding	19	17

Supplemental Disclosures - Sche

Supplemental Disclosures - Schedules of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Payables And Accruals [Abstract]
Accrued personnel costs	$ 13,155	$ 10,903
Accrued sublicense fees and royalties	12,160	4,542
Accrued external research and development expenses	9,738	5,791
Accrued purchases of property and equipment	7,853	1,328
Accrued income taxes payable	3,135
Accrued external general and administrative expenses	2,865	2,053
Other accrued expenses and current liabilities	176	229
Accrued expenses and other current liabilities	$ 49,082	$ 24,846

Selected Quarterly Financial _3

Selected Quarterly Financial Information (Unaudited) - Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 21,445	$ 98,912	$ 16,566	$ 17,644	$ 11,768	$ 14,700	$ 7,881	$ 884	$ 154,567	$ 35,233	$ 218,505
Cost of revenues	10,257	17,364	4,684	3,409	3,791	2,494	1,927	29
Research and development expense	47,180	43,968	38,111	37,035	33,807	35,692	29,483	25,203	166,294	124,185	83,873
General and administrative expense	17,571	15,859	15,554	14,833	14,450	12,402	13,405	11,558	63,817	51,815	36,850
Total operating expenses	75,096	84,961	58,399	55,344	52,092	50,596	44,753	36,790	273,800	184,231	130,405
Net income (loss)	$ (46,241)	$ 8,791	$ (33,762)	$ (40,038)	$ (26,464)	$ (34,584)	$ (1,457)	$ (32,228)	$ (111,250)	$ (94,733)	$ 99,937
Net income (loss) per share:
Basic net income (loss) per share	$ (1.24)	$ 0.24	$ (0.91)	$ (1.08)	$ (0.72)	$ (0.94)	$ (0.04)	$ (0.89)	$ (2.98)	$ (2.58)	$ 2.99
Diluted net income (loss) per share	$ (1.24)	$ 0.23	$ (0.91)	$ (1.08)	$ (0.72)	$ (0.94)	$ (0.04)	$ (0.89)	$ (2.98)	$ (2.58)	$ 2.73