Assembly Biosciences (ASMB) 10-K 25 Feb 21 2020 FY Annual report Financial data

Document And Entity Information

Document And Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Feb. 22, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Transition Report	false
Document Period End Date	Dec. 31, 2020
Entity File Number	001-35005
Entity Registrant Name	ASSEMBLY BIOSCIENCES, INC.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	20-8729264
Entity Address, Address Line One	331 Oyster Point Blvd.
Entity Address, Address Line Two	Fourth Floor
Entity Address, City or Town	South San Francisco
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94080
City Area Code	833
Local Phone Number	509-4583
Amendment Flag	false
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Entity Central Index Key	0001426800
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Trading Symbol	ASMB
Security Exchange Name	NASDAQ
Title of 12(b) Security	Common Stock, $0.001 Par Value
Entity Common Stock, Shares Outstanding		38,246,092
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Public Float			$ 756.9
Documents Incorporated by Reference	Part III of this Annual Report on Form 10-K incorporates information by reference to portions of the definitive proxy statement for the Company’s Annual Meeting of Stockholders to be held in 2021, to be filed within 120 days of the registrant’s fiscal year ended December 31, 2020.

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current assets
Cash and cash equivalents	$ 59,444	$ 46,732
Marketable securities	156,969	227,311
Accounts receivable from collaborations	1,230	3,374
Prepaid expenses and other current assets	6,850	5,363
Total current assets	224,493	282,780
Property and equipment, net	1,600	1,830
Operating lease right-of-use (ROU) assets	9,131	11,975
Other assets	6,392	1,684
Indefinite-lived intangible asset	29,000	29,000
Goodwill	12,638	12,638
Total assets	283,254	339,907
Current liabilities
Accounts payable	4,598	1,731
Accrued clinical expenses	4,444	4,826
Other accrued expenses	11,987	8,286
Deferred revenue - short-term		6,411
Operating lease liabilities - short-term	3,404	3,186
Total current liabilities	24,433	24,440
Deferred tax liabilities	2,531	2,531
Deferred revenue - long-term	8,987	30,637
Operating lease liabilities - long-term	6,725	9,082
Total liabilities	42,676	66,690
Commitments and contingencies
Stockholders' equity
Preferred stock, $0.001 par value; 5,000,000 shares authorized; no shares issued or outstanding
Common stock, $0.001 par value; 100,000,000 shares authorized as of December 31, 2020 and 2019; 34,026,680 and 32,558,307 shares issued and outstanding as of December 31, 2020 and 2019, respectively	34	32
Additional paid-in capital	742,387	712,807
Accumulated other comprehensive loss	(270)	(201)
Accumulated deficit	(501,573)	(439,421)
Total stockholders' equity	240,578	273,217
Total liabilities and stockholders' equity	$ 283,254	$ 339,907

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2020	Dec. 31, 2019
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	34,026,680	32,558,307
Common stock, shares outstanding	34,026,680	32,558,307

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement [Abstract]
Collaboration revenue	$ 79,105	$ 15,963	$ 14,804
Operating expenses:
Research and development	106,823	85,757	72,741
General and administrative	37,058	32,919	34,798
Total operating expenses	143,881	118,676	107,539
Loss from operations	(64,776)	(102,713)	(92,735)
Other income
Interest and other income	2,624	4,305	3,083
Total other income	2,624	4,305	3,083
Loss before income taxes	(62,152)	(98,408)	(89,652)
Income tax benefit (expense)		774	(1,099)
Net loss	(62,152)	(97,634)	(90,751)
Other comprehensive income
Unrealized gain (loss) on marketable securities, net of tax	(69)	146	45
Comprehensive loss	$ (62,221)	$ (97,488)	$ (90,706)
Net loss per share, basic and diluted	$ (1.75)	$ (3.72)	$ (3.98)
Weighted average common shares outstanding, basic and diluted	35,427,120	26,258,790	22,801,644

CONSOLIDATED STATEMENTS OF CHAN

CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit
Balance at Dec. 31, 2017	$ 113,120	$ 20	$ 364,528	$ (392)	$ (251,036)
Balance (in shares) at Dec. 31, 2017		20,137,974
Sale of common stock, net of underwriters' discount and costs	155,425	$ 4	155,421
Sale of common stock, net of underwriters' discount and costs (in shares)		4,600,000
Issuance of common stock upon exercise of stock options	3,960	$ 1	3,959
Issuance of common stock upon exercise of stock options ( in shares )		735,030
Issuance of common stock under Employee Stock Purchase Plan (ESPP)	408		408
Issuance of common stock under Employee Stock Purchase Plan (ESPP) (in shares)		21,483
Issuance of shares of common stock for settlement of restricted stock units (RSUs)(in shares)		938
Unrealized gain on marketable securities, net of tax	45			45
Stock-based compensation	28,446		28,446
Net loss	(90,751)				(90,751)
Balance at Dec. 31, 2018	210,653	$ 25	552,762	(347)	(341,787)
Balance (in shares) at Dec. 31, 2018		25,495,425
Sale of common stock and pre-funded warrants, net of underwriters' discount and costs	134,661	$ 6	134,655
Sale of common stock and pre-funded warrants, net of underwriters' discount and costs (in shares)		6,287,878
Issuance of common stock upon exercise of stock options	4,238	$ 1	4,237
Issuance of common stock upon exercise of stock options ( in shares )		585,292
Settlement of RSUs for cash	(4)		(4)
Issuance of common stock under Employee Stock Purchase Plan (ESPP)	747		747
Issuance of common stock under Employee Stock Purchase Plan (ESPP) (in shares)		59,370
Issuance of shares of common stock for settlement of restricted stock units (RSUs)(in shares)		130,342
Unrealized gain on marketable securities, net of tax	146			146
Stock-based compensation	20,410		20,410
Net loss	(97,634)				(97,634)
Balance at Dec. 31, 2019	273,217	$ 32	712,807	(201)	(439,421)
Balance (in shares) at Dec. 31, 2019		32,558,307
Sale of Common Stock Net of Commissions and Fees	5,452	$ 1	5,451
Sale of common stock, net of commissions and fees (in shares)		892,840
Issuance of common stock upon exercise of stock options	1,467	$ 1	1,466
Issuance of common stock upon exercise of stock options ( in shares )		175,579
Issuance of common stock under Employee Stock Purchase Plan (ESPP)	680		680
Issuance of common stock under Employee Stock Purchase Plan (ESPP) (in shares)		86,812
Issuance of shares of common stock for settlement of restricted stock units (RSUs)(in shares)		313,142
Unrealized gain on marketable securities, net of tax	(69)			(69)
Stock-based compensation	21,983		21,983
Net loss	(62,152)				(62,152)
Balance at Dec. 31, 2020	$ 240,578	$ 34	$ 742,387	$ (270)	$ (501,573)
Balance (in shares) at Dec. 31, 2020		34,026,680

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities
Net loss	$ (62,152,000)	$ (97,634,000)	$ (90,751,000)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	691,000	494,000	643,000
Non-cash IPR&D expense	1,750,000	0	0
Stock-based compensation	21,853,000	20,558,000	28,485,000
Net accretion and amortization of investments in marketable securities	(13,000)	(1,735,000)	(229,000)
Non-cash rent expense	5,214,000	4,454,000	0
Deferred income tax (benefit) expense	0	(774,000)	1,099,000
Loss on disposal of fixed assets	9,000	102,000	0
Other	0	(5,000)	0
Changes in operating assets and liabilities:
Accounts receivable from collaborations	2,144,000	(944,000)	(156,000)
Prepaid expenses and other current assets	(1,487,000)	(3,685,000)	(1,094,000)
Other assets	(4,708,000)	1,664,000	(3,008,000)
Accounts payable	2,867,000	(1,962,000)	1,569,000
Accrued clinical expenses	(382,000)	1,265,000	3,119,000
Other accrued expenses	3,831,000	2,016,000	382,000
Deferred revenue	(28,061,000)	(3,612,000)	(5,125,000)
Deferred rent	0	0	108,000
Operating lease liabilities	(4,513,000)	(4,269,000)	0
Net cash used in operating activities	(62,957,000)	(84,067,000)	(64,958,000)
Cash flows from investing activities
Purchases of property and equipment	(470,000)	(1,554,000)	(340,000)
Purchase of IPR&D	(1,750,000)	0	0
Purchases of marketable securities	(193,188,000)	(281,334,000)	(183,941,000)
Proceeds from maturities of marketable securities	221,617,000	203,911,000	48,884,000
Proceeds from sale of marketable securities	41,861,000	28,659,000	0
Net cash provided by (used in) investing activities	68,070,000	(50,318,000)	(135,397,000)
Cash flows from financing activities
Proceeds from common stock and pre-funded warrants sold, net of underwriters' discount, commissions and costs	5,452,000	134,661,000	155,425,000
Proceeds from the issuance of common stock under ESPP	680,000	747,000	408,000
Proceeds from the exercise of stock options	1,467,000	4,238,000	3,960,000
Net cash provided by financing activities	7,599,000	139,646,000	159,793,000
Net increase (decrease) in cash and cash equivalents	12,712,000	5,261,000	(40,562,000)
Cash and cash equivalents at the beginning of the period	46,732,000	41,471,000	82,033,000
Cash and cash equivalents at the end of the period	59,444,000	46,732,000	41,471,000
Supplemental non-cash investing and financing activities
Operating lease liabilities arising from obtaining right-of-use assets	$ 1,302,000	$ 15,261,000	$ 0

Nature of Business

Nature of Business	12 Months Ended
	Dec. 31, 2020
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of Business	Note 1 - Nature of Business Overview Assembly Biosciences, Inc., together with its subsidiaries (Assembly or the Company), incorporated in Delaware in October 2005, is a clinical-stage biotechnology company advancing a novel class of oral therapeutic candidates for the treatment of hepatitis B virus (HBV) infection. The Company operates in one segment and is headquartered in South San Francisco, California with operations in California, Connecticut and China. The Company’s research and development programs are pursuing multiple drug candidates that inhibit the HBV replication cycle and block the generation of covalently closed circular DNA (cccDNA), with the aim of discovering and developing finite and curative therapies for patients with HBV. Assembly has discovered multiple novel core inhibitors, which are small molecules that directly target and allosterically modify the HBV core (HBc) protein. In December 2020, the Company and its Board of Directors approved a plan to wind down its Microbiome program in order to prioritize and focus its resources entirely on discovering and developing finite and curative therapies for HBV. The Microbiome program had been developing a novel class of oral live microbial biotherapeutics candidates designed to treat disorders associated with the microbiome. Liquidity The Company has not derived any revenue from product sales to date and currently has no approved products. Once a product has been developed, it will need to be approved for sale by the U.S. Food and Drug Administration (FDA) or an applicable foreign regulatory agency. Since inception, the Company’s operations have been financed primarily through the sale of equity securities, the proceeds from the exercise of warrants and stock options, the issuance of debt, and upfront payments related to collaboration agreements. The Company has incurred losses from operations since inception and expects to continue to incur substantial losses for the next several years as it continues its product development efforts. Management believes the Company currently has sufficient funds to meet its operating requirements for at least the next twelve months following the date that these consolidated financial statements are issued. If the Company cannot generate significant cash from its operations, it intends to obtain any additional funding it requires through strategic relationships, public or private equity or debt financings, grants or other arrangements (see Note 7 for recent sales of common stock). The Company cannot assure such funding will be available on reasonable terms, if at all. Market volatility resulting from the novel coronavirus disease (COVID-19) pandemic or other factors could also adversely impact the Company’s ability to access capital when and as needed. If the Company is unable to generate enough revenue from its collaborations, secure additional sources of funding or receive full and timely collections of amounts due, it may be necessary to significantly reduce the current rate of spending through reductions in staff and delaying, scaling back, or stopping certain research and development programs, including more costly clinical trials.

Summary of Significant Accounti

Summary of Significant Accounting Policies and Recent Accounting Pronouncements	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Summary of Significant Accounting Policies and Recent Accounting Pronouncements	Note 2 - Summary of Significant Accounting Policies and Recent Accounting Pronouncements Basis of Presentation These consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States (U.S. GAAP) and include the accounts of the Company and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. Use of Estimates The preparation of consolidated financial statements in conformity with the accounting principles generally accepted in the United States of America (U.S. GAAP) requires management to make estimates and assumptions that may affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and reported amounts of expenses during the reporting period. Actual results could differ from those estimates. Significant estimates inherent in the preparation of the accompanying consolidated financial statements include estimates for the cost-based input of revenue recognition and standalone selling price estimates for allocation of transaction price to performance obligations revenue recognition, estimates of costs incurred but not yet invoiced for clinical trial accruals, recoverability and useful lives of our long-lived assets, the estimated fair value of our indefinite-lived intangible assets, the estimated fair value of our reporting unit for purposes of evaluating goodwill impairment, provisions for income taxes, amounts receivable under collaboration agreements, measurement of operating lease liabilities, and the fair value of stock options, stock appreciation rights, and RSUs granted to employees, directors, and consultants. The Company’s estimates could be affected by external conditions, including those unique to the Company and general economic conditions. It is reasonably possible that these external factors could have an effect on the Company’s estimates and could cause actual results to differ from those estimates and assumptions. Other Risks and Uncertainties In March 2020, the World Health Organization declared the COVID-19 outbreak a pandemic. To date, the Company’s operations have not been significantly impacted by the COVID-19 pandemic. However, the Company cannot at this time predict the specific extent, duration, or full impact the COVID-19 pandemic will have on its business, operations, strategy, prospects and financial condition and results. The impact of the COVID-19 pandemic on the Company’s financial performance will depend on future developments, including the duration and spread of the outbreak and related governmental advisories and restrictions. These developments and the impact of the COVID-19 pandemic on the financial markets and the overall economy are highly uncertain. If the financial markets and/or the overall economy are impacted for an extended period, the Company’s results may be adversely affected. Cash and Cash Equivalents All highly liquid investments, including money market funds, with original maturities of three months or less at the time of purchase are considered to be cash equivalents. All of the Company’s cash equivalents have liquid markets and high credit ratings. The Company maintains its cash in bank deposits and other accounts, the balances of which, at times and at December 31, 2020 and 2019, exceed federally insured limits. Investments in Marketable Securities The Company invests its excess cash in debt securities with high credit ratings including but not limited to money market funds classified as cash equivalents, asset backed securities, securities issued by the U.S. government and its agencies, corporate debt securities and commercial paper. The Company has designated its investments in marketable securities as available-for-sale and measures these securities at their respective fair values. Marketable securities are classified as short-term or long-term based on the maturity date and their availability to meet current operating requirements. Marketable securities that mature in one year or less are classified as short-term available-for-sale securities and are reported as a component of current assets. Securities that are classified as available-for-sale are measured at fair value with temporary unrealized gains and losses reported in other comprehensive loss, and as a component of stockholders' equity until their disposition. The Company reviews all available-for-sale securities at each period end to determine if they remain available-for-sale based on their current intent and ability to sell the security if it is required to do so. Realized gains and losses from the sale of marketable securities, if any, are calculated using the specific-identification method. Marketable securities are subject to a periodic impairment review. The Company may recognize an impairment charge when a decline in the fair value of investments below the cost basis is determined to be other-than-temporary. In determining whether a decline in market value is other-than-temporary, various factors are considered, including the cause, duration of time and severity of the impairment, any adverse changes in the investees’ financial condition, and the Company’s intent and ability to hold the security for a period of time sufficient to allow for an anticipated recovery in market value. To date, there have been no declines in value deemed to be other than temporary for any of our investments in marketable securities. Goodwill and Indefinite-Lived Intangible Asset Goodwill and indefinite-lived intangible assets are reviewed for impairment at least annually in the fourth quarter, and more frequently if events or other changes in circumstances indicate that the carrying amount of the assets may not be recoverable. Impairment of goodwill and indefinite-lived intangibles is determined to exist when the fair value is less than the carrying value of the net assets being tested. Goodwill The difference between the purchase price and the fair value of assets acquired and liabilities assumed in a business combination is allocated to goodwill. Goodwill is evaluated for impairment on an annual basis as of October 1, and more frequently if indicators are present or changes in circumstances suggest that impairment may exist. In performing each annual impairment assessment and any interim impairment assessment, the Company determines if it should qualitatively assess whether it is more likely than not that the fair value of goodwill is less than its carrying amount (the qualitative impairment test). If the Company concludes it is more likely than not that the fair value of the reporting unit is less than its carrying amount, or elect not to use the qualitative impairment test, a quantitative impairment test is performed The Company annual or interim quantitative impairment testing is performed by comparing the estimated As of October 1, 2020 the Company has determined through its quantitative impairment test that the fair value of its goodwill significantly exceeded the carrying value of its single reporting unit and concluded that goodwill was not impaired. In November 2020, after the Company’s public announcement that it became clear that patients who stopped therapy in Study 211 had not achieved meaningful sustained virologic response rates most of the patients had relapsed, the Company’s stock price declined significantly. Due to a sustained decline in the Company’s stock price during the remainder of the fourth quarter of 2020, the Company determined these factors were an indication of a triggering event of impairment and an interim goodwill impairment test was performed as of December 31, 2020. However, the interim quantitative impairment test still determined the fair value, when considering a control premium based on a range of recent acquisitions of entities similar to the Company which were made on a non-minority basis, exceeded the carrying value of its single reporting unit, and concluded that goodwill was still not impaired. Indefinite-Lived Intangible Asset The Company’s indefinite-lived intangible asset consists of IPR&D acquired in a business combination that are used in research and development activities but have not yet reached technological feasibility, regardless of whether they have alternative future use. The primary basis for determining the technological feasibility or completion of these projects is obtaining regulatory approval to market the underlying products in an applicable geographic region. The Company classifies IPR&D acquired in a business combination as an indefinite-lived intangible asset until the completion or abandonment of the associated research and development efforts. Upon completion of the associated research and development efforts, the Company performed a final test for impairment and will determine the useful life of the technology and begin amortizing the assets to reflect their use over their remaining lives. Upon permanent abandonment, the Company would write-off the remaining carrying amount of the associated IPR&D intangible asset. Indefinite-lived intangible assets are not amortized, but instead are reviewed for impairment at least annually, or more frequently if events occur or circumstances change that would indicate the carrying amount may be impaired. In performing each annual impairment assessment and any interim impairment assessment, the Company determines if it should qualitatively assess whether it is more likely than not that the fair value of its IPR&D asset is less than its carrying amount (the qualitative impairment test). If the Company concludes that is the case, or elect not to use qualitative impairment test, the Company would proceed with quantitatively determining the fair value of the IPR&D asset and comparing its fair value to its carrying value to determine the amount of impairment, if any (the quantitative impairment test). When performing the quantitative impairment assessment, the Company uses the income approach to determine the fair value of its IPR&D asset. This approach calculates fair value by estimating the after-tax cash flows attributable to an in-process project over its useful life and then discounting these after-tax cash flows back to a present value. This estimate includes judgmental assumptions regarding the estimates that market participants would make in evaluating the IPR&D asset, including the probability of successfully completing clinical trials and obtaining regulatory approval to market the IPR&D asset, the timing of and the expected costs to complete IPR&D projects, future net cash flows from potential drug sales, which are based on estimates of the sales price of the drug, the size of the patient population and cure rate, our competitive position in the marketplace, and appropriate discount and tax rates. Any impairment to be recorded is calculated as the difference between the fair value of the IPR&D asset as of the date of the assessment with the carrying value of the IPR&D asset on its consolidated balance sheet. In performing the qualitative impairment test, the Company considers the results of the most recent quantitative impairment test and identifies the most relevant drivers of the fair value for the IPR&D asset. The most relevant drivers of fair value identified are consistent with the assumptions used in the quantitative estimate of the IPR&D asset discussed below. Using these drivers of fair value, the Company identifies events and circumstances that may have an effect on the fair value of the IPR&D asset since the last time the IPR&D’s fair value was quantitatively determined. The Company then weighs these factors to determine and conclude if it is not more likely than not that the IPR&D asset is impaired. If it is more likely than not that the IPR&D asset is impaired, the Company proceeds with quantitatively determining the fair value of the IPR&D asset. For the Company’s 2020 impairment test, the Company performed a qualitative test and concluded it was more-likely-than-not that the fair value of its IPR&D asset exceeded its carrying value and no further testing was required. This was based on a decrease in the probability of success based on the impact of the Study 211 and dual combination VBR and NrtI therapy’s ability to serve as a finite and curative therapy for chronic HBV infection offset by an increase in the probability of success of 2158 and 3733 based on their advancement into Phase 2 and Phase 1 trials during 2020, respectively and the significance of the future net cash flows from potential drug sales for a finite and curative therapy for chronic HBV infection as primarily driven by the number of patients who will be diagnosed and treated and the Company’s competitive position in the marketplace. The Company did not recognize any IPR&D impairment in any of the periods presented . Leases All of the Company’s leases are operating leases for facilities and equipment. Prior to January 1, 2019, the Company recognized related rent expense on a straight-line basis over the term of the lease. Incentives granted under the Company’s facilities lease, including allowances for leasehold improvements and rent holidays, were recognized as reductions to rental expense on a straight-line basis over the term of the lease. Deferred rent consisted of the difference between cash payments and the rent expense recognized. Subsequent to the adoption of the new leasing standard on January 1, 2019, the Company recognizes a lease asset for its right to use the underlying asset and a lease liability for the corresponding lease obligation. The Company determines whether an arrangement is or contains a lease at contract inception. Operating leases with a duration greater than one year are included in operating lease right-of-use assets, operating lease liabilities - short-term, and operating lease liabilities - long-term in the Company’s consolidated balance sheets. Operating lease right-of-use assets and liabilities are recognized at the lease commencement date based on the present value of lease payments over the lease term. In determining the net present value of lease payments, the Company uses its incremental borrowing rate based on the information available at the lease commencement date. The incremental borrowing rate represents the interest rate the Company would incur at lease commencement to borrow an amount equal to the lease payments on a collateralized basis over the term of a lease. The Company considers a lease term to be the noncancelable period that it has the right to use the underlying asset, including any periods where it is reasonably assured the Company will exercise the option to extend the contract. Periods covered by an option to extend are included in the lease term if the lessor controls the exercise of that option. The operating lease right-of-use assets also include any lease payments made and exclude lease incentives. Lease expense is recognized on a straight-line basis over the expected lease term. Variable lease expenses are recorded when incurred. The Company has elected not to separate lease and non-lease components for its leased assets and accounts for all lease and non-lease components of its agreements as a single lease component. Impairment of Long-Lived Assets The Company monitors the carrying value of long-lived assets, including ROU operating lease assets, for potential impairment and tests the recoverability of such assets whenever events or changes in circumstances indicate that the carrying amounts may not be recoverable. If a change in circumstance occurs, the Company performs a test of recoverability by comparing the carrying value of the asset or asset group to its undiscounted expected future cash flows. If cash flows cannot be separately and independently identified for a single asset, the Company will determine whether impairment has occurred for the group of assets for which the Company can identify the projected cash flows. If the carrying values are in excess of undiscounted expected future cash flows, the Company measures any impairment by comparing the fair value of the asset or asset group to its carrying value. In conjunction with the decision to wind down the Microbiome program in December 2020, the Company evaluated its ROU assets and property and equipment used in the Microbiome program for impairment. The Company determined the carrying value was no longer recoverable and, based on a fair value of the assets determined from market quotes, recorded an impairment loss of $0.7 million included in research and development expenses. The Company did not recognize any impairment on its long-lived assets in 2019 or 2018. Property and Equipment, Net Property and equipment are stated at cost and consist of lab and office equipment, leasehold improvements and computer hardware and software. The Company records depreciation under the straight-line method over the estimated useful lives of its property and equipment ranging from three to seven years. Leasehold improvements are amortized over the remaining terms of the respective leases or the estimated useful life of the leasehold improvements, whichever is less. Maintenance and repair costs are expensed as incurred. Fair Value Measurements The Company follows accounting guidance on fair value measurements for financial instruments measured on a recurring basis, as well as for certain assets and liabilities that are initially recorded at their estimated fair values. Fair value is defined as the exit price, or the amount that would be received from selling an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. The Company uses the following three-level hierarchy that maximizes the use of observable inputs and minimizes the use of unobservable inputs to value its financial instruments: Level 1: Observable inputs such as unadjusted quoted prices in active markets for identical instruments. Level 2: Quoted prices for similar instruments that are directly or indirectly observable in the marketplace. Level 3: Significant unobservable inputs which are supported by little or no market activity and that are financial instruments whose values are determined using pricing models, discounted cash flow methodologies, or similar techniques, as well as instruments for which the determination of fair value requires significant judgment or estimation. Financial instruments measured at fair value are classified in their entirety based on the lowest level of input that is significant to the fair value measurement. The assessment of the significance of a particular input to the fair value measurement in its entirety requires us to make judgments and consider factors specific to the asset or liability. The use of different assumptions and/or estimation methodologies may have a material effect on estimated fair values. Accordingly, the fair value estimates disclosed or initial amounts recorded may not be indicative of the amount that the Company or holders of the instruments could realize in a current market exchange. The carrying amounts of cash equivalents and marketable securities approximate their fair value based upon quoted market prices. Certain of the Company’s financial instruments are not measured at fair value on a recurring basis, but are recorded at amounts that approximate their fair value due to their liquid or short-term nature, such as cash, accounts receivable, accounts payable and accrued expenses. The following tables present the fair value of the Company’s financial assets measured at fair value on a recurring basis using the above input categories (in thousands): December 31, 2020 Level 1 Level 2 Level 3 Estimated Fair Value Cash equivalents Money market funds $ 47,553 $ — $ — $ 47,553 U.S. and foreign commercial paper — 6,498 — 6,498 Total cash equivalents 47,553 6,498 — 54,051 Short-term investments U.S. and foreign corporate debt securities — 16,939 — 16,939 Asset-backed securities — 12,675 — 12,675 U.S. treasury securities — 23,999 — 23,999 U.S. and foreign commercial paper — 103,356 — 103,356 Total short-term investments — 156,969 — 156,969 Total assets measured at fair value $ 47,553 $ 163,467 $ — $ 211,020 December 31, 2019 Level 1 Level 2 Level 3 Estimated Fair Value Cash equivalents Money market fund $ 33,095 $ — $ — $ 33,095 U.S. and foreign corporate debt securities — 4,999 — 4,999 U.S. and foreign commercial paper — 4,484 — 4,484 Total cash equivalents 33,095 9,483 — 42,578 Short-term investments U.S. and foreign corporate debt securities — 72,486 — 72,486 Asset-backed securities — 34,025 — 34,025 U.S. treasury securities — 44,714 — 44,714 U.S. and foreign commercial paper — 76,086 — 76,086 Total short-term investments — 227,311 — 227,311 Total assets measured at fair value $ 33,095 $ 236,794 $ — $ 269,889 Money market funds are highly liquid and actively traded marketable securities that generally transact at a stable $1.00 net asset value representing its estimated fair value. The Company estimates the fair value of its U.S. and foreign corporate debt securities, asset backed securities, U.S. treasury securities and U.S. and foreign commercial paper by taking into consideration valuations obtained from third-party pricing services. The pricing services utilize industry standard valuation models, including both income and market-based approaches, for which all significant inputs are observable, either directly or indirectly, to estimate fair value. These inputs include reported trades of and broker/dealer quotes on the same or similar securities, issuer credit spreads; benchmark securities; prepayment/default projections based on historical data; and other observable inputs. There have been no transfers between Level 1, Level 2 or Level 3 for any of the periods presented. See Note 3 for further information regarding the carrying value of our investments in marketable securities. Revenue Recognition and Accounts Receivable from Collaborations The Company analyzes its collaboration arrangements to assess whether such arrangements, or transactions between arrangement participants, involve joint operating activities performed by parties that are both active participants in the activities and exposed to significant risks and rewards dependent on the commercial success of such activities or are more akin to a vendor-customer relationship. In making this evaluation, the Company considers whether the activities of the collaboration are considered to be distinct and deemed to be within the scope of the collaborative arrangement accounting standard and those that are more reflective of a vendor-customer relationship and, therefore, within the scope of the revenue with contracts with customers accounting standard. This assessment is performed throughout the life of the arrangement based on changes in the responsibilities of all parties in the arrangement. For elements of collaboration arrangements that are not accounted for pursuant to the revenue from contracts with customers accounting standard, an appropriate recognition method is determined and applied consistently, generally by analogy to the revenue from contracts with customers accounting standard. Amounts related to transactions with a counterparty in a collaborative arrangement that is not a customer are presented as collaboration revenue and on a separate line item from revenue recognized from contracts with customers, if any, in the Company’s consolidated statements of operations and comprehensive loss. Under certain collaborative arrangements, the Company has been reimbursed for a portion of its research and development expenses or participates in the cost-sharing of such research and development expenses. Such reimbursements and cost-sharing arrangements are reflected as a reduction of research and development expense in the Company’s consolidated statements of operations and comprehensive loss, as the Company does not consider performing these activities for reimbursement to be a part of its ongoing major or central operations. For arrangements or transactions between arrangement participants determined to be within the scope of the contracts with customers accounting standard, the Company evaluates the term of the arrangement and recognizes revenue when the customer obtains control of promised goods or services in a contract for an amount that reflects the consideration the Company expects to receive in exchange for those goods or services. For contracts with customers, the Company applies the following five-step model in order to determine this amount: (1) identification of the promised goods or services in the contract; (2) determination of whether the promised goods or services are performance obligations, including whether they are distinct in the context of the contract; (3) measurement of the transaction price, including the constraint on variable consideration; (4) allocation of the transaction price to the performance obligations; and (5) recognition of revenue when (or as) the Company satisfies each performance obligation. The Company has provided standard indemnification and protection of licensed intellectual property for its customer. These provisions are part of assurance that the licenses meet the agreements, representations and are not obligations to provide goods or services. The Company only applies the five-step model to contracts when it is probable the Company will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. As part of the accounting for contracts with customers, the Company must develop assumptions that require judgment to determine the standalone selling price of each performance obligation identified in the contract. The Company then allocates the total transaction price to each performance obligation based on the estimated standalone selling prices of each performance obligation. The Company recognizes the amount of the transaction price that is allocated to the respective performance obligation when the performance obligation is satisfied or as it is satisfied as revenue. Upfront License Fees If a license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company recognizes revenues from nonrefundable, upfront license fees based on the relative value prescribed to the license compared to the total value of the arrangement. The revenue is recognized when the license is transferred to the collaborator and the collaborator is able to use and benefit from the license. For licenses that are not distinct from other obligations identified in the arrangement, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time. If the combined performance obligation is satisfied over time, the Company applies an appropriate method of measuring progress for purposes of recognizing revenue from nonrefundable, upfront license fees. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Development and Regulatory Milestone Payments Depending on facts and circumstances, the Company may record revenues from certain milestones in a reporting period before the milestone is achieved if the Company concludes achievement of the milestone is probable and recognition of revenue related to the milestone will not result in a significant reversal in amounts recognized in future periods. The Company records a corresponding contract asset when this conclusion is reached. Milestone payments that have not been included in the transaction price to date are fully constrained. The Company re-evaluates the probability of achievement of such milestones and any related constraint each reporting period. The Company adjusts its estimate of the overall transaction price, including the amount of collaborative revenue that was recorded, if necessary. Research and Development Service Payments Under the Research, Development, Collaboration and License Agreement (Allergan Agreement) with Allergan Pharmaecuticals International Limited (Allergan), the Company was reimbursed at a certain percentage for performing research and development services based on hours worked by the Company’s employees, at a fixed contractual rate per hour, and third-party pass-through costs the Company incurred on a quarterly basis. Research and development service payments were included in the transaction price in the reporting period the Company concluded it was probable that recording revenue in the period would not result in a significant reversal in amounts recognized in future periods. Accounts receivable were recorded when the right to the research and development service payment consideration became unconditional. The Company recorded the full reimbursed portion of these expenses as collaboration revenue associated with the Allergan Agreement in its consolidated statements of operations as the Company consider performing research and development services to be a part of its ongoing and central operations. Sales-based Milestone and Royalty Payments The Company’s customer may be required to pay the Company sales-based milestone payments or royalties on future sales of commercial products. The Company recognizes revenues related to sales-based milestone and royalty payments upon the later to occur of (i) achievement of the collaborator’s underlying sales or (ii) satisfaction of any performance obligation(s) related to these sales, in each case assuming the Company’s licensed intellectual property is deemed to be the predominant item to which the sales-based milestones and/or royalties relate. The Company receives payments from its customer based on billing schedules established in each contract. Upfront payments and fees are recorded as deferred revenue upon receipt or when due until the Company performs its obligations under the arrangement. If the related performance obligation is expected to be satisfied within the next twelve months, these amounts will be classified in current liabilities. The Company recognizes a contract asset relating to its conditional right to consideration that is not subject to a constraint. Amounts are recorded as accounts receivable when the Company’s right to consideration is unconditional. A net contract asset or liability is presented for each contract with a customer. The Company does not assess whether a contract has a significant financing component if the expectation at contract inception is such that the period between payment by the customer and the transfer of the promised goods or services to the customer will be one year or less. At December 31, 2020 and 2019, all accounts receivable are deemed collectible. Stock-Based Compensation The Company measures stock-based compensation to employees, consultants, and Board members at fair value on the grant date of the award. The fair value of RSUs is determined based on the number of shares granted and the quoted market price of the Company’s common stock on the date of grant. Compensation cost is recognized as expense on a straight-line basis over the requisite service period of the award. Stock-based awards with graded vesting schedules are recognized using the accelerated attribution method on a straight-line basis over the requisite service period for each separately vesting portion of the award. For awards that have a performance condition, compensation cost is measured based on the fair value of the award on the grant date, the date performance targets are established, and is expensed over the requisite service period for each separately vesting tranche when achievement of the performance condition becomes probable. The Company assesses the probability of the performance conditions being met on a continuous basis. Forfeitures are recognized when they occur. The Company estimates the fair value of stock option grants that do not contain market-based vesting conditions using the Black-Scholes option pricing model . T he assumptions used in estimating the fair value of these awards, such as expected term, expected dividend yield , volatility and risk-free interest rate, represent management’s best estimates and involve inherent uncertainties and the application of management’s judgment. The Company is also required to make estimates as to the probability of achieving the s

Investments in Marketable Secur

Investments in Marketable Securities	12 Months Ended
	Dec. 31, 2020
Investments Debt And Equity Securities [Abstract]
Investments in Marketable Securities	Note 3 - Investments in Marketable Securities Investments in marketable available-for-sale securities consisted of the following (in thousands): December 31, 2020 Amortized Cost Gross Unrealized Gain (1) Gross Unrealized Loss (1) Estimated Fair Value Cash equivalents Money market funds $ 47,553 $ — $ — $ 47,553 U.S. and foreign commercial paper 6,498 — — 6,498 Total cash equivalents 54,051 — — 54,051 Short-term investments U.S. and foreign corporate debt securities 16,939 3 (3 ) 16,939 Asset-backed securities 12,674 2 (1 ) 12,675 U.S. treasury securities 23,997 2 — 23,999 U.S. and foreign commercial paper 103,356 — — 103,356 Total short-term investments 156,966 7 (4 ) 156,969 Total cash equivalents and investments $ 211,017 $ 7 $ (4 ) $ 211,020 December 31, 2019 Amortized Cost Gross Unrealized Gain (1) Gross Unrealized Loss (1) Estimated Fair Value Cash equivalents Money market funds $ 33,095 $ — $ — $ 33,095 U.S. and foreign corporate debt securities 5,000 — (1 ) 4,999 U.S. and foreign commercial paper 4,484 — — 4,484 Total cash equivalents 42,579 — (1 ) 42,578 Short-term investments U.S. and foreign corporate debt securities 72,452 38 (4 ) 72,486 Asset-backed securities 34,008 17 — 34,025 U.S. treasury securities 44,692 24 (2 ) 44,714 U.S. and foreign commercial paper 76,086 — — 76,086 Total short-term investments 227,238 79 (6 ) 227,311 Total cash equivalents and investments $ 269,817 $ 79 $ (7 ) $ 269,889 (1) Gross unrealized gain (loss) is pre-tax. As of December 31, 2020, the contractual term to maturity of short-term marketable securities held by the Company is less than one year. There were no long-term marketable securities held by the Company as of December 31, 2020. Realized gains and losses for the years ended December 31, 2020, 2019 and 2018 were not significant. None of the Company’s investments have been in a continuous unrealized loss position for more than 12 months as of December 31, 2020. See Note 2 for further information regarding the fair value of our investments in marketable securities.

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2020
Property Plant And Equipment [Abstract]
Property and Equipment, Net	Note 4 - Property and Equipment, Net Property and equipment consist of the following (in thousands): As of December 31, 2020 2019 Lab equipment $ 247 $ 247 Office equipment 699 699 Leasehold improvement 2,490 2,084 Total property and equipment 3,436 3,030 Less: Accumulated depreciation (1,836 ) (1,200 ) Property and equipment, net $ 1,600 $ 1,830 Depreciation expense for the years ended December 31, 2020, 2019 and 2018 was $0.7 million, $0.5 million, and $0.6 million, respectively, and was recorded in both research and development expense and general and administrative expense in the consolidated statements of operations and comprehensive loss. Primarily all of property and equipment is located in the U.S.

Other Accrued Expenses

Other Accrued Expenses	12 Months Ended
	Dec. 31, 2020
Payables And Accruals [Abstract]
Other Accrued Expenses	Note 5 – Other Accrued Expenses Other accrued expenses consist of the following (in thousands): As of December 31, 2020 2019 Other accrued expenses: Accrued compensation $ 7,016 $ 5,312 Accrued restructuring charges 4,164 2,094 Accrued professional fees and other 807 880 Total other accrued expenses $ 11,987 $ 8,286

Restructurings

Restructurings	12 Months Ended
	Dec. 31, 2020
Restructuring And Related Activities [Abstract]
Restructurings	Note 6 – Restructurings 2019 Restructuring In November 2019, the Company’s Board of Directors approved the relocation of the Company corporate headquarters to South San Francisco, California which became effective January 1, 2020. The Company accrued restructuring charges of $2.1 million in 2019 related to one-time termination severance payments and other employee-related costs associated with the relocation plan. This represents the total amount expected to be incurred in connection with the relocation. 2020 Restructuring In December 2020, the Company and its Board of Directors determined that it was in the Company’s best interest to wind down its Microbiome program, enabling the Company to prioritize resources and focus on the advancement of its pipeline of novel core inhibitors for chronic HBV infection. The Company expects to complete the wind-down of the Microbiome program in early 2021. The following table summarizes the Company’s estimates of costs incurred and expected to be incurred (in thousands): Total Restructuring Cost Employee Severance and Related Benefits Asset Impairment and Other Costs Total estimated restructuring costs to be incurred $ 8,040 $ 6,461 $ 1,579 Restructuring costs incurred during the period: Total restructuring costs incurred for the year ended December 31, 2019 2,094 2,094 — Total restructuring costs incurred for the year ended December 31, 2020 5,684 4,105 1,579 Cumulative restructuring costs incurred through December 31, 2020 $ 7,778 $ 6,199 $ 1,579 The following table presents the activity in the accrued restructuring charges during the period, all of which are related to employee severance and related benefits (in thousands): Accrued balance as of January 1, 2019 $ — Costs incurred 2,094 Accrued balance as of December 31, 2019 $ 2,094 Costs incurred 3,843 Reductions for cash payments (1,773 ) Accrued balance as of December 31, 2020 $ 4,164 The Company also recognized $0.3 million of accelerated vesting for stock-based compensation for employees subject to the restructuring activities during 2020. The Company expects the accrued restructuring liability to be fully paid in 2021. The following table presents where the restructuring charges were recognized (in thousands): Year Ended December 31, 2020 2019 Research and development $ 5,486 $ 433 General and administrative 198 1,661 Total $ 5,684 $ 2,094

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Stockholders' Equity	Note 7 - Stockholders’ Equity The Company is authorized to issue 5,000,000 shares of preferred stock as of December 31, 2020 and 2019, respectively. As of December 31, 2020 and 2019, no shares of preferred stock were issued and outstanding. The Company is authorized to issue 100,000,000 shares of common stock as of December 31, 2020 and 2019, respectively. Sale of Common Stock and Pre-Funded Warrants In December 2017, the Company filed a registration statement on Form S-3 with the SEC, File No. 333-222366, that became effective January 10, 2018 (the 2018 Registration Statement). The 2018 Registration Statement gave the Company the ability to sell any combination of the securities described in the 2018 Registration Statement in one or more offerings up to an aggregate offering price of $250.0 million. In connection with the filing of the 2018 Registration Statement, the Company entered into a sales agreement that gave the Company the ability to sell shares of its common stock having an aggregate offering price of up to $75.0 million through “at the market offerings” (the 2017 ATM). The 2017 ATM was terminated effective September 6, 2020, and no shares were sold under the 2017 ATM prior to the termination. In July 2018, the Company sold to various investors an aggregate of 4,600,000 shares of common stock in a public offering at $36.00 per share, which included the exercise in full by the underwriters of their option to purchase 600,000 additional shares of common stock. The Company received aggregate net proceeds of $155.4 million from the offering and the option exercise, after deducting underwriting discounts and commissions and offering expenses payable by the Company. In December 2019, the Company sold to various investors an aggregate of 6,287,878 shares of common stock at a public offering price of $16.50 per share, which included the exercise in full by the underwriters of their option to purchase 1,136,363 shares of common stock, and pre-funded warrants to purchase 2,424,242 shares of common stock at a public offering price of $16.499. The Company received aggregate net proceeds of $134.7 million from the offering and the option exercise, after deducting underwriting discounts and commissions and offering expenses payable by the Company. The pre-funded warrants became immediately exercisable upon issuance at an exercise price of $0.001 per share, but under their terms, the outstanding pre-funded warrants to purchase shares of the Company’s common stock generally may not be exercised if the holder’s ownership of the Company’s common stock would exceed 19.99% following such exercise. The exercise price and number of shares of common stock issuable upon the exercise of the pre-funded warrants (Warrant Shares) are subject to adjustment in the event of any stock dividends and splits, reverse stock split, recapitalization, reorganization or similar transaction, as described in the pre-funded warrant agreements. Under certain circumstances, the pre-funded warrants may be exercisable on a “cashless” basis. Both the pre-funded warrants and the Warrant Shares are registered securities. The pre-funded warrants were classified as a component of permanent stockholders’ equity within additional paid-in-capital and were recorded at the issuance date using a relative fair value allocation method. The pre-funded warrants are equity classified because they are freestanding financial instruments that are legally detachable and separately exercisable from the equity instruments, are immediately exercisable, do not embody an obligation for the Company to repurchase its shares, permit the holders to receive a fixed number of common shares upon exercise, are indexed to the Company’s common stock and meet the equity classification criteria. In addition, such pre-funded warrants do not provide any guarantee of value or return. The Company valued the pre-funded warrants at issuance, concluding their sales price approximated their fair value, and allocated net proceeds from the sale proportionately to the common stock and pre-funded warrants of which $37.5 million allocated to the pre-funded warrants and recorded as a component of additional paid-in-capital. In August 2020, the Company filed a shelf registration statement on Form S-3 with the SEC, File No. 333-248469, that became effective on September 4, 2020 (the 2020 Registration Statement). The Company may from time to time sell any combination of the securities described in the 2020 Registration Statement in one or more offerings up to an aggregate offering price of $300.0 million. In connection with the filing of the 2020 Registration Statement, the Company entered into a sales agreement under which the Company may offer and sell shares of its common stock having an aggregate offering price of up to $100.0 million through “at-the-market” offerings (2020 ATM), which shares are included in the $300.0 million of securities registered pursuant to the 2020 Registration Statement. During the year ended December 31, 2020, the Company issued and sold 892,840 shares of common stock under the 2020 Registration Statement, for which the Company received net proceeds of $5.5 million, after deducting commissions, fees and expenses. The Company carried forward registration fees paid with respect to $21.4 million of securities that remained available under the 2018 Registration Statement. As such, as of the effectiveness of the 2020 Registration Statement, the 2018 Registration Statement was deemed terminated. Common Stock Warrants As of December 31, 2020, the following warrants to purchase shares of the Company’s common stock were issued and outstanding: Issue date Expiration date Exercise Price per Share December 31, 2020 December 31, 2019 September 10, 2010 September 10, 2020 $ 30.000 — 15,296 December 16, 2019 None $ 0.001 2,424,242 2,424,242 2,424,242 2,439,538 There were no warrants exercised during the years ended December 31, 2020, 2019 and 2018. During the year ended December 31, 2020, 15,296 warrants to purchase common stock expired unexercised. During the year ended December 31, 2018, 1,613 warrants to purchase common stock expired unexercised.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	Note 8 - Stock-Based Compensation Equity Incentive Plans In May 2018, the Company’s stockholders approved (1) the Assembly Biosciences, Inc. 2018 Stock Incentive Plan (the 2018 Plan) pursuant to which the Company reserved 1,900,000 shares of its common stock for issuance in connection with equity incentive awards and (2) the Assembly Biosciences, Inc. Employee Stock Purchase Plan (the 2018 ESPP) pursuant to which the Company reserved 400,000 shares of its common stock for issuance in connection with purchases by employees pursuant to this plan. In May 2019, the Company’s stockholders approved an amendment to the 2018 Plan that increased the aggregate shares of common stock reserved under the 2018 Plan to 3,000,000. In June 2020, the Company’s stockholders approved an amendment to the 2018 Plan that increased the aggregate number of shares of common stock reserved under the 2018 Plan to 4,600,000. As of December 31, 2020, the Company had awards outstanding under the following shareholder approved plans: 2010 Equity Incentive Plan (the 2010 Plan), which has been frozen; the Amended and Restated 2014 Stock Incentive Plan (the 2014 Plan); and the 2018 Plan. Shares of common stock underlying awards that are forfeited under the 2010 Plan on or after June 2, 2016 will become available for issuance under the 2014 Plan. As of December 31, 2020, the Company also had awards outstanding under the Assembly Biosciences, Inc. 2017 Inducement Award Plan (the 2017 Plan), the 2019 Inducement Award Plan (the 2019 Plan) and the Assembly Biosciences, Inc. 2020 Inducement Award Plan (the 2020 Plan). The Company issues new shares of common stock to settle options exercised or vested RSUs. The Company also issues new shares of common stock in connection with purchases of shares of common stock by eligible employees under the Company’s 2018 ESPP. Stock Plan Activity Stock Options The following table summarizes the stock option activity and related information for 2020: Number of Shares Weighted Average Exercise Price Per Share Weighted Average Remaining Contractual Term (Years) Total Intrinsic Value (in thousands) Outstanding as of December 31, 2019 5,613,353 $ 15.90 Granted 1,649,340 16.07 Exercised (175,579 ) 9.75 Forfeited (390,522 ) 22.81 Outstanding as of December 31, 2020 6,696,592 $ 15.70 6.9 $ 1,856 Exercisable as of December 31, 2020 3,926,732 $ 14.70 5.5 $ 1,813 The weighted-average grant-date fair value of options granted was $10.64, $10.55 and $30.84 during the years ended December 31, 2020, 2019 and 2018, respectively. The total intrinsic value of options exercised in 2020, 2019 and 2018 was $2.0 million, $5.0 million and $31.1 million, respectively. RSUs The following table summarizes RSU activity and related information for 2020: Number of RSU's Weighted Average Fair Value Per RSU at Grant Price Nonvested as of December 31, 2019 758,718 $ 25.47 Granted 536,164 16.21 Vested (297,133 ) 23.65 Forfeited (110,881 ) 26.46 Nonvested as of December 31, 2020 886,868 (1) $ 20.36 (1) Includes 140,000 RSUs that have vested but are subject to deferred settlement, which have a weighted average remaining contractual term of 2.2 years. The total fair value of RSUs vested and settled during 2020, 2019 and 2018 was $7.8 million, $5.7 million, and $5.3 million, respectively. The total intrinsic value of RSUs vested and settled during 2020 and 2019 was $4.4 million and $2.9 million, respectively. The total intrinsic value of RSUs vested and settled during 2018 was nominal. As of December 31, 2020, RSUs outstanding include 100,000 RSUs granted in September 2019 to the Company’s chief executive officer with performance-based conditions. The 100,000 awards with an aggregate fair value of $1.2 million vest upon performance conditions not yet deemed probable and accordingly no stock-based compensation expense has been recognized as of December 31, 2020. In July 2020, the performance condition for 45,000 RSUs granted in December 2017 to a former executive officer was met associated with the execution of an HBV business development transaction. The Company recognized $0.7 million as a cumulative catch-up adjustment of stock-based compensation expense for this award for the year ended December 31, 2020. In the second quarter of 2019, 100,000 RSUs granted to a former officer were forfeited due to his departure. These RSUs had a grant date fair value of $2.4 million and were vesting over time but would have accelerated upon the achievement of certain performance-based conditions. The Company reversed the previously recognized expense of $0.5 million related to these forfeited awards upon the departure of the former officer in 2019. Employee Stock Purchase Plan The 2018 ESPP provides for the purchase by employees of up to an aggregate of 400,000 shares of the Company’s common stock at a discount to the market price. Subject to the annual statutory limits and the 2018 ESPP’s limit of 1,000 shares of common stock per offering, an eligible employee may participate through payroll deductions of up to 15% of such employee’s compensation for each pay period Eligible employees can purchase the Company’s common stock at the end of a predetermined offering period at 85% of the lower of the fair market value at the beginning or end of the offering period. Under the 2018 ESPP, the offering periods end on the last business day occurring on or before May 14 or November 14. The ESPP is compensatory and results in stock-based compensation expense. In November 2018, employees purchased 21,483 shares of common stock under the 2018 ESPP. In May and November 2019, employees purchased 36,804 and 22,566 shares of common stock, respectively, under the 2018 ESPP. In May and November 2020, employees purchased 42,266 and 44,546 shares of common stock, respectively, under the 2018 ESPP. As of December 31, 2020, 232,335 shares of common stock are available for future sale under the Company’s 2018 ESPP. Stock-based compensation expense recorded in connection with the 2018 ESPP was $0.5 million, $0.4 million and $0.2 million for the years December 31, 2020, 2019 and 2018, respectively. Valuation Assumptions The Company used the Black-Scholes option-pricing model for determining the estimated fair value and stock-based compensation related to stock options and ESPP purchase rights. A summary of the assumptions used to estimate the fair values of stock options grants for the years presented is as follows: Year Ended December 31, 2020 2019 2018 Exercise price $5.62 - $23.30 $9.31 - $23.04 $23.78 - $57.53 Expected volatility 66.4% - 92.3% 66.5% - 83.2% 75.6% - 86.1% Risk-free interest rate 0.26% - 1.44% 1.36% - 2.65% 2.56% - 3.04% Expected term (years) 5.5 - 7.5 5.5 - 7.5 5.5 - 7.0 Expected dividend yield 0% 0% 0% The risk-free interest rate assumption was based on the rates for U.S. Treasury zero-coupon bonds with maturities similar to those of the expected term of the stock option being valued. The expected dividend yield was zero as the Company currently does not intend to pay dividends in the foreseeable future. The weighted average expected term of options was calculated using the simplified method as prescribed by accounting guidance for stock-based compensation due to the Company’s limited history of relevant stock option exercise activity. The expected volatility was calculated based on the Company’s historical stock prices, supplemented as necessary with historical volatility of the common stock of several peer companies with characteristics similar to those of the Company. The fair value of ESPP purchase rights were not material for any period presented. Stock-Based Compensation Expense The Company recognized stock-based compensation expense included in the consolidated statement of operations and comprehensive loss for the periods presented (in thousands): Year Ended December 31, 2020 2019 2018 Research and development $ 11,380 $ 11,376 $ 11,820 General and administrative 10,473 (1) 9,182 (2) 16,665 Total stock-based compensation expense $ 21,853 $ 20,558 $ 28,485 (1) (2) As of December 31, 2020 20.4 1.8

Collaboration Agreements

Collaboration Agreements	12 Months Ended
	Dec. 31, 2020
Collaboration Agreement [Abstract]
Collaboration Agreements	Note 9 - Collaboration Agreements Allergan Agreement In January 2017, the Company and Allergan entered into the Allergan Agreement to develop and commercialize select microbiome gastrointestinal disease therapies. Pursuant to the Allergan Agreement, the Company granted Allergan an exclusive worldwide license to certain of its intellectual property, including its intellectual property arising under the Allergan Agreement, to develop and commercialize licensed compounds for ulcerative colitis (UC), Crohn’s disease, and two compounds for irritable bowel syndrome (IBS). Allergan and the Company also agreed to collaborate on research and development activities with respect to the licensed compounds in accordance with a mutually agreed upon research and development plan. Per the terms of the Allergan Agreement, Allergan could select backups and additional target indications to add to the licenses granted for additional consideration and also had the ability to enter into a contract manufacturing agreement with the Company for compound supply at cost plus an agreed upon margin. In addition, the Company participated on a Joint Development Committee (JDC) and Joint Patent Committee (JPC). Allergan had the right to terminate the Allergan Agreement at any time upon advance written notice. In June 2020, following its acquisition of Allergan, AbbVie Inc. (AbbVie), on behalf of Allergan, gave written notice of termination of the Allergan Agreement effective 120 days following the delivery of notice, on October 10, 2020. Upon termination, the licenses granted by the Company and its know-how reverted to the Company. Under the terms of the Allergan Agreement, AbbVie was obligated to continue to reimburse the Company for certain research and development costs through October 10, 2020. Upon effectiveness of the termination, such reimbursements ceased. Due to the delivery of the termination notice, the Company determined there were no further enforceable rights and obligations under the Allergan Agreement beyond June 2020 and the remaining $36.0 million of deferred revenue was recognized in 2020. Allergan paid the Company an upfront non-refundable payment of $50.0 million which was received in 2017. Additionally, the Company was eligible to receive variable consideration in the form of research and development cost reimbursements, up to $631.0 million related to seven development milestones and up to $2.14 billion related to 12 commercial development and sales milestones in connection with the successful development and commercialization of licensed compounds. In addition, the Company was eligible to receive tiered royalties at rates ranging from the mid-single digits to the mid-teens based on net sales. Allergan and the Company agreed to share research and development costs up to an aggregate of $75.0 million through proof-of-concept (POC) studies on a ⅔, ⅓ basis, respectively, and Allergan agreed to assume all post-POC development costs. In the event any pre-POC development costs would have exceeded $75.0 million in the aggregate, the Company may have elected either (a) to fund ⅓ of such costs in excess of $75.0 million or (b) to allow Allergan to deduct from future development milestone payments ⅓ of the development costs funded by Allergan in excess of $75.0 million plus a premium of 25%. The Company had an option to co-promote the licensed programs in the U.S. and China, subject to certain conditions set forth in the Allergan Agreement. The Company concluded that Allegan was a customer, and the contract was not subject to accounting literature on collaborative arrangements. This is because the Company granted to Allergan licenses to its intellectual property and agreed to perform research and development services, all of which are outputs of the Company’s ongoing activities, in exchange for consideration. The Company identified the following material promises under the Allergan Agreement: (1) grant of a licenses to intellectual property for the four initial indications, inclusive of the related technology know-how (Licenses) and (2) the obligation to perform research development services through POC (Development Services). The Company’s participation on the JDC and JPC were considered to be immaterial in the context of the contract. The Company’s co-promotion option was not considered to be a performance obligation. Allergan’s selection of backups or additional target indications to add to the licenses granted for additional consideration and ability to enter into a contract manufacturing agreement with the Company for compound supply at cost plus an agreed upon margin were not considered to be performance obligations as the Company concluded the options were not offered at a discount that exceeds discounts available to other customers, and therefore were not material rights. The grant of additional licensing rights upon option exercises and contract manufacturing agreements were to be accounted for as separate contracts when or if they occurred. The Company concluded the Licenses each were considered to be functional as they had significant standalone functionality and were capable of being distinct. However, the Company determined that each of the Licenses individually were not distinct from the Development Services within the context of the agreement. This is because Allergan was dependent on the Company to execute the Development Services, which it was uniquely able to perform, in order for Allergan to benefit from the Licenses. As such, t he Company determined that it had four performance obligations under the Allergan Agreement associated with the grant of the four compound Licenses combined with the performance of the Development Services for each of the four compound indications. The Company determined that the four performance obligations would have be en performed over the duration of the contract, which began in February 2017 and ends upon receipt of the termination notice . The Company used a cost-based input method to measure proportional performance and to calculate the corresponding amount of revenue to recognize. The Company believe d this wa s the best measure of progress because other measures do not reflect how the Company transfers its performance obligation to Allegan. In applying the cost-based input method of revenue recognition, the Company measure d costs incurred relative to budgeted costs to fulfill the four performance obligations. These costs consist ed primarily of third-party contract costs and internal labor costs. Revenue will be recognized based on actual costs incurred as a percentage of total budgeted costs as the Company complete d its performance obligations. To allocate transaction price among the four performance obligations, the Company estimated their standalone selling price (SSP) using an income-based valuation approach for the estimated value a licensor of the compounds would receive considering the stage of the compounds’ development. A change in the assumptions used to determine its best estimate of selling price for the four performance obligations would not have had a significant effect on the allocation of consideration received to the four performance obligations. The transaction price at the inception of the agreement and upon adoption of the revenue from contracts with customers guidance was limited to the $50.0 million upfront payment. Of this amount, the Company allocated $12.5 million to each of the four performance obligations. Research and development cost reimbursement payments were included in the transaction price in the reporting period that the Company concludes that it was probable that recording revenue in the period would not have resulted in a significant reversal in amounts recognized. The variable consideration related to the remaining development and commercialization milestone payments was not included in the transaction price as these were fully constrained. As part of the Company’s evaluation of the development and commercialization milestones constraint, the Company determined the achievement of such milestones was contingent upon success in future clinical trials and regulatory approvals which was not within its control and uncertain at this stage. Any variable consideration related to sales-based milestones (including royalties) would have been recognized when the related sales occur as they were determined to relate predominantly to the license granted to Allergan. The Company re-evaluated the transaction price in each reporting period and as uncertain events were resolved or other changes in circumstances occurred. The Company did not incur any significant incremental costs of obtaining the Allergan contract. For the year ended December 31, 2020, 2019, and 2018 the Company recognized $48.1 million , BeiGene Agreement In July 2020, the Company and BeiGene, Ltd. (BeiGene) entered into a Collaboration Agreement (the BeiGene Agreement) to develop and commercialize the Company’s novel core inhibitor product candidates VBR, ABI-H2158 and ABI-H3733 for chronic HBV infection (the Licensed Product Candidates) in the People’s Republic of China, Hong Kong, Taiwan and Macau (the Territory). Under the agreement, the Company and BeiGene are collaborating on certain global clinical studies and both the Company and BeiGene will independently conduct other clinical studies in their own respective territories. BeiGene agreed to pay all development and regulatory costs for the Licensed Product Candidates in the Territory up to an aggregate of $45.0 million. Development and regulatory costs for the Licensed Product Candidates for the Territory in excess of $45.0 million will be shared equally by the Company and BeiGene. If the Company conducts certain ancillary trials outside of the plan to develop these candidates in the Territory, BeiGene may elect to obtain access to the know-how and clinical data resulting for such ancillary trials and shall reimburse the Company proportionally for the Territory of the costs of such trials. Activities under the BeiGene Agreement will be governed by a joint steering committee (JSC) consisting of equal representatives from each party to the agreement. All decisions of the JSC are to be made by consensus with final decision-making authority granted to each party based on key areas of the collaboration for which they are responsible. During the term of the BeiGene Agreement, neither party will commercialize any competing products in the Territory. The Company will be responsible for manufacturing and supply of the candidates to be used in and outside of the Territory, although the parties may approve BeiGene to take on some or all of the commercial supply activities of the applicable Licensed Products in the Territory. The Company is not obligated to perform pre-phase 3 clinical trial development work outside the Territory on ABI-H2158 and ABI-H3733 but must provide BeiGene pre-Phase 3 clinical trial know-how and development results if and when such development efforts are completed. If, after ABI-H2158 and ABI-H3733 reach the end of Phase 2 clinical trials, the Company and BeiGene are unable to mutually agree on the terms of a Phase 3 global study, BeiGene may elect to terminate the BeiGene Agreement solely as it relates to that compound, as applicable. Such a termination would result in the Company regaining all rights to the applicable compound in the Territory. In addition, BeiGene may terminate the BeiGene Agreement for convenience at any time upon 90 days’ advance written notice to us. The BeiGene Agreement also contains customary provisions for termination by either party, including in the event of breach of the BeiGene Agreement, subject to cure. Pursuant to the terms of the BeiGene Agreement, the Company received an upfront cash payment of $40.0 million from BeiGene for the delivery of exclusive, royalty-bearing licenses to develop and commercialize the Licensed Product Candidates in the Territory, and the Company is eligible to receive up to approximately $500.0 million in cash milestone payments, comprised of up to $113.8 million for development and regulatory milestones and up to $385.0 million in net sales milestones. In addition, the Company is eligible to receive tiered royalties at percentages ranging from the mid-teens to the low thirties of net sales. The collaborative arrangements guidance as both parties are active participants and are exposed to significant risks and rewards dependent on the success of commercializing the Licensed Product Candidates in the Territory but that the unit of account related to the delivery of Licensed Product Candidates is within the scope of the contract with customers guidance. The remaining units of account related to participation on the JSC and subcommittees, clinical supply and other in Territory and global development activities (the Collaboration Activities) are within the scope of the collaborative arrangements guidance. Commercial supply will be evaluated as a separate contract when the agreement is executed and a purchase order is received from BeiGene. The Company identified the following material promises related to the contract with customers unit of account under the BeiGene Agreement ABI-H2158 , and 3) the transfer of the ABI-H3733 License. The Company concluded each of these licenses to be functional BeiGene . The ABI-H2158 and ABI-H3733 Licenses have a The agreed upon prices for the clinical and commercial supply of the Licensed Product Candidates to BeiGene do not represent material rights, and therefore are not performance obligations, and such pricing on an aggregate basis represents the standalone selling price an entity would typically pay for such a product in that region or market. There are also no minimum purchase commitments. The Company estimated the standalone selling price (SSP) of the Licenses using an income-based valuation approach for the estimated value a licensor of the compounds would receive considering the stage of the compound’s development. The Company believes a change in the assumptions used to determine its best estimate of SSP would not have a significant value on the allocation of consideration received. The transaction price at the inception of the agreement was limited to the $40.0 million upfront payment. The variable consideration related to the remaining development and commercialization milestone payments has not been included in the transaction price as these were fully constrained as of December 31, 2020. As part of the Company’s evaluation of the development and commercialization milestones constraint, the Company determined the achievement of such milestones are contingent upon success in future clinical trials and regulatory approvals which are not within its control and uncertain at this stage. Any variable consideration related to sales-based milestones (including royalties) will be recognized when the related sales occur as they were determined to relate predominantly to the Licensed Product Candidates granted to BeiGene. The Company will reevaluate the transaction price in each reporting period as uncertain events are resolved or other changes in circumstances occur. During the year ended December 31, 2020, the Company recognized $31.0 million as collaboration revenue for the amount allocated to the VBR License as substantial completion of the license technology transfer has occurred. The remaining transaction price allocated to the ABI-H2158 and ABI-H3733 Licenses of $9.0 million was recorded as a long-term deferred revenue contract liability on the consolidated balance sheet as of December 31, 2020. Revenue for these performance obligations will be recognized when the Company provides pre-Phase 3 clinical trial know-how and development results for these compounds to BeiGene or a termination of the BeiGene Agreement for the respective compound. Payments to, or reimbursements from, BeiGene related to the Collaboration Activities will be accounted for as an increase to or reduction of research and development expenses when incurred or realized, respectively. During the year ended December 31, 2020, the Company did not recognize any increase or reduction of research and development expense under the BeiGene Agreement. The Company incurred $3.5 million in incremental costs of obtaining the BeiGene Agreement. These contract costs have been capitalized and are being recognized consistent with the pattern of recognition of revenue associated with the Licensed Product Candidates. As of December 31, 2020, $2.7 million has been amortized to general and administrative expenses and $0.8 million is included in other assets on the condensed consolidated balance sheet. Arbutus Agreement In August 2020, the Company and Arbutus Biopharma Corporation (Arbutus) entered into a Collaboration Agreement (Arbutus Agreement) to conduct a randomized, multi-center, open-label Phase 2 clinical trial to explore the safety, PK and antiviral activity of the triple combination of VBR, AB-729 and an NrtI compared to the double combinations of VBR with an NrtI and AB-729 with an NrtI. The Arbutus Agreement is within the scope of the collaborative arrangements guidance as both parties are active participants and are exposed to significant risks and rewards dependent on the success of the collaborative activity. Arbutus is not a customer as it does not obtain an output from the collaborative activities as they were not provided an exclusive license to VBR or the ability to manufacture VBR, and the Company does not consider performing such collaborative activities to be a part of its ongoing activities. The revenue from contracts with customers guidance was considered by analogy in determining the unit of account, and the recognition and measurement of such unit of account for collaborative activities under the Agreement and concluded there is one activity, to run open-label Phase 2 clinical trial is akin to performance obligation related to collaborative activities. Reimbursements and cost-sharing portions of this performance obligation will be reflected as a reduction of research and development expense when realized in the Company’s consolidated statements of operations, as the Company does not consider performing research and development services for reimbursement to be a part of its ongoing major or central operations. During the year ended December 31 Contract Liabilities The following tables present changes in the Company’s contract liabilities (in thousands): Balance at Beginning of Period Additions Deductions Balance at End of Period Year Ended December 31, 2020 Contract liabilities: Deferred revenue $ 37,048 $ 40,000 $ (68,061 ) $ 8,987 Year Ended December 31, 2019 Contract liabilities: Deferred revenue $ 40,660 $ — $ (3,612 ) $ 37,048 Year Ended December 31, 2020 2019 2018 Collaboration revenue recognized in the period from Amounts included in deferred revenue at the beginning of the period $ 37,048 $ 3,612 $ 5,125 Performance obligations satisfied in previous periods — — —

Strategic License Agreements

Strategic License Agreements	12 Months Ended
	Dec. 31, 2020
Strategic License Agreements [Abstract]
Strategic License Agreements	Note 10 – Strategic License Agreements HBV Research Agreement with Indiana University Since September 2013, the Company has been party to an exclusive License Agreement dated September 3, 2013 with Indiana University Research and Technology Corporation (IURTC) from whom it has licensed aspects of the Company’s HBV program held by IURTC. The license agreement requires the Company to make milestone payments based upon the successful accomplishment of clinical and regulatory milestones. The aggregate amount of all performance milestone payments under the IURTC license agreement, should all milestones through development be met, is $0.8 million, with a portion related to the first performance milestone having been paid. The Company is obligated to pay IURTC royalty payments based on net sales of the licensed technology as well as a portion of any sublicensing revenue Assembly receives. The Company is also required to pay diligence maintenance fees each year to the extent that the royalty, sublicensing, and milestone payments to IURTC are less than the diligence maintenance fee for that year. A performance milestone totaling $0.1 million was determined to have occurred under this agreement and was paid during the year ended December 31, 2020. Additionally, the Company paid IURTC $0.7 million as a sublicensing fee during the year ended December 31, 2020. The Company made $0.1 million in milestone payments for the year ended December 31, 2018. No milestone payments were incurred or accrued for under this agreement as of and for the year ended December 31, 2019. The milestone and license fees are included in research and development expenses in the consolidated statements of operations and comprehensive loss. Microbiome Targeted Colonic Delivery Platform In November 2013, the Company entered into a License and Collaboration Agreement with Therabiome, LLC (Therabiome), for all intellectual property and know-how owned or controlled by Therabiome relating to the oral delivery of pharmaceutical drugs to specific sites in the intestine, using a pH sensitive controlled release capsule-in-capsule technology. The Company will be solely responsible for all research and development activities with respect to any product it develops under the license. The Company must pay Therabiome clinical and regulatory milestones for each product or therapy advanced from the platform for U.S. regulatory milestones. The Company also must pay Therabiome lesser amounts for foreign regulatory milestones, which vary by country and region. The Company also must pay Therabiome royalties on annual net sales of a product in the low to mid-single digit percentages plus, once annual net sales exceed certain thresholds, a one-time cash payment upon reaching the thresholds. Therabiome must pay the Company royalties on annual net sales of any product Therabiome is permitted to develop using the intellectual property in the low double to mid-double digit percentages, depending on the level of development or involvement the Company had in the product. Two regulatory milestones resulting in payments totaling $0.4 million were determined to have occurred under this agreement and were paid in the year ended December 31, 2019. No amounts were incurred or accrued for this agreement as of and for the years ended December 31, 2020 and 2018. In connection with the wind-down of the Microbiome program, the License and Collaboration Agreement with Therabiome was terminated in January 2021. Door Agreement In November 2020, the Company and Door Pharmaceuticals, LLC (Door) entered into an exclusive, two-year For the period ended December 31, 2020, the Company incurred $0.3 million of research and development funding in addition to the $1.8 million up-front payment. Under the consolidation accounting standard, the Company determined that Door is a VIE. The Company does not have the power to direct the activities that most significantly affect the economic performance of Door and as such the Company is not the primary beneficiary and consolidation is not required. As of December 31, 2020, the Company has not provided financial or other support to Door that was not contractually required.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Income Taxes	Note 11 - Income Taxes There was no current income tax provision for the years ended December 31, 2020, 2019 and 2018. The deferred income tax expense for the year ended December 31, 2020 was not material. The Company recognized deferred income tax benefit of $0.8 million for the year ended December 31, 2019 and a deferred income tax expense of $1.1 million for the year ended December 31, 2018. The effective tax rate of our provision for income taxes differs from the federal statutory rate as follows: As of December 31, 2020 2019 2018 Statutory federal income tax rate 21.0 % 21.0 % 21.0 % State taxes, net of federal tax benefit 6.1 5.6 7.1 Stock based compensation 0.7 (3.8 ) 6.7 Research and development tax credits 2.2 5.6 2.4 State rate change — (1.4 ) 2.5 Uncertain tax positions 2.2 (2.1 ) (4.8 ) Return to provision adjustments 4.3 (2.5 ) 4.4 Other 0.1 (0.2 ) 0.8 Change in valuation allowance (36.6 ) (21.4 ) (41.3 ) Income taxes provision (benefit) 0.0 % 0.8 % (1.2 )% Significant components of the Company’s deferred taxes are as follows (in thousands): As of December 31, 2020 2019 Deferred tax assets: Federal and state-operating loss carryforwards $ 108,236 $ 81,584 Stock-based compensation 16,563 12,347 Intangible assets 1,463 1,774 Deferred revenue — 9,503 Operating lease liabilities 2,600 3,147 Research and development credits 8,872 7,499 Other 854 702 Total deferred tax assets 138,588 116,556 Valuation allowance (131,332 ) (108,577 ) Deferred tax asset, net of valuation allowance $ 7,256 $ 7,979 Deferred tax liabilities: In-process research and development $ (7,443 ) $ (7,439 ) Operating lease right-of-use assets (2,344 ) (3,071 ) Total deferred tax liabilities (9,787 ) (10,510 ) Net deferred tax liability $ (2,531 ) $ (2,531 ) The Company maintains a valuation allowance on deferred tax assets due to the uncertainty regarding the ability to utilize these deferred tax assets in the future. The increase in the valuation allowance for both the years ended December 31, 2020 and 2019 is primarily due to an increase in the Company’s federal and state-operating loss carryforwards. The in-process research and development deferred tax liability was recorded in connection with the merger with Assembly Pharmaceuticals, Inc. in 2014 and relates to the difference between the carrying amount of in-process research and development for financial statement purposes relative to the amount used for income tax purposes. As of December 31, 2020, the Company had potentially utilizable gross federal net operating loss carryforwards of $378.8 million with $264.1 million of net operating losses that carry forward indefinitely and $114.7 million of net operating losses which begin to expire in 2027 if not utilized. There are state net operating loss carryforwards of $446.8 million with $0.7 million carrying forward indefinitely and $446.1 million beginning to expire in 2031. In addition, the Company has federal research and development credit carryforwards of $8.5 million which begin to expire in 2028 if not utilized and California research and development credit carryforwards of $3.6 million, which will carryforward indefinitely. Pursuant to Internal Revenue Code (IRC), Section 382 and 383, use of the Company’s U.S. federal and state net operating loss and research and development income tax credit carryforwards may be limited in the event of a cumulative change in ownership of more than 50.0% within a three-year period. The Company has performed an ownership change study through December 31, 2019 and has determined that a “change in ownership” as defined by IRC Section 382 and the rules and regulations promulgated thereunder, did occur in December 2010, January 2013 and October 2014. The Company has adjusted its net operating loss carryovers to appropriately reflect any attributes which will expire due to the limitation. The Company has not performed any additional analysis for IRC Sections 382 and 383 and there is a risk that additional changes in ownership could have occurred since December 31, 2018. If a change in ownership were to have occurred, additional net operating loss and tax credit carryforwards could be eliminated or restricted. If eliminated, the related asset would be removed from the deferred tax asset schedule with a corresponding reduction in the valuation allowance. The following table summarizes activity related to the Company’s gross unrecognized tax benefits (in thousands): As of December 31, 2020 2019 2018 Balances as of beginning of year $ 6,070 $ 4,613 $ — Increases related to prior year tax positions — 15 3,679 Decreases related to prior year tax positions (4,162 ) (934 ) — Increases related to current year tax positions 747 2,376 934 Balances as of end of year $ 2,655 $ 6,070 $ 4,613 The unrecognized tax benefits, if recognized, would not have an impact on the Company’s effective tax rate assuming the Company continues to maintain a full valuation allowance position. Based on prior year’s operations and experience, the Company does not expect a significant change to its unrecognized tax benefits over the next twelve months. The unrecognized tax benefits may increase or change during the next year for unexpected or unusual items for items that arise in the ordinary course of business. In subsequent periods, any interest and penalties related to uncertain tax positions will be recognized as a component of income tax expense. The Company files income tax returns in the U.S. federal, California and other state jurisdictions and is not currently under examination by federal, state, or local taxing authorities for any open tax years. Due to net operating loss carryforwards, all years effectively remain open for income tax examination by tax authorities in the U.S. and states in which the Company files tax returns.

Leases

Leases	12 Months Ended
	Dec. 31, 2020
Lease Cost [Abstract]
Leases	Note 12 - Leases Operating Leases The Company leases corporate office and laboratory space in South San Francisco, California under a sub-sublease that expires in December 2023. The sub-sublease contains scheduled rent increases over the lease term. Prior to moving into the South San Francisco office and laboratory space in February 2019, the Company leased office and laboratory space in San Francisco, California, under a sublease that expired in February 2019. The Company also leases office space for administrative functions in Carmel, Indiana under a lease agreement that expires in August 2023. In February 2021, the Company subleased substantially all of the office space under the Carmel, Indiana lease. The Company also leases office and laboratory space in Groton, Connecticut that supports the Microbiome program under a lease that expires in March 2021. Due to the wind-down of the Microbiome program, the lease will not be renewed. The Company’s China subsidiary leases office space and lab space in Shanghai. The lab space expired in December 2020 and the office space expires March 2021, neither of which are being renewed. Additionally, the Company’s China subsidiary leases office space in Beijing under a lease agreement that expires in December 2021. Certain lease contracts contain renewal clauses that the Company assesses on a case by case basis. The Company also leases certain laboratory equipment accounted for as operating leases expiring at various dates, with the final lease expiring in 2023. In February 2021, the Company purchased substantially all of the leased equipment used for the Microbiome program from its leasing agency and sold them to a third party. The loss on the sale is equal to the impairment loss the Company recognized on these assets for the year ended December 31, 2020 of $0.7 million. When the Company cannot determine the implicit rate in its leasing arrangements, the Company uses its incremental borrowing rate as the discount rate when measuring operating lease liabilities. The incremental borrowing rate represents an estimate of the interest rate the Company would incur at lease commencement to borrow an amount equal to the lease payments on a collateralized basis over the term of a lease within a particular currency environment. At December 31, 2020, the Company had operating lease liabilities of $10.1 million and right-of-use assets of $9.1 million. The following summarizes quantitative information about the Company’s operating leases (in thousands): Year Ended December 31, 2020 2019 Lease cost Operating lease cost $ 5,214 $ 4,454 Short-term lease cost 401 609 Variable lease cost 1,468 1,193 Total lease cost $ 7,083 $ 6,256 Year Ended December 31, 2020 2019 Operating cash flows from operating leases $ 4,513 $ 4,269 Right-of-use assets exchanged for new operating lease liabilities $ 1,302 $ 15,261 As of December 31, 2020, the weighted-average remaining lease term for operating leases was 2.7 years and the weighted-average discount rate for operating leases was 9.2%. As of December 31, 2020, the maturities of the Company’s operating lease liabilities were as follows (in thousands): 2021 $ 4,369 2022 3,905 2023 3,502 Total 11,776 Less: present value discount (1,647 ) Operating lease liabilities $ 10,129 Operating lease costs were $7.1 million, $6.3 million and $4.2 million for the years ended December 31, 2020, 2019 and 2018, respectively.

Employee Benefit Plan

Employee Benefit Plan	12 Months Ended
	Dec. 31, 2020
Compensation And Retirement Disclosure [Abstract]
Employee Benefit Plan	Note 13 - Employee Benefit Plan In January 2018, the Company established a defined contribution 401(k) plan (the Plan) for all employees who are at least 21 years of age. Employees are eligible to participate in the Plan upon commencement of employment. Under the terms of the Plan, employees may make voluntary contributions as a percentage of compensation. The Plan also permits the Company to make discretionary matching contributions. In 2020, 2019 and 2018, the Company made discretionary matching contributions of $0.9 million, $0.7 million and $0.7 million, respectively.

Selected Quarterly Financial Da

Selected Quarterly Financial Data	12 Months Ended
	Dec. 31, 2020
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Data	Note 14 - Selected Quarterly Financial Data (Unaudited) The following table contains quarterly financial information for the four quarters of 2020 and 2019 which has been prepared in accordance with GAAP for interim financial information. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair statement of the information for the periods presented. 2020 Quarter Ended March 31 June 30 September 30 December 31 (in thousands except for per share amounts) Collaboration revenue $ 4,081 $ 39,376 $ 34,611 $ 1,037 Operating expenses $ 31,775 $ 32,797 $ 38,630 $ 40,679 Interest and other income $ 1,039 $ 691 $ 670 $ 224 Net income (loss) $ (26,655 ) $ 7,270 $ (3,349 ) $ (39,418 ) Unrealized gain (loss) from marketable securities, net of tax $ 115 $ 190 $ (262 ) $ (112 ) Basic net income (loss) per common share $ (0.76 ) $ 0.21 $ (0.09 ) $ (1.11 ) Diluted net income (loss) per common share $ (0.76 ) $ 0.19 $ (0.09 ) $ (1.11 ) 2019 Quarter Ended March 31 June 30 September 30 December 31 (in thousands except for per share amounts) Collaboration revenue $ 3,885 $ 3,080 $ 4,231 $ 4,767 Operating expenses $ 32,221 $ 22,780 $ 30,224 $ 33,451 Interest and other income $ 1,276 $ 1,182 $ 983 $ 859 Net loss $ (27,052 ) $ (18,503 ) $ (24,995 ) $ (27,084 ) Unrealized gain (loss) from marketable securities, net of tax $ 108 $ 52 $ (18 ) $ 4 Basic and diluted net loss per common share $ (1.05 ) $ (0.72 ) $ (0.96 ) $ (0.99 )

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2020
Subsequent Events [Abstract]
Subsequent Events	Note 15 – Subsequent Events Subsequent to December 31, 2020, the Company sold 4,177,080 shares of common stock through its 2020 ATM resulting in net proceeds of $25.5 million.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies and Recent Accounting Pronouncements (Policies)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation These consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States (U.S. GAAP) and include the accounts of the Company and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation.
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in conformity with the accounting principles generally accepted in the United States of America (U.S. GAAP) requires management to make estimates and assumptions that may affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and reported amounts of expenses during the reporting period. Actual results could differ from those estimates. Significant estimates inherent in the preparation of the accompanying consolidated financial statements include estimates for the cost-based input of revenue recognition and standalone selling price estimates for allocation of transaction price to performance obligations revenue recognition, estimates of costs incurred but not yet invoiced for clinical trial accruals, recoverability and useful lives of our long-lived assets, the estimated fair value of our indefinite-lived intangible assets, the estimated fair value of our reporting unit for purposes of evaluating goodwill impairment, provisions for income taxes, amounts receivable under collaboration agreements, measurement of operating lease liabilities, and the fair value of stock options, stock appreciation rights, and RSUs granted to employees, directors, and consultants. The Company’s estimates could be affected by external conditions, including those unique to the Company and general economic conditions. It is reasonably possible that these external factors could have an effect on the Company’s estimates and could cause actual results to differ from those estimates and assumptions.
Other Risks And Uncertainties	Other Risks and Uncertainties In March 2020, the World Health Organization declared the COVID-19 outbreak a pandemic. To date, the Company’s operations have not been significantly impacted by the COVID-19 pandemic. However, the Company cannot at this time predict the specific extent, duration, or full impact the COVID-19 pandemic will have on its business, operations, strategy, prospects and financial condition and results. The impact of the COVID-19 pandemic on the Company’s financial performance will depend on future developments, including the duration and spread of the outbreak and related governmental advisories and restrictions. These developments and the impact of the COVID-19 pandemic on the financial markets and the overall economy are highly uncertain. If the financial markets and/or the overall economy are impacted for an extended period, the Company’s results may be adversely affected.
Cash and Cash Equivalents	Cash and Cash Equivalents All highly liquid investments, including money market funds, with original maturities of three months or less at the time of purchase are considered to be cash equivalents. All of the Company’s cash equivalents have liquid markets and high credit ratings. The Company maintains its cash in bank deposits and other accounts, the balances of which, at times and at December 31, 2020 and 2019, exceed federally insured limits.
Investments in Marketable Securities	Investments in Marketable Securities The Company invests its excess cash in debt securities with high credit ratings including but not limited to money market funds classified as cash equivalents, asset backed securities, securities issued by the U.S. government and its agencies, corporate debt securities and commercial paper. The Company has designated its investments in marketable securities as available-for-sale and measures these securities at their respective fair values. Marketable securities are classified as short-term or long-term based on the maturity date and their availability to meet current operating requirements. Marketable securities that mature in one year or less are classified as short-term available-for-sale securities and are reported as a component of current assets. Securities that are classified as available-for-sale are measured at fair value with temporary unrealized gains and losses reported in other comprehensive loss, and as a component of stockholders' equity until their disposition. The Company reviews all available-for-sale securities at each period end to determine if they remain available-for-sale based on their current intent and ability to sell the security if it is required to do so. Realized gains and losses from the sale of marketable securities, if any, are calculated using the specific-identification method. Marketable securities are subject to a periodic impairment review. The Company may recognize an impairment charge when a decline in the fair value of investments below the cost basis is determined to be other-than-temporary. In determining whether a decline in market value is other-than-temporary, various factors are considered, including the cause, duration of time and severity of the impairment, any adverse changes in the investees’ financial condition, and the Company’s intent and ability to hold the security for a period of time sufficient to allow for an anticipated recovery in market value. To date, there have been no declines in value deemed to be other than temporary for any of our investments in marketable securities.
Goodwill and Indefinite-Lived Intangible Asset	Goodwill and Indefinite-Lived Intangible Asset Goodwill and indefinite-lived intangible assets are reviewed for impairment at least annually in the fourth quarter, and more frequently if events or other changes in circumstances indicate that the carrying amount of the assets may not be recoverable. Impairment of goodwill and indefinite-lived intangibles is determined to exist when the fair value is less than the carrying value of the net assets being tested. Goodwill The difference between the purchase price and the fair value of assets acquired and liabilities assumed in a business combination is allocated to goodwill. Goodwill is evaluated for impairment on an annual basis as of October 1, and more frequently if indicators are present or changes in circumstances suggest that impairment may exist. In performing each annual impairment assessment and any interim impairment assessment, the Company determines if it should qualitatively assess whether it is more likely than not that the fair value of goodwill is less than its carrying amount (the qualitative impairment test). If the Company concludes it is more likely than not that the fair value of the reporting unit is less than its carrying amount, or elect not to use the qualitative impairment test, a quantitative impairment test is performed The Company annual or interim quantitative impairment testing is performed by comparing the estimated As of October 1, 2020 the Company has determined through its quantitative impairment test that the fair value of its goodwill significantly exceeded the carrying value of its single reporting unit and concluded that goodwill was not impaired. In November 2020, after the Company’s public announcement that it became clear that patients who stopped therapy in Study 211 had not achieved meaningful sustained virologic response rates most of the patients had relapsed, the Company’s stock price declined significantly. Due to a sustained decline in the Company’s stock price during the remainder of the fourth quarter of 2020, the Company determined these factors were an indication of a triggering event of impairment and an interim goodwill impairment test was performed as of December 31, 2020. However, the interim quantitative impairment test still determined the fair value, when considering a control premium based on a range of recent acquisitions of entities similar to the Company which were made on a non-minority basis, exceeded the carrying value of its single reporting unit, and concluded that goodwill was still not impaired. Indefinite-Lived Intangible Asset The Company’s indefinite-lived intangible asset consists of IPR&D acquired in a business combination that are used in research and development activities but have not yet reached technological feasibility, regardless of whether they have alternative future use. The primary basis for determining the technological feasibility or completion of these projects is obtaining regulatory approval to market the underlying products in an applicable geographic region. The Company classifies IPR&D acquired in a business combination as an indefinite-lived intangible asset until the completion or abandonment of the associated research and development efforts. Upon completion of the associated research and development efforts, the Company performed a final test for impairment and will determine the useful life of the technology and begin amortizing the assets to reflect their use over their remaining lives. Upon permanent abandonment, the Company would write-off the remaining carrying amount of the associated IPR&D intangible asset. Indefinite-lived intangible assets are not amortized, but instead are reviewed for impairment at least annually, or more frequently if events occur or circumstances change that would indicate the carrying amount may be impaired. In performing each annual impairment assessment and any interim impairment assessment, the Company determines if it should qualitatively assess whether it is more likely than not that the fair value of its IPR&D asset is less than its carrying amount (the qualitative impairment test). If the Company concludes that is the case, or elect not to use qualitative impairment test, the Company would proceed with quantitatively determining the fair value of the IPR&D asset and comparing its fair value to its carrying value to determine the amount of impairment, if any (the quantitative impairment test). When performing the quantitative impairment assessment, the Company uses the income approach to determine the fair value of its IPR&D asset. This approach calculates fair value by estimating the after-tax cash flows attributable to an in-process project over its useful life and then discounting these after-tax cash flows back to a present value. This estimate includes judgmental assumptions regarding the estimates that market participants would make in evaluating the IPR&D asset, including the probability of successfully completing clinical trials and obtaining regulatory approval to market the IPR&D asset, the timing of and the expected costs to complete IPR&D projects, future net cash flows from potential drug sales, which are based on estimates of the sales price of the drug, the size of the patient population and cure rate, our competitive position in the marketplace, and appropriate discount and tax rates. Any impairment to be recorded is calculated as the difference between the fair value of the IPR&D asset as of the date of the assessment with the carrying value of the IPR&D asset on its consolidated balance sheet. In performing the qualitative impairment test, the Company considers the results of the most recent quantitative impairment test and identifies the most relevant drivers of the fair value for the IPR&D asset. The most relevant drivers of fair value identified are consistent with the assumptions used in the quantitative estimate of the IPR&D asset discussed below. Using these drivers of fair value, the Company identifies events and circumstances that may have an effect on the fair value of the IPR&D asset since the last time the IPR&D’s fair value was quantitatively determined. The Company then weighs these factors to determine and conclude if it is not more likely than not that the IPR&D asset is impaired. If it is more likely than not that the IPR&D asset is impaired, the Company proceeds with quantitatively determining the fair value of the IPR&D asset. For the Company’s 2020 impairment test, the Company performed a qualitative test and concluded it was more-likely-than-not that the fair value of its IPR&D asset exceeded its carrying value and no further testing was required. This was based on a decrease in the probability of success based on the impact of the Study 211 and dual combination VBR and NrtI therapy’s ability to serve as a finite and curative therapy for chronic HBV infection offset by an increase in the probability of success of 2158 and 3733 based on their advancement into Phase 2 and Phase 1 trials during 2020, respectively and the significance of the future net cash flows from potential drug sales for a finite and curative therapy for chronic HBV infection as primarily driven by the number of patients who will be diagnosed and treated and the Company’s competitive position in the marketplace. The Company did not recognize any IPR&D impairment in any of the periods presented .
Leases	Leases All of the Company’s leases are operating leases for facilities and equipment. Prior to January 1, 2019, the Company recognized related rent expense on a straight-line basis over the term of the lease. Incentives granted under the Company’s facilities lease, including allowances for leasehold improvements and rent holidays, were recognized as reductions to rental expense on a straight-line basis over the term of the lease. Deferred rent consisted of the difference between cash payments and the rent expense recognized. Subsequent to the adoption of the new leasing standard on January 1, 2019, the Company recognizes a lease asset for its right to use the underlying asset and a lease liability for the corresponding lease obligation. The Company determines whether an arrangement is or contains a lease at contract inception. Operating leases with a duration greater than one year are included in operating lease right-of-use assets, operating lease liabilities - short-term, and operating lease liabilities - long-term in the Company’s consolidated balance sheets. Operating lease right-of-use assets and liabilities are recognized at the lease commencement date based on the present value of lease payments over the lease term. In determining the net present value of lease payments, the Company uses its incremental borrowing rate based on the information available at the lease commencement date. The incremental borrowing rate represents the interest rate the Company would incur at lease commencement to borrow an amount equal to the lease payments on a collateralized basis over the term of a lease. The Company considers a lease term to be the noncancelable period that it has the right to use the underlying asset, including any periods where it is reasonably assured the Company will exercise the option to extend the contract. Periods covered by an option to extend are included in the lease term if the lessor controls the exercise of that option. The operating lease right-of-use assets also include any lease payments made and exclude lease incentives. Lease expense is recognized on a straight-line basis over the expected lease term. Variable lease expenses are recorded when incurred. The Company has elected not to separate lease and non-lease components for its leased assets and accounts for all lease and non-lease components of its agreements as a single lease component.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets The Company monitors the carrying value of long-lived assets, including ROU operating lease assets, for potential impairment and tests the recoverability of such assets whenever events or changes in circumstances indicate that the carrying amounts may not be recoverable. If a change in circumstance occurs, the Company performs a test of recoverability by comparing the carrying value of the asset or asset group to its undiscounted expected future cash flows. If cash flows cannot be separately and independently identified for a single asset, the Company will determine whether impairment has occurred for the group of assets for which the Company can identify the projected cash flows. If the carrying values are in excess of undiscounted expected future cash flows, the Company measures any impairment by comparing the fair value of the asset or asset group to its carrying value. In conjunction with the decision to wind down the Microbiome program in December 2020, the Company evaluated its ROU assets and property and equipment used in the Microbiome program for impairment. The Company determined the carrying value was no longer recoverable and, based on a fair value of the assets determined from market quotes, recorded an impairment loss of $0.7 million included in research and development expenses. The Company did not recognize any impairment on its long-lived assets in 2019 or 2018.
Property and Equipment, Net	Property and Equipment, Net Property and equipment are stated at cost and consist of lab and office equipment, leasehold improvements and computer hardware and software. The Company records depreciation under the straight-line method over the estimated useful lives of its property and equipment ranging from three to seven years. Leasehold improvements are amortized over the remaining terms of the respective leases or the estimated useful life of the leasehold improvements, whichever is less. Maintenance and repair costs are expensed as incurred.
Fair Value Measurements	Fair Value Measurements The Company follows accounting guidance on fair value measurements for financial instruments measured on a recurring basis, as well as for certain assets and liabilities that are initially recorded at their estimated fair values. Fair value is defined as the exit price, or the amount that would be received from selling an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. The Company uses the following three-level hierarchy that maximizes the use of observable inputs and minimizes the use of unobservable inputs to value its financial instruments: Level 1: Observable inputs such as unadjusted quoted prices in active markets for identical instruments. Level 2: Quoted prices for similar instruments that are directly or indirectly observable in the marketplace. Level 3: Significant unobservable inputs which are supported by little or no market activity and that are financial instruments whose values are determined using pricing models, discounted cash flow methodologies, or similar techniques, as well as instruments for which the determination of fair value requires significant judgment or estimation. Financial instruments measured at fair value are classified in their entirety based on the lowest level of input that is significant to the fair value measurement. The assessment of the significance of a particular input to the fair value measurement in its entirety requires us to make judgments and consider factors specific to the asset or liability. The use of different assumptions and/or estimation methodologies may have a material effect on estimated fair values. Accordingly, the fair value estimates disclosed or initial amounts recorded may not be indicative of the amount that the Company or holders of the instruments could realize in a current market exchange. The carrying amounts of cash equivalents and marketable securities approximate their fair value based upon quoted market prices. Certain of the Company’s financial instruments are not measured at fair value on a recurring basis, but are recorded at amounts that approximate their fair value due to their liquid or short-term nature, such as cash, accounts receivable, accounts payable and accrued expenses. The following tables present the fair value of the Company’s financial assets measured at fair value on a recurring basis using the above input categories (in thousands): December 31, 2020 Level 1 Level 2 Level 3 Estimated Fair Value Cash equivalents Money market funds $ 47,553 $ — $ — $ 47,553 U.S. and foreign commercial paper — 6,498 — 6,498 Total cash equivalents 47,553 6,498 — 54,051 Short-term investments U.S. and foreign corporate debt securities — 16,939 — 16,939 Asset-backed securities — 12,675 — 12,675 U.S. treasury securities — 23,999 — 23,999 U.S. and foreign commercial paper — 103,356 — 103,356 Total short-term investments — 156,969 — 156,969 Total assets measured at fair value $ 47,553 $ 163,467 $ — $ 211,020 December 31, 2019 Level 1 Level 2 Level 3 Estimated Fair Value Cash equivalents Money market fund $ 33,095 $ — $ — $ 33,095 U.S. and foreign corporate debt securities — 4,999 — 4,999 U.S. and foreign commercial paper — 4,484 — 4,484 Total cash equivalents 33,095 9,483 — 42,578 Short-term investments U.S. and foreign corporate debt securities — 72,486 — 72,486 Asset-backed securities — 34,025 — 34,025 U.S. treasury securities — 44,714 — 44,714 U.S. and foreign commercial paper — 76,086 — 76,086 Total short-term investments — 227,311 — 227,311 Total assets measured at fair value $ 33,095 $ 236,794 $ — $ 269,889 Money market funds are highly liquid and actively traded marketable securities that generally transact at a stable $1.00 net asset value representing its estimated fair value. The Company estimates the fair value of its U.S. and foreign corporate debt securities, asset backed securities, U.S. treasury securities and U.S. and foreign commercial paper by taking into consideration valuations obtained from third-party pricing services. The pricing services utilize industry standard valuation models, including both income and market-based approaches, for which all significant inputs are observable, either directly or indirectly, to estimate fair value. These inputs include reported trades of and broker/dealer quotes on the same or similar securities, issuer credit spreads; benchmark securities; prepayment/default projections based on historical data; and other observable inputs. There have been no transfers between Level 1, Level 2 or Level 3 for any of the periods presented. See Note 3 for further information regarding the carrying value of our investments in marketable securities.
Revenue Recognition and Accounts Receivable From Collaborations	Revenue Recognition and Accounts Receivable from Collaborations The Company analyzes its collaboration arrangements to assess whether such arrangements, or transactions between arrangement participants, involve joint operating activities performed by parties that are both active participants in the activities and exposed to significant risks and rewards dependent on the commercial success of such activities or are more akin to a vendor-customer relationship. In making this evaluation, the Company considers whether the activities of the collaboration are considered to be distinct and deemed to be within the scope of the collaborative arrangement accounting standard and those that are more reflective of a vendor-customer relationship and, therefore, within the scope of the revenue with contracts with customers accounting standard. This assessment is performed throughout the life of the arrangement based on changes in the responsibilities of all parties in the arrangement. For elements of collaboration arrangements that are not accounted for pursuant to the revenue from contracts with customers accounting standard, an appropriate recognition method is determined and applied consistently, generally by analogy to the revenue from contracts with customers accounting standard. Amounts related to transactions with a counterparty in a collaborative arrangement that is not a customer are presented as collaboration revenue and on a separate line item from revenue recognized from contracts with customers, if any, in the Company’s consolidated statements of operations and comprehensive loss. Under certain collaborative arrangements, the Company has been reimbursed for a portion of its research and development expenses or participates in the cost-sharing of such research and development expenses. Such reimbursements and cost-sharing arrangements are reflected as a reduction of research and development expense in the Company’s consolidated statements of operations and comprehensive loss, as the Company does not consider performing these activities for reimbursement to be a part of its ongoing major or central operations. For arrangements or transactions between arrangement participants determined to be within the scope of the contracts with customers accounting standard, the Company evaluates the term of the arrangement and recognizes revenue when the customer obtains control of promised goods or services in a contract for an amount that reflects the consideration the Company expects to receive in exchange for those goods or services. For contracts with customers, the Company applies the following five-step model in order to determine this amount: (1) identification of the promised goods or services in the contract; (2) determination of whether the promised goods or services are performance obligations, including whether they are distinct in the context of the contract; (3) measurement of the transaction price, including the constraint on variable consideration; (4) allocation of the transaction price to the performance obligations; and (5) recognition of revenue when (or as) the Company satisfies each performance obligation. The Company has provided standard indemnification and protection of licensed intellectual property for its customer. These provisions are part of assurance that the licenses meet the agreements, representations and are not obligations to provide goods or services. The Company only applies the five-step model to contracts when it is probable the Company will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. As part of the accounting for contracts with customers, the Company must develop assumptions that require judgment to determine the standalone selling price of each performance obligation identified in the contract. The Company then allocates the total transaction price to each performance obligation based on the estimated standalone selling prices of each performance obligation. The Company recognizes the amount of the transaction price that is allocated to the respective performance obligation when the performance obligation is satisfied or as it is satisfied as revenue. Upfront License Fees If a license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company recognizes revenues from nonrefundable, upfront license fees based on the relative value prescribed to the license compared to the total value of the arrangement. The revenue is recognized when the license is transferred to the collaborator and the collaborator is able to use and benefit from the license. For licenses that are not distinct from other obligations identified in the arrangement, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time. If the combined performance obligation is satisfied over time, the Company applies an appropriate method of measuring progress for purposes of recognizing revenue from nonrefundable, upfront license fees. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Development and Regulatory Milestone Payments Depending on facts and circumstances, the Company may record revenues from certain milestones in a reporting period before the milestone is achieved if the Company concludes achievement of the milestone is probable and recognition of revenue related to the milestone will not result in a significant reversal in amounts recognized in future periods. The Company records a corresponding contract asset when this conclusion is reached. Milestone payments that have not been included in the transaction price to date are fully constrained. The Company re-evaluates the probability of achievement of such milestones and any related constraint each reporting period. The Company adjusts its estimate of the overall transaction price, including the amount of collaborative revenue that was recorded, if necessary. Research and Development Service Payments Under the Research, Development, Collaboration and License Agreement (Allergan Agreement) with Allergan Pharmaecuticals International Limited (Allergan), the Company was reimbursed at a certain percentage for performing research and development services based on hours worked by the Company’s employees, at a fixed contractual rate per hour, and third-party pass-through costs the Company incurred on a quarterly basis. Research and development service payments were included in the transaction price in the reporting period the Company concluded it was probable that recording revenue in the period would not result in a significant reversal in amounts recognized in future periods. Accounts receivable were recorded when the right to the research and development service payment consideration became unconditional. The Company recorded the full reimbursed portion of these expenses as collaboration revenue associated with the Allergan Agreement in its consolidated statements of operations as the Company consider performing research and development services to be a part of its ongoing and central operations. Sales-based Milestone and Royalty Payments The Company’s customer may be required to pay the Company sales-based milestone payments or royalties on future sales of commercial products. The Company recognizes revenues related to sales-based milestone and royalty payments upon the later to occur of (i) achievement of the collaborator’s underlying sales or (ii) satisfaction of any performance obligation(s) related to these sales, in each case assuming the Company’s licensed intellectual property is deemed to be the predominant item to which the sales-based milestones and/or royalties relate. The Company receives payments from its customer based on billing schedules established in each contract. Upfront payments and fees are recorded as deferred revenue upon receipt or when due until the Company performs its obligations under the arrangement. If the related performance obligation is expected to be satisfied within the next twelve months, these amounts will be classified in current liabilities. The Company recognizes a contract asset relating to its conditional right to consideration that is not subject to a constraint. Amounts are recorded as accounts receivable when the Company’s right to consideration is unconditional. A net contract asset or liability is presented for each contract with a customer. The Company does not assess whether a contract has a significant financing component if the expectation at contract inception is such that the period between payment by the customer and the transfer of the promised goods or services to the customer will be one year or less. At December 31, 2020 and 2019, all accounts receivable are deemed collectible.
Stock-Based Compensation	Stock-Based Compensation The Company measures stock-based compensation to employees, consultants, and Board members at fair value on the grant date of the award. The fair value of RSUs is determined based on the number of shares granted and the quoted market price of the Company’s common stock on the date of grant. Compensation cost is recognized as expense on a straight-line basis over the requisite service period of the award. Stock-based awards with graded vesting schedules are recognized using the accelerated attribution method on a straight-line basis over the requisite service period for each separately vesting portion of the award. For awards that have a performance condition, compensation cost is measured based on the fair value of the award on the grant date, the date performance targets are established, and is expensed over the requisite service period for each separately vesting tranche when achievement of the performance condition becomes probable. The Company assesses the probability of the performance conditions being met on a continuous basis. Forfeitures are recognized when they occur. The Company estimates the fair value of stock option grants that do not contain market-based vesting conditions using the Black-Scholes option pricing model . T he assumptions used in estimating the fair value of these awards, such as expected term, expected dividend yield , volatility and risk-free interest rate, represent management’s best estimates and involve inherent uncertainties and the application of management’s judgment. The Company is also required to make estimates as to the probability of achieving the specific performance conditions. If actual results are not consistent with the Company’s assumptions and judgments used in making these estimates, the Company may be required to increase or decrease compensation expense, which could be material to the Company’s consolidated results of operations. Prior to January 1, 2019, the Company remeasured the fair value of the non-employee awards at each reporting period prior to vesting and finally at the vesting date of the award. Changes in the estimated fair value of these non-employee awards were recognized as compensation expense in the period of change. After January 1, 2019, the Company recognizes non-employee compensation costs over the requisite service period based on a measurement of fair value for each stock award.
Research and Development Expense and Accruals	Research and Development Expense and Accruals Research and development costs include personnel-related costs, outside contracted services including clinical study costs, facilities costs, fees paid to consultants, milestone payments prior to FDA approval, license fees prior to FDA approval, professional services, travel costs, dues and subscriptions, depreciation and materials used in clinical trials and research and development and costs incurred under the Collaboration Agreement. Research and development costs are expensed as incurred unless there is an alternative future use in other research and development projects. Payments made prior to the receipt of goods or services to be used in research and development are capitalized until the goods or services are received. Such payments are evaluated for current or long-term classification based on when they will be realized or consumed. Assets acquired as part of an asset acquisition that are used in research and development or are IPR&D are immediately expensed as research and development unless there is an alternative future use in other research and development projects. The Company records expenses related to clinical studies and manufacturing development activities based on its estimates of the services received and efforts expended pursuant to contracts with multiple contract research organizations (CROs) and manufacturing vendors that conduct and manage these activities on its behalf. The financial terms of these agreements are subject to negotiation, vary from contract to contract, and may result in uneven payment flows. There may be instances in which payments made to the Company’s vendors will exceed the level of services provided and result in a prepayment of the expense. Payments under some of these contracts depend on factors such as the successful enrollment of subjects and the completion of clinical study milestones. In amortizing or accruing service fees, the Company estimates the time period over which services will be performed, enrollment of subjects, number of sites activated and the level of effort to be expended in each period. If the actual timing of the performance of services or the level of effort varies from the Company’s estimate, the Company will adjust the accrued or prepaid expense balance accordingly. To date, there have been no material differences from the Company’s estimates to the amounts actually incurred. The Company has entered and may continue to enter into license agreements to access and utilize certain technology. In each case, the Company evaluates if the license agreement results in the acquisition of an asset or a business. To date, none of the Company’s license agreements have been considered to be acquisitions of businesses. For asset acquisitions, the upfront payments to acquire such licenses, as well as any future milestone payments, are immediately recognized as research and development expense when paid, provided there is no alternative future use of the rights in other research and development projects. These license agreements may also include contingent consideration in the form of cash payments to be made for future milestone events. The Company assesses whether such contingent consideration meets the definition of a derivative and to date the Company has determined that such contingent consideration are not derivatives.
Restructurings Charges	Restructuring Charges The Company recognizes restructuring charges related to reorganization plans that have been committed to by management and when liabilities have been incurred. In connection with these activities, the Company records restructuring charges at fair value for (1) contractual employee termination benefits when obligations are associated to services already rendered, rights to such benefits have vested, and payment of benefits is probable and can be reasonably estimated, (2) one-time employee termination benefits when management has committed to a plan of termination, the plan identifies the employees and their expected termination dates, the details of termination benefits are complete, it is unlikely changes to the plan will be made or the plan will be withdrawn and communication to such employees has occurred, and (3) contract termination costs when a contract is terminated before the end of its term. One-time employee termination benefits are recognized in their entirety when communication has occurred, and future services are not required. If future services are required, the costs are recorded ratably over the remaining period of service. Contract termination costs to be incurred over the remaining contract term without economic benefit are recorded in their entirety when the contract is canceled. The recognition of restructuring charges requires the Company to make certain judgments and estimates regarding the nature, timing and amount of costs associated with the planned reorganization plan. To the extent the Company’s actual results differ from its estimates and assumptions, the Company may be required to revise the estimates of future accrued restructuring liabilities, requiring the recognition of additional restructuring charges or the reduction of accrued restructuring liabilities already recognized. Such changes to previously estimated amounts may be material to the consolidated financial statements. Changes in the estimates of the restructuring charges are recorded in the period the change is determined. At the end of each reporting period, the Company evaluates the remaining accrued restructuring balances to ensure that no excess accruals are retained, and the utilization of t he provisions are for their intended purpose in accordance with developed restructuring plans.
Variable Interest Entities	Variable Interest Entities The Company reviews agreements it enters into with third party entities, pursuant to which we may have a variable interest in the entity, in order to determine if the entity is a variable interest entity (VIE). If the entity is a VIE, the Company assesses whether or not it is the primary beneficiary of that entity. In determining whether the Company is the primary beneficiary of an entity, the Company applies a qualitative approach that determines whether it has both (1) the power to direct the economically significant activities of the entity and (2) the obligation to absorb losses of, or the right receive benefits from, the entity that could potentially be significant to that entity. If the Company were to determine it is the primary beneficiary of a VIE, the Company would consolidate the statements of operations and financial condition of the VIE into its consolidated financial statements. The Company’s determination about whether it should consolidate such VIEs is made continuously as changes to existing relationships or future transactions may result in a consolidation event. As of December 31, 2020, the Company did not consolidate any entities it had determined to be VIEs.
Income Taxes	Income Taxes The Company records income taxes using the asset and liability method. Deferred income tax assets and liabilities are recognized for the future tax effects attributable to temporary differences between the financial statement carrying amounts of existing assets and liabilities and their respective income tax bases, and operating loss and tax credit carryforwards. The effect of a change in tax rates on deferred tax assets and liabilities is recognized in income in the period that includes the enactment date. The Company establishes a valuation allowance if it is more likely than not that the deferred tax assets will not be realized based on an evaluation of objective verifiable evidence. For tax positions that are more likely than not of being sustained upon audit, the Company recognizes the largest amount of the benefit that is greater than 50% likely of being realized. For tax positions that are not more likely than not of being sustained upon audit, the Company does not recognize any portion of the benefit. The Company recognizes and measures uncertain tax positions using a two-step approach set forth in authoritative guidance. The first step is to evaluate the tax position taken or expected to be taken by determining whether the weight of available evidence indicates that it is more likely than not that the tax position will be sustained in an audit, including resolution of any related appeals or litigation processes. The second step is to measure the tax benefit as the largest amount that is more than 50% likely to be realized upon ultimate settlement. Significant judgment is required to evaluate uncertain tax positions. The Company evaluates uncertain tax positions on a regular basis. The evaluations are based on a number of factors, including changes in facts and circumstances, changes in tax law, correspondence with tax authorities during the course of the audit, and effective settlement of audit issues. The provision for income taxes includes the effects of any accruals that the Company believes are appropriate. It is the Company’s policy to recognize interest and penalties related to income tax matters in income tax expense. No interest or penalties related to uncertain tax positions has been incurred or accrued for any periods presented. In March 2020, the Families First Coronavirus Response Act (FFCR Act) and the Coronavirus Aid, Relief, and Economic Security Act (CARES Act) were each enacted in response to the COVID-19 pandemic. The FFCR Act and the CARES Act contain numerous income tax provisions relating to refundable payroll tax credits, deferment of employer side social security payments, net operating loss carryback periods, alternative minimum tax credit refunds, modifications to the net interest deduction limitations and technical corrections to tax depreciation methods for qualified improvement property. In June 2020, Assembly Bill 85 (A.B. 85) was signed into California law. A.B. 85 provides for a three-year suspension of the use of net operating losses for medium and large businesses and a three-year cap on the use of business incentive tax credits to offset no more than $5.0 million of tax per year. A.B. 85 suspends the use of net operating losses for taxable years 2020, 2021 and 2022 for certain taxpayers with taxable income of $1.0 million or more. The carryover period for any net operating losses that are suspended under this provision will be extended. A.B. 85 also requires that business incentive tax credits including carryovers may not reduce the applicable tax by more than $5.0 million for taxable years 2020, 2021 and 2022. In December 2020, the Consolidated Appropriations Act, 2021 (CAA) was signed into law. The CAA included additional funding through tax credits as part of its economic package for 2021. The FFCR Act, CARES Act, A.B. 85 did and CAA not have a material impact on the Company’s consolidated financial statements; however, the Company continues to examine the impacts the FFCR Act, CARES Act, A.B. 85 and CAA may have on its business, results of operations, financial condition and liquidity.
Net Loss per Share	Net Loss per Share Basic net loss per common share excludes dilution and is computed by dividing net loss by the weighted average number of common shares outstanding during the period. Diluted net loss per common share reflects the potential dilution that could occur if securities or other contracts to issue common stock were exercised or converted into common stock or resulted in the issuance of common stock that then shared in the earnings of the entity unless inclusion of such shares would be anti-dilutive. Since the Company has only incurred losses, basic and diluted net loss per share is the same. In December 2019, the Company sold 6,287,878 shares of common stock as well as pre-funded warrants to purchase up to 2,424,242 shares of common stock (see Note 7). The pre-funded warrants are exercisable for shares of common stock at a price of $0.001 per share. The shares of common stock into which the pre-funded warrants may be exercised are considered outstanding for the purposes of computing earnings per share because the shares may be issued for little or no consideration, they are fully vested, and are exercisable after the original issuance date. Securities excluded from the computation of diluted loss from per share because including them would have been antidilutive are as follows: Year Ended December 31, 2020 2019 2018 Warrants to purchase common stock — 15,296 15,296 Options to purchase common stock 6,696,592 5,613,353 4,637,145 Common stock subject to purchase under our ESPP 44,223 11,342 21,483 Unvested RSUs 746,868 630,384 568,005 Total 7,487,683 6,270,375 5,241,929
Comprehensive Loss	Comprehensive Loss Comprehensive loss is comprised of net loss and adjustments for the change in unrealized gains and losses on investments in available-for-sale marketable securities. The Company displays comprehensive loss and its components in the consolidated statements of operations and comprehensive loss, net of tax effects if any.
Concentrations of Risk	Concentrations of Risk Credit Risk Financial instruments which potentially subject the Company to credit risk consist primarily of cash, cash equivalents and marketable securities. The Company holds these investments in highly rated financial institutions, and, by policy, limits the amounts of credit exposure to any one financial institution. These amounts at times may exceed federally insured limits. The Company has not experienced any credit losses in such accounts and does not believe it is exposed to any significant credit risk on these funds. The Company has no off-balance sheet concentrations of credit risk, such as foreign currency exchange contracts, option contracts or other hedging arrangements. Supplier Risk Certain materials and key components the Company utilizes in its operations are obtained through single suppliers. Since the suppliers of key components and materials must be named in a New Drug Application (NDA) filed with the FDA for a product
Adoption of Recent Accounting Pronouncements	Adoption of Recent Accounting Pronouncements In January 2017, the Financial Accounting Standards Board (FASB) issued ASU 2017-04, Intangibles-Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment Financial Instruments – Credit Losses (Topic 326), Derivatives and Hedging (Topic 815), and Leases (Topic 842): Effective Dates On January 1, 2020, the Company adopted ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement, On January 1, 2020, the Company adopted ASU 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 In December 2019, the FASB issued ASU No. 2019-12, Simplifying the Accounting for Income Taxes Income Taxes (Topic 740) Accounting Pronouncements to Be Adopted In June 2016, the FASB issued ASU 2016-13, Financial Instruments - Credit Losses: Measurement of Credit Losses on Financial Instruments In April, May and November 2019, the FASB issued additional amendments to the new accounting standard related to transition and clarification. Financial Instruments—Credit Losses (Topic 326), Derivatives and Hedging (Topic 815), and Leases (Topic 842): Effective Dates In August 2020, the FASB issued ASU 2020-06, Debt—Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging—Contracts in Entity’s Own Equity (Subtopic 815-40): Accounting for Convertible Instruments and Contracts in an Entity’s Own Equity The ASU is effective for interim and annual periods beginning after December 15, 2021, with early adoption permitted after December 15, 2020. Adoption of the ASU can either be on a modified retrospective or full retrospective basis. The Company is currently evaluating the impacts of ASU 2020-06 on its consolidated financial statements and related disclosures. In October 2020, the FASB issued ASU 2020-10, Codification Improvements – Disclosures. This ASU improves consistency by amending the codification to include all disclosure guidance in the appropriate disclosure sections and clarifies application of various provisions in the Codification by amending and adding new headings, cross referencing to other accounting standards, and refining or correcting termination. This ASU is effective for fiscal years beginning after December 15, 2020. The adoption of the ASU will not impact the Company’s consolidated financial statements or related disclosures.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies and Recent Accounting Pronouncements (Tables)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Schedule of Fair Value of Financial Assets	The following tables present the fair value of the Company’s financial assets measured at fair value on a recurring basis using the above input categories (in thousands): December 31, 2020 Level 1 Level 2 Level 3 Estimated Fair Value Cash equivalents Money market funds $ 47,553 $ — $ — $ 47,553 U.S. and foreign commercial paper — 6,498 — 6,498 Total cash equivalents 47,553 6,498 — 54,051 Short-term investments U.S. and foreign corporate debt securities — 16,939 — 16,939 Asset-backed securities — 12,675 — 12,675 U.S. treasury securities — 23,999 — 23,999 U.S. and foreign commercial paper — 103,356 — 103,356 Total short-term investments — 156,969 — 156,969 Total assets measured at fair value $ 47,553 $ 163,467 $ — $ 211,020 December 31, 2019 Level 1 Level 2 Level 3 Estimated Fair Value Cash equivalents Money market fund $ 33,095 $ — $ — $ 33,095 U.S. and foreign corporate debt securities — 4,999 — 4,999 U.S. and foreign commercial paper — 4,484 — 4,484 Total cash equivalents 33,095 9,483 — 42,578 Short-term investments U.S. and foreign corporate debt securities — 72,486 — 72,486 Asset-backed securities — 34,025 — 34,025 U.S. treasury securities — 44,714 — 44,714 U.S. and foreign commercial paper — 76,086 — 76,086 Total short-term investments — 227,311 — 227,311 Total assets measured at fair value $ 33,095 $ 236,794 $ — $ 269,889
Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share	Securities excluded from the computation of diluted loss from per share because including them would have been antidilutive are as follows: Year Ended December 31, 2020 2019 2018 Warrants to purchase common stock — 15,296 15,296 Options to purchase common stock 6,696,592 5,613,353 4,637,145 Common stock subject to purchase under our ESPP 44,223 11,342 21,483 Unvested RSUs 746,868 630,384 568,005 Total 7,487,683 6,270,375 5,241,929

Investments in Marketable Sec_2

Investments in Marketable Securities (Tables)	12 Months Ended
	Dec. 31, 2020
Investments Debt And Equity Securities [Abstract]
Schedule of Investments in Marketable Available-for-Sale Securities	Investments in marketable available-for-sale securities consisted of the following (in thousands): December 31, 2020 Amortized Cost Gross Unrealized Gain (1) Gross Unrealized Loss (1) Estimated Fair Value Cash equivalents Money market funds $ 47,553 $ — $ — $ 47,553 U.S. and foreign commercial paper 6,498 — — 6,498 Total cash equivalents 54,051 — — 54,051 Short-term investments U.S. and foreign corporate debt securities 16,939 3 (3 ) 16,939 Asset-backed securities 12,674 2 (1 ) 12,675 U.S. treasury securities 23,997 2 — 23,999 U.S. and foreign commercial paper 103,356 — — 103,356 Total short-term investments 156,966 7 (4 ) 156,969 Total cash equivalents and investments $ 211,017 $ 7 $ (4 ) $ 211,020 December 31, 2019 Amortized Cost Gross Unrealized Gain (1) Gross Unrealized Loss (1) Estimated Fair Value Cash equivalents Money market funds $ 33,095 $ — $ — $ 33,095 U.S. and foreign corporate debt securities 5,000 — (1 ) 4,999 U.S. and foreign commercial paper 4,484 — — 4,484 Total cash equivalents 42,579 — (1 ) 42,578 Short-term investments U.S. and foreign corporate debt securities 72,452 38 (4 ) 72,486 Asset-backed securities 34,008 17 — 34,025 U.S. treasury securities 44,692 24 (2 ) 44,714 U.S. and foreign commercial paper 76,086 — — 76,086 Total short-term investments 227,238 79 (6 ) 227,311 Total cash equivalents and investments $ 269,817 $ 79 $ (7 ) $ 269,889 (1) Gross unrealized gain (loss) is pre-tax.

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 consist of the following (in thousands): As of December 31, 2020 2019 Lab equipment $ 247 $ 247 Office equipment 699 699 Leasehold improvement 2,490 2,084 Total property and equipment 3,436 3,030 Less: Accumulated depreciation (1,836 ) (1,200 ) Property and equipment, net $ 1,600 $ 1,830

Other Accrued Expenses (Tables)

Other Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2020
Payables And Accruals [Abstract]
Schedule of Other Accrued Expenses	Other accrued expenses consist of the following (in thousands): As of December 31, 2020 2019 Other accrued expenses: Accrued compensation $ 7,016 $ 5,312 Accrued restructuring charges 4,164 2,094 Accrued professional fees and other 807 880 Total other accrued expenses $ 11,987 $ 8,286

Restructurings (Tables)

Restructurings (Tables) - 2020 Restructuring	12 Months Ended
	Dec. 31, 2020
Restructuring Cost And Reserve [Line Items]
Summary of Estimates of Restructuring Costs	The following table summarizes the Company’s estimates of costs incurred and expected to be incurred (in thousands): Total Restructuring Cost Employee Severance and Related Benefits Asset Impairment and Other Costs Total estimated restructuring costs to be incurred $ 8,040 $ 6,461 $ 1,579 Restructuring costs incurred during the period: Total restructuring costs incurred for the year ended December 31, 2019 2,094 2,094 — Total restructuring costs incurred for the year ended December 31, 2020 5,684 4,105 1,579 Cumulative restructuring costs incurred through December 31, 2020 $ 7,778 $ 6,199 $ 1,579
Schedule of Accrued Restructuring Charges	The following table presents the activity in the accrued restructuring charges during the period, all of which are related to employee severance and related benefits (in thousands): Accrued balance as of January 1, 2019 $ — Costs incurred 2,094 Accrued balance as of December 31, 2019 $ 2,094 Costs incurred 3,843 Reductions for cash payments (1,773 ) Accrued balance as of December 31, 2020 $ 4,164
Schedule of Restructuring Charges	The following table presents where the restructuring charges were recognized (in thousands): Year Ended December 31, 2020 2019 Research and development $ 5,486 $ 433 General and administrative 198 1,661 Total $ 5,684 $ 2,094

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Summary of Warrants to Purchase Shares of Company's Common Stock	As of December 31, 2020, the following warrants to purchase shares of the Company’s common stock were issued and outstanding: Issue date Expiration date Exercise Price per Share December 31, 2020 December 31, 2019 September 10, 2010 September 10, 2020 $ 30.000 — 15,296 December 16, 2019 None $ 0.001 2,424,242 2,424,242 2,424,242 2,439,538

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Stock Option Activity and Related Information	The following table summarizes the stock option activity and related information for 2020: Number of Shares Weighted Average Exercise Price Per Share Weighted Average Remaining Contractual Term (Years) Total Intrinsic Value (in thousands) Outstanding as of December 31, 2019 5,613,353 $ 15.90 Granted 1,649,340 16.07 Exercised (175,579 ) 9.75 Forfeited (390,522 ) 22.81 Outstanding as of December 31, 2020 6,696,592 $ 15.70 6.9 $ 1,856 Exercisable as of December 31, 2020 3,926,732 $ 14.70 5.5 $ 1,813
Summary of Restricted Stock Units and Related Information	The following table summarizes RSU activity and related information for 2020: Number of RSU's Weighted Average Fair Value Per RSU at Grant Price Nonvested as of December 31, 2019 758,718 $ 25.47 Granted 536,164 16.21 Vested (297,133 ) 23.65 Forfeited (110,881 ) 26.46 Nonvested as of December 31, 2020 886,868 (1) $ 20.36 (1) Includes 140,000 RSUs that have vested but are subject to deferred settlement, which have a weighted average remaining contractual term of 2.2 years.
Summary of Assumptions Used to Estimate Fair Values of Stock Options Grants	A summary of the assumptions used to estimate the fair values of stock options grants for the years presented is as follows: Year Ended December 31, 2020 2019 2018 Exercise price $5.62 - $23.30 $9.31 - $23.04 $23.78 - $57.53 Expected volatility 66.4% - 92.3% 66.5% - 83.2% 75.6% - 86.1% Risk-free interest rate 0.26% - 1.44% 1.36% - 2.65% 2.56% - 3.04% Expected term (years) 5.5 - 7.5 5.5 - 7.5 5.5 - 7.0 Expected dividend yield 0% 0% 0%
Schedule of Recognized Stock-Based Compensation Expense	The Company recognized stock-based compensation expense included in the consolidated statement of operations and comprehensive loss for the periods presented (in thousands): Year Ended December 31, 2020 2019 2018 Research and development $ 11,380 $ 11,376 $ 11,820 General and administrative 10,473 (1) 9,182 (2) 16,665 Total stock-based compensation expense $ 21,853 $ 20,558 $ 28,485 (1) (2)

Collaboration Agreements (Table

Collaboration Agreements (Tables)	12 Months Ended
	Dec. 31, 2020
Collaboration Agreement [Abstract]
Schedule of Changes in Contract Liabilities	The following tables present changes in the Company’s contract liabilities (in thousands): Balance at Beginning of Period Additions Deductions Balance at End of Period Year Ended December 31, 2020 Contract liabilities: Deferred revenue $ 37,048 $ 40,000 $ (68,061 ) $ 8,987 Year Ended December 31, 2019 Contract liabilities: Deferred revenue $ 40,660 $ — $ (3,612 ) $ 37,048
Schedule of Collaboration Revenue Recognized with the Period	Year Ended December 31, 2020 2019 2018 Collaboration revenue recognized in the period from Amounts included in deferred revenue at the beginning of the period $ 37,048 $ 3,612 $ 5,125 Performance obligations satisfied in previous periods — — —

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Effective Income Tax Rate of Provision for Income Taxes	The effective tax rate of our provision for income taxes differs from the federal statutory rate as follows: As of December 31, 2020 2019 2018 Statutory federal income tax rate 21.0 % 21.0 % 21.0 % State taxes, net of federal tax benefit 6.1 5.6 7.1 Stock based compensation 0.7 (3.8 ) 6.7 Research and development tax credits 2.2 5.6 2.4 State rate change — (1.4 ) 2.5 Uncertain tax positions 2.2 (2.1 ) (4.8 ) Return to provision adjustments 4.3 (2.5 ) 4.4 Other 0.1 (0.2 ) 0.8 Change in valuation allowance (36.6 ) (21.4 ) (41.3 ) Income taxes provision (benefit) 0.0 % 0.8 % (1.2 )%
Components of Deferred Taxes	Significant components of the Company’s deferred taxes are as follows (in thousands): As of December 31, 2020 2019 Deferred tax assets: Federal and state-operating loss carryforwards $ 108,236 $ 81,584 Stock-based compensation 16,563 12,347 Intangible assets 1,463 1,774 Deferred revenue — 9,503 Operating lease liabilities 2,600 3,147 Research and development credits 8,872 7,499 Other 854 702 Total deferred tax assets 138,588 116,556 Valuation allowance (131,332 ) (108,577 ) Deferred tax asset, net of valuation allowance $ 7,256 $ 7,979 Deferred tax liabilities: In-process research and development $ (7,443 ) $ (7,439 ) Operating lease right-of-use assets (2,344 ) (3,071 ) Total deferred tax liabilities (9,787 ) (10,510 ) Net deferred tax liability $ (2,531 ) $ (2,531 )
Activity Related to Gross Unrecognized Tax Benefits	The following table summarizes activity related to the Company’s gross unrecognized tax benefits (in thousands): As of December 31, 2020 2019 2018 Balances as of beginning of year $ 6,070 $ 4,613 $ — Increases related to prior year tax positions — 15 3,679 Decreases related to prior year tax positions (4,162 ) (934 ) — Increases related to current year tax positions 747 2,376 934 Balances as of end of year $ 2,655 $ 6,070 $ 4,613

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2020
Lease Cost [Abstract]
Schedule of Quantitative Information of Operating Leases	The following summarizes quantitative information about the Company’s operating leases (in thousands): Year Ended December 31, 2020 2019 Lease cost Operating lease cost $ 5,214 $ 4,454 Short-term lease cost 401 609 Variable lease cost 1,468 1,193 Total lease cost $ 7,083 $ 6,256 Year Ended December 31, 2020 2019 Operating cash flows from operating leases $ 4,513 $ 4,269 Right-of-use assets exchanged for new operating lease liabilities $ 1,302 $ 15,261
Schedule of Maturities Operating Lease Liabilities	As of December 31, 2020, the maturities of the Company’s operating lease liabilities were as follows (in thousands): 2021 $ 4,369 2022 3,905 2023 3,502 Total 11,776 Less: present value discount (1,647 ) Operating lease liabilities $ 10,129

Selected Quarterly Financial _2

Selected Quarterly Financial Data (Tables)	12 Months Ended
	Dec. 31, 2020
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Information	The following table contains quarterly financial information for the four quarters of 2020 and 2019 which has been prepared in accordance with GAAP for interim financial information. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair statement of the information for the periods presented. 2020 Quarter Ended March 31 June 30 September 30 December 31 (in thousands except for per share amounts) Collaboration revenue $ 4,081 $ 39,376 $ 34,611 $ 1,037 Operating expenses $ 31,775 $ 32,797 $ 38,630 $ 40,679 Interest and other income $ 1,039 $ 691 $ 670 $ 224 Net income (loss) $ (26,655 ) $ 7,270 $ (3,349 ) $ (39,418 ) Unrealized gain (loss) from marketable securities, net of tax $ 115 $ 190 $ (262 ) $ (112 ) Basic net income (loss) per common share $ (0.76 ) $ 0.21 $ (0.09 ) $ (1.11 ) Diluted net income (loss) per common share $ (0.76 ) $ 0.19 $ (0.09 ) $ (1.11 ) 2019 Quarter Ended March 31 June 30 September 30 December 31 (in thousands except for per share amounts) Collaboration revenue $ 3,885 $ 3,080 $ 4,231 $ 4,767 Operating expenses $ 32,221 $ 22,780 $ 30,224 $ 33,451 Interest and other income $ 1,276 $ 1,182 $ 983 $ 859 Net loss $ (27,052 ) $ (18,503 ) $ (24,995 ) $ (27,084 ) Unrealized gain (loss) from marketable securities, net of tax $ 108 $ 52 $ (18 ) $ 4 Basic and diluted net loss per common share $ (1.05 ) $ (0.72 ) $ (0.96 ) $ (0.99 )

Nature of Business - Additional

Nature of Business - Additional Information (Details)	12 Months Ended
	Dec. 31, 2020segmentproduct
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Operating segments (in segments) | segment	1
FDA approved products (in products) | product	0

Summary of Significant Accoun_4

Summary of Significant Accounting Policies and Recent Accounting Pronouncements - Additional Information (Details) - USD ($)	1 Months Ended		12 Months Ended
	Dec. 31, 2019	Jul. 31, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Impairment loss			$ 700,000	$ 0	$ 0
Fair value assets transfers between level 1, level 2 or level 3	$ 0		0	0
Uncertain tax positions, income tax interest or penalties incurred			0	0	0
Uncertain tax positions, income tax interest or penalties accrued	$ 0		$ 0	$ 0	$ 0
ASU 2017-04
Change in Accounting Principle, Accounting Standards Update, Adoption Date			Jan. 1, 2020
Change in Accounting Principle, Accounting Standards Update, Adopted [true false]			true
Change in Accounting Principle, Accounting Standards Update, Immaterial Effect [true false]			true
ASU 2018-13
Change in Accounting Principle, Accounting Standards Update, Adoption Date			Jan. 1, 2020
Change in Accounting Principle, Accounting Standards Update, Adopted [true false]			true
Change in Accounting Principle, Accounting Standards Update, Immaterial Effect [true false]			true
ASU 2018-18
Change in Accounting Principle, Accounting Standards Update, Adoption Date			Jan. 1, 2020
Change in Accounting Principle, Accounting Standards Update, Adopted [true false]			true
Change in Accounting Principle, Accounting Standards Update, Immaterial Effect [true false]			true
ASU 2019-12
Change in Accounting Principle, Accounting Standards Update, Adoption Date			Jan. 1, 2020
Change in Accounting Principle, Accounting Standards Update, Adopted [true false]			true
Change in Accounting Principle, Accounting Standards Update, Immaterial Effect [true false]			true
Common Stock
Number of shares sold	6,287,878				4,600,000
Public Offering
Number of shares sold	6,287,878	4,600,000
Number of pre-funded warrants sold to purchase of common stock	2,424,242			2,424,242
Pre-funded warrants exercisable price per share	$ 0.001			$ 0.001
Maximum
Estimated useful lives of property and equipment			7 years
Minimum
Estimated useful lives of property and equipment			3 years

Summary of Significant Accoun_5

Summary of Significant Accounting Policies and Recent Accounting Pronouncements - Schedule of Fair Value of Financial Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Short-term investments
Short-term investments	$ 156,969	$ 227,311
Recurring
Cash equivalents
Total cash equivalents	54,051	42,578
Short-term investments
Short-term investments	156,969	227,311
Total assets measured at fair value	211,020	269,889
Recurring | U.S. and foreign commercial paper
Cash equivalents
Total cash equivalents	6,498	4,484
Short-term investments
Short-term investments	103,356	76,086
Recurring | U.S and foreign corporate debt securities
Cash equivalents
Total cash equivalents		4,999
Short-term investments
Short-term investments	16,939	72,486
Recurring | Asset-backed securities
Short-term investments
Short-term investments	12,675	34,025
Recurring | U.S. treasury securities
Short-term investments
Short-term investments	23,999	44,714
Recurring | Money market fund
Cash equivalents
Total cash equivalents	47,553	33,095
Recurring | Level 1
Cash equivalents
Total cash equivalents	47,553	33,095
Short-term investments
Short-term investments	0	0
Total assets measured at fair value	47,553	33,095
Recurring | Level 1 | U.S. and foreign commercial paper
Cash equivalents
Total cash equivalents	0	0
Short-term investments
Short-term investments	0	0
Recurring | Level 1 | U.S and foreign corporate debt securities
Cash equivalents
Total cash equivalents		0
Short-term investments
Short-term investments	0	0
Recurring | Level 1 | Asset-backed securities
Short-term investments
Short-term investments	0	0
Recurring | Level 1 | U.S. treasury securities
Short-term investments
Short-term investments	0	0
Recurring | Level 1 | Money market fund
Cash equivalents
Total cash equivalents	47,553	33,095
Recurring | Level 2
Cash equivalents
Total cash equivalents	6,498	9,483
Short-term investments
Short-term investments	156,969	227,311
Total assets measured at fair value	163,467	236,794
Recurring | Level 2 | U.S. and foreign commercial paper
Cash equivalents
Total cash equivalents	6,498	4,484
Short-term investments
Short-term investments	103,356	76,086
Recurring | Level 2 | U.S and foreign corporate debt securities
Cash equivalents
Total cash equivalents		4,999
Short-term investments
Short-term investments	16,939	72,486
Recurring | Level 2 | Asset-backed securities
Short-term investments
Short-term investments	12,675	34,025
Recurring | Level 2 | U.S. treasury securities
Short-term investments
Short-term investments	23,999	44,714
Recurring | Level 2 | Money market fund
Cash equivalents
Total cash equivalents	0	0
Recurring | Level 3
Cash equivalents
Total cash equivalents	0	0
Short-term investments
Short-term investments	0	0
Total assets measured at fair value	0	0
Recurring | Level 3 | U.S. and foreign commercial paper
Cash equivalents
Total cash equivalents	0	0
Short-term investments
Short-term investments	0	0
Recurring | Level 3 | U.S and foreign corporate debt securities
Cash equivalents
Total cash equivalents		0
Short-term investments
Short-term investments	0	0
Recurring | Level 3 | Asset-backed securities
Short-term investments
Short-term investments	0	0
Recurring | Level 3 | U.S. treasury securities
Short-term investments
Short-term investments	0	0
Recurring | Level 3 | Money market fund
Cash equivalents
Total cash equivalents	$ 0	$ 0

Summary of Significant Accoun_6

Summary of Significant Accounting Policies and Recent Accounting Pronouncements - Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive securities excluded from computation of loss per share	7,487,683	6,270,375	5,241,929
Warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive securities excluded from computation of loss per share		15,296	15,296
Options
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive securities excluded from computation of loss per share	6,696,592	5,613,353	4,637,145
Common stock subject to purchase under ESPP
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive securities excluded from computation of loss per share	44,223	11,342	21,483
RSUs
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive securities excluded from computation of loss per share	746,868	630,384	568,005

Investments in Marketable Sec_3

Investments in Marketable Securities - Schedule of Investments in Marketable Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Investments in marketable securities:
Amortized Cost	$ 59,444	$ 46,732
Estimated Fair Value	156,969	227,311
Amortized Cost	211,017	269,817
Gross Unrealized Gain	7	79
Gross Unrealized Loss	(4)	(7)
Estimated Fair Value	211,020	269,889
Cash equivalents
Investments in marketable securities:
Amortized Cost	54,051	42,579
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	(1)
Estimated Fair Value	54,051	42,578
Cash equivalents | Money market fund
Investments in marketable securities:
Amortized Cost	47,553	33,095
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Estimated Fair Value	47,553	33,095
Cash equivalents | U.S and foreign corporate debt securities
Investments in marketable securities:
Amortized Cost		5,000
Gross Unrealized Gain		0
Gross Unrealized Loss		(1)
Estimated Fair Value		4,999
Cash equivalents | U.S. and foreign commercial paper
Investments in marketable securities:
Amortized Cost	6,498	4,484
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Estimated Fair Value	6,498	4,484
Short-term investments
Investments in marketable securities:
Amortized Cost	156,966	227,238
Gross Unrealized Gain	7	79
Gross Unrealized Loss	(4)	(6)
Estimated Fair Value	156,969	227,311
Short-term investments | U.S and foreign corporate debt securities
Investments in marketable securities:
Amortized Cost	16,939	72,452
Gross Unrealized Gain	3	38
Gross Unrealized Loss	(3)	(4)
Estimated Fair Value	16,939	72,486
Short-term investments | Asset-backed securities
Investments in marketable securities:
Amortized Cost	12,674	34,008
Gross Unrealized Gain	2	17
Gross Unrealized Loss	(1)	0
Estimated Fair Value	12,675	34,025
Short-term investments | U.S. treasury securities
Investments in marketable securities:
Amortized Cost	23,997	44,692
Gross Unrealized Gain	2	24
Gross Unrealized Loss	0	(2)
Estimated Fair Value	23,999	44,714
Short-term investments | U.S. and foreign commercial paper
Investments in marketable securities:
Amortized Cost	103,356	76,086
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Estimated Fair Value	$ 103,356	$ 76,086

Investments in Marketable Sec_4

Investments in Marketable Securities - Additional Information (Details)	12 Months Ended
	Dec. 31, 2020USD ($)
Investments in marketable securities:
Long-term marketable securities	$ 0
Short-term investments
Investments in marketable securities:
Available for sale securities maturity term	less than one year

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment
Total property and equipment	$ 3,436	$ 3,030
Less: Accumulated depreciation	(1,836)	(1,200)
Property and equipment, net	1,600	1,830
Lab equipment
Property, Plant and Equipment
Total property and equipment	247	247
Office equipment
Property, Plant and Equipment
Total property and equipment	699	699
Leasehold improvement
Property, Plant and Equipment
Total property and equipment	$ 2,490	$ 2,084

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Abstract]
Depreciation	$ 0.7	$ 0.5	$ 0.6

Other Accrued Expenses - Schedu

Other Accrued Expenses - Schedule of Other Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Other accrued expenses:
Accrued compensation	$ 7,016	$ 5,312
Accrued restructuring charges	4,164	2,094
Accrued professional fees and other	807	880
Total other accrued expenses	$ 11,987	$ 8,286

Restructurings - Additional Inf

Restructurings - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Restructuring Cost And Reserve [Line Items]
Accrued restructuring charges	$ 4,164	$ 2,094
2019 Restructuring
Restructuring Cost And Reserve [Line Items]
Accrued restructuring charges		$ 2,100
2020 Restructuring
Restructuring Cost And Reserve [Line Items]
Accrued restructuring charges expected to be paid year	2021
Share-based compensation, accelerated cost	$ 300

Restructurings - Summary of Est

Restructurings - Summary of Estimates of Restructuring Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Restructuring Cost And Reserve [Line Items]
Total estimated restructuring costs to be incurred	$ 8,040
Total restructuring costs incurred	5,684	$ 2,094
Cumulative restructuring costs incurred through December 31, 2020	7,778
Employee Severance and Related Benefits
Restructuring Cost And Reserve [Line Items]
Total estimated restructuring costs to be incurred	6,461
Total restructuring costs incurred	4,105	$ 2,094
Cumulative restructuring costs incurred through December 31, 2020	6,199
Asset Impairment and Other Costs
Restructuring Cost And Reserve [Line Items]
Total estimated restructuring costs to be incurred	1,579
Total restructuring costs incurred	1,579
Cumulative restructuring costs incurred through December 31, 2020	$ 1,579

Restructurings - Schedule of Ac

Restructurings - Schedule of Accrued Restructuring Charges (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Restructuring And Related Activities [Abstract]
Accrued balance	$ 2,094
Costs incurred	5,684	$ 2,094
Costs incurred	3,843
Reductions for cash payments	(1,773)
Accrued balance	$ 4,164	$ 2,094

Restructurings - Schedule of Re

Restructurings - Schedule of Restructuring Charges (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Restructuring Cost And Reserve [Line Items]
Restructuring charges	$ 5,684	$ 2,094
Research and Development
Restructuring Cost And Reserve [Line Items]
Restructuring charges	5,486	433
General and Administrative
Restructuring Cost And Reserve [Line Items]
Restructuring charges	$ 198	$ 1,661

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	Feb. 25, 2021	Sep. 06, 2020	Dec. 31, 2019	Jul. 31, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Aug. 31, 2020	Dec. 31, 2017
Subsidiary Sale Of Stock [Line Items]
Preferred stock, shares authorized			5,000,000		5,000,000	5,000,000
Common stock, shares authorized			100,000,000		100,000,000	100,000,000
Preferred stock, shares issued			0		0	0
Preferred stock, shares outstanding			0		0	0
Initial public offering value of shares authorized								$ 300	$ 250
Warrants exercisable description					their terms, the outstanding pre-funded warrants to purchase shares of the Company’s common stock generally may not be exercised if the holder’s ownership of the Company’s common stock would exceed 19.99% following such exercise.
Proceeds from sale of pre-funded warrants			$ 37.5
Common stock remains available for sale			$ 21.4			$ 21.4
Number of warrants exercised					0	0	0
Warrant to Purchase of Common Stock					15,296		1,613
Maximum
Subsidiary Sale Of Stock [Line Items]
Initial public offering value of shares authorized								300
Ownership percentage to exercise warrant			19.99%
2017 At the Market Offerings
Subsidiary Sale Of Stock [Line Items]
Number of shares sold		0
2017 At the Market Offerings | Maximum
Subsidiary Sale Of Stock [Line Items]
Initial public offering value of shares authorized									$ 75
Public Offering
Subsidiary Sale Of Stock [Line Items]
Number of shares sold			6,287,878	4,600,000
Sale of stock, price per share			$ 16.50	$ 36		$ 16.50
Net proceeds from the issuance of common stock through equity plans			$ 134.7	$ 155.4
Number of pre-funded warrants sold to purchase of common stock			2,424,242			2,424,242
Sale of stock, price per warrant			$ 16.499
Warrants exercisable price per share			$ 0.001			$ 0.001
Additional Shares Sold by Underwriters
Subsidiary Sale Of Stock [Line Items]
Number of shares sold			1,136,363	600,000
2020 At the Market Offerings
Subsidiary Sale Of Stock [Line Items]
Number of shares sold	4,177,080				892,840
Net proceeds from the issuance of common stock through equity plans	$ 25.5				$ 5.5
2020 At the Market Offerings | Maximum
Subsidiary Sale Of Stock [Line Items]
Initial public offering value of shares authorized								$ 100

Stockholders' Equity - Summary

Stockholders' Equity - Summary of Warrants to Purchase Shares of Company's Common Stock (Details) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Subsidiary Sale Of Stock [Line Items]
Number of warrants outstanding	2,424,242	2,439,538
Warrants Issued on September 10, 2010
Subsidiary Sale Of Stock [Line Items]
Issue date	Sep. 10, 2010
Expiration date	Sep. 10, 2020
Exercise Price per Share	$ 30
Number of warrants outstanding		15,296
Warrants Issued on December 16, 2019
Subsidiary Sale Of Stock [Line Items]
Issue date	Dec. 16, 2019
Exercise Price per Share	$ 0.001
Number of warrants outstanding	2,424,242	2,424,242

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Details) - USD ($)	1 Months Ended							3 Months Ended	12 Months Ended
	Nov. 30, 2020	Jul. 31, 2020	May 31, 2020	Nov. 30, 2019	Sep. 30, 2019	May 31, 2019	Nov. 30, 2018	Jun. 30, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jun. 30, 2020	May 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award
Shares reserved for issuance (in shares)						3,000,000						4,600,000	1,900,000
Weighted-average grant-date fair value of options granted									$ 10.64	$ 10.55	$ 30.84
Total intrinsic value of options exercised									$ 2,000,000	$ 5,000,000	$ 31,100,000
Stock-based compensation expense									21,853,000	20,558,000	28,485,000
Unrecognized stock-based compensation expense									$ 20,400,000
Unrecognized stock-based compensation expense expected to be recognized, term									1 year 9 months 18 days
2018 ESPP
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation expense									$ 500,000	400,000	$ 200,000
Maximum shares available for purchase by employees (in shares)									400,000
Maximum number of common stock to be issued per offering									1,000
Percentage of payroll deductions in ESPP									15.00%
Purchase price after discount (in percent)									85.00%
Number of common stock purchased by employees	44,546		42,266	22,566		36,804	21,483
Common stock, capital shares reserved for future issuance									232,335
Former Officer
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation expense									$ (1,700,000)	(3,600,000)
RSUs
Share-based Compensation Arrangement by Share-based Payment Award
Total fair value of awards vested and settled									7,800,000	5,700,000
Total intrinsic value of awards vested and settled									$ 4,400,000	$ 2,900,000
Number of awards vested and settled									297,133
Granted									536,164
Forfeited									110,881
RSUs | Executives
Share-based Compensation Arrangement by Share-based Payment Award
Granted		45,000			100,000				100,000
Total fair value of awards									$ 1,200
Stock-based compensation expense									0
Cumulative catch-up adjustment of stock-based compensation expense									$ 700,000
RSUs | Former Officer
Share-based Compensation Arrangement by Share-based Payment Award
Total fair value of awards								$ 2,400,000
Stock-based compensation expense								$ (500,000)
Forfeited								100,000
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award
Shares reserved for issuance (in shares)													400,000

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity and Related Information (Details) - Options $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)$ / sharesshares
Number of Shares
Options, Outstanding at beginning of period | shares	5,613,353
Option, Granted | shares	1,649,340
Option, Exercised | shares	(175,579)
Option, Forfeited | shares	(390,522)
Option, Outstanding at end of period | shares	6,696,592
Options, Exercisable | shares	3,926,732
Weighted Average Exercise Price Per Share
Options, Outstanding at beginning of period | $ / shares	$ 15.90
Option, Granted | $ / shares	16.07
Option, Exercised | $ / shares	9.75
Option, Forfeited | $ / shares	22.81
Option, Outstanding at end of period | $ / shares	15.70
Options, Exercisable | $ / shares	$ 14.70
Options Outstanding, Weighted Average Remaining Contractual Term (Years)	6 years 10 months 24 days
Options Exercisable, Weighted Average Remaining Contractual Term (Years)	5 years 6 months
Total Intrinsic Value
Option, Outstanding at end of period | $	$ 1,856
Options, Exercisable | $	$ 1,813

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Restricted Stock Units and Related Information (Details) - RSUs	12 Months Ended
	Dec. 31, 2020$ / sharesshares
Number of Shares
Beginning of period | shares	758,718
Granted | shares	536,164
Vested | shares	(297,133)
Forfeited | shares	(110,881)
Ending of period | shares	886,868
Weighted Average Fair Value Per RSU at Grant Price
Beginning of period | $ / shares	$ 25.47
Granted | $ / shares	16.21
Vested | $ / shares	23.65
Forfeited | $ / shares	26.46
Ending of period | $ / shares	$ 20.36

Stock-Based Compensation - Su_3

Stock-Based Compensation - Summary of Restricted Stock Units and Related Information (Parenthetical) (Details) - RSUs	12 Months Ended
	Dec. 31, 2020shares
Share-based Compensation Arrangement by Share-based Payment Award
Vested and subject to deferred settlement	140,000
Weighted average remaining contractual term subject to deferred settlement	2 years 2 months 12 days

Stock-Based Compensation - Su_4

Stock-Based Compensation - Summary of Assumptions Used to Estimate Fair Values of Stock Options Grants (Details) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award
Expected dividend yield	0.00%	0.00%	0.00%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award
Exercise price	$ 5.62	$ 9.31	$ 23.78
Expected volatility	66.40%	66.50%	75.60%
Risk-free interest rate	0.26%	1.36%	2.56%
Expected term (years)	5 years 6 months	5 years 6 months	5 years 6 months
Expected dividend yield	0.00%
Maximum
Share-based Compensation Arrangement by Share-based Payment Award
Exercise price	$ 23.30	$ 23.04	$ 57.53
Expected volatility	92.30%	83.20%	86.10%
Risk-free interest rate	1.44%	2.65%	3.04%
Expected term (years)	7 years 6 months	7 years 6 months	7 years
Expected dividend yield	0.00%

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Recognized Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Compensation expense:
Stock-based compensation expense	$ 21,853	$ 20,558	$ 28,485
Research and Development
Compensation expense:
Stock-based compensation expense	11,380	11,376	11,820
General and Administrative
Compensation expense:
Stock-based compensation expense	$ 10,473	$ 9,182	$ 16,665

Stock-Based Compensation - Sc_2

Stock-Based Compensation - Schedule of Recognized Stock-Based Compensation Expense (Parenthetical) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Compensation expense:
Stock-based compensation expense	$ 21,853	$ 20,558	$ 28,485
Former Officer
Compensation expense:
Stock-based compensation expense	$ (1,700)	$ (3,600)

Collaboration Agreements - Addi

Collaboration Agreements - Additional Information (Details)	Feb. 10, 2017USD ($)	Dec. 31, 2020USD ($)	Sep. 30, 2020USD ($)	Jun. 30, 2020USD ($)	Mar. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Sep. 30, 2019USD ($)	Jun. 30, 2019USD ($)	Mar. 31, 2019USD ($)	Dec. 31, 2020USD ($)Milestone	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Jul. 17, 2020USD ($)
Collaboration Agreements:
Collaboration revenue		$ 1,037,000	$ 34,611,000	$ 39,376,000	$ 4,081,000	$ 4,767,000	$ 4,231,000	$ 3,080,000	$ 3,885,000	$ 79,105,000	$ 15,963,000	$ 14,804,000
Contract with customer, liability, current						6,411,000					6,411,000
Contract with customer, liability, noncurrent		8,987,000				30,637,000				8,987,000	30,637,000
Contract liability/deferred revenue		8,987,000				37,048,000				8,987,000	37,048,000	40,660,000
Accounts receivable from collaboration		1,230,000				3,374,000				1,230,000	3,374,000
Research and development expense										106,823,000	85,757,000	72,741,000
Allergan
Collaboration Agreements:
Agreed to share maximum amount development cost	$ 75,000,000
Premium percentage on agreed contribution development cost amount	25.00%
Allergan Agreement
Collaboration Agreements:
Deferred revenue, cumulative catch up revenue recognized										36,000,000
Collaborative Arrangement
Collaboration Agreements:
Collaboration revenue	$ 50,000,000									12,500,000
Cost of reimbursable expenses										48,100,000	16,000,000	$ 14,800,000
Contract with customer, liability, current						6,400,000					6,400,000
Contract with customer, liability, noncurrent						30,600,000					30,600,000
Contract liability/deferred revenue		0								0
Accounts receivable from collaboration		1,000,000				$ 3,400,000				1,000,000	$ 3,400,000
Collaborative Arrangement | Milestone Scenario One
Collaboration Agreements:
Potential milestone payments										$ 631,000,000
Number of milestones | Milestone										7
Collaborative Arrangement | Milestone Scenario Two
Collaboration Agreements:
Potential milestone payments										$ 2,140,000,000
Number of milestones | Milestone										12
BeiGene Agreement
Collaboration Agreements:
Collaboration revenue										$ 31,000,000
Contract with customer, liability, noncurrent		$ 9,000,000								9,000,000
Agreed to pay maximum amount development and regulatory costs										45,000,000
Upfront cash payment received										40,000,000
Research and development expense										0
Incremental costs										3,500,000
BeiGene Agreement | Other Assets
Collaboration Agreements:
Incremental costs										800,000
BeiGene Agreement | General and Administrative
Collaboration Agreements:
Incremental costs										2,700,000
BeiGene Agreement | Maximum
Collaboration Agreements:
Potential milestone payments										500,000,000
BeiGene Agreement | Development and Regulatory | Maximum
Collaboration Agreements:
Potential milestone payments										113,800,000
BeiGene Agreement | Sales Milestone Payments | Maximum
Collaboration Agreements:
Potential milestone payments										385,000,000
Bei Gene Limited
Collaboration Agreements:
Contract liability/deferred revenue													$ 40,000,000
Arbutus Agreement
Collaboration Agreements:
Research and development expense										$ 200,000

Collaboration Agreements - Sche

Collaboration Agreements - Schedule of Changes in Contract Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Collaboration Agreement [Abstract]
Balance at Beginning of Period	$ 37,048	$ 40,660
Additions	40,000	0
Deductions	(68,061)	(3,612)
Balance at End of Period	$ 8,987	$ 37,048

Collaboration Agreements - Sc_2

Collaboration Agreements - Schedule of Collaboration Revenue Recognized with the Period (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Collaboration revenue recognized in the period from
Amounts included in deferred revenue at the beginning of the period	$ 37,048	$ 3,612	$ 5,125
Performance obligations satisfied in previous periods	$ 0	$ 0	$ 0

Strategic License Agreements -

Strategic License Agreements - Additional Information (Details)	1 Months Ended	12 Months Ended
	Nov. 30, 2020USD ($)	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)Milestone	Dec. 31, 2018USD ($)
Milestone and Research Agreements:
Potential milestone payment		$ 800,000
Sublicensing Fee		700,000
Performance Milestone Payment		100,000
Payment of milestone			$ 0	$ 100,000
Microbiome Targeted Colonic Delivery Program
Milestone and Research Agreements:
Potential milestone payment			$ 400,000
Accrued expenses		0		$ 0
Number of milestones | Milestone			2
Door Agreement
Milestone and Research Agreements:
License agreement term	2 years
Up-front payment	$ 1,800,000
Research and development funding charges		$ 300,000
Door Agreement | Maximum
Milestone and Research Agreements:
Success-based milestones	35,000,000
Exercise and annual fees	2,000,000
Door Agreement | Minimum
Milestone and Research Agreements:
Exercise and annual fees	$ 100,000

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Taxes [Line Items]
Current income tax provision	$ 0	$ 0	$ 0
Deferred income tax expense (benefit)	0	(774,000)	$ 1,099,000
Deferred tax assets, operating loss carryforwards, federal	378,800,000
Deferred tax assets, operating loss carryforwards, state and local	446,800,000
Deferred tax assets, tax credit carryforwards, research	8,872,000	$ 7,499,000
Domestic Tax Authority
Income Taxes [Line Items]
Deferred tax assets, operating loss carryforwards, not subject to expiration	264,100,000
Deferred tax assets, operating loss carryforwards, subject to expiration	$ 114,700,000
Operating loss carry forwards expiration period	2027
Deferred tax assets, tax credit carryforwards, research	$ 8,500,000
Domestic Tax Authority | Research Tax Credit Carryforward
Income Taxes [Line Items]
Operating loss carry forwards expiration period	2028
State and Local Jurisdiction
Income Taxes [Line Items]
Deferred tax assets, operating loss carryforwards, not subject to expiration	$ 700,000
Deferred tax assets, operating loss carryforwards, subject to expiration	$ 446,100,000
Operating loss carry forwards expiration period	2031
California Franchise Tax Board
Income Taxes [Line Items]
Deferred tax assets, tax credit carryforwards, research	$ 3,600,000

Income Taxes - Effective Income

Income Taxes - Effective Income Tax Rate of Provision for Income Taxes (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Statutory federal income tax rate	21.00%	21.00%	21.00%
State taxes, net of federal tax benefit	6.10%	5.60%	7.10%
Stock based compensation	0.70%	(3.80%)	6.70%
Research and development tax credits	2.20%	5.60%	2.40%
State rate change	0.00%	(1.40%)	2.50%
Uncertain tax positions	2.20%	(2.10%)	(4.80%)
Return to provision adjustments	4.30%	(2.50%)	4.40%
Other	0.10%	(0.20%)	0.80%
Change in valuation allowance	(36.60%)	(21.40%)	(41.30%)
Income taxes provision (benefit)	0.00%	0.80%	(1.20%)

Income Taxes - Components of De

Income Taxes - Components of Deferred Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Federal and state-operating loss carryforwards	$ 108,236	$ 81,584
Stock-based compensation	16,563	12,347
Intangible assets	1,463	1,774
Deferred revenue		9,503
Operating lease liabilities	2,600	3,147
Research and development credits	8,872	7,499
Other	854	702
Total deferred tax assets	138,588	116,556
Valuation allowance	(131,332)	(108,577)
Deferred tax asset, net of valuation allowance	7,256	7,979
Deferred tax liabilities:
In-process research and development	(7,443)	(7,439)
Operating lease right-of-use assets	(2,344)	(3,071)
Total deferred tax liabilities	(9,787)	(10,510)
Net deferred tax liability	$ (2,531)	$ (2,531)

Income Taxes - Activity Related

Income Taxes - Activity Related to Gross Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Balances as of beginning of year	$ 6,070	$ 4,613	$ 0
Increases related to prior year tax positions	0	15	3,679
Decreases related to prior year tax positions	(4,162)	(934)	0
Increases related to current year tax positions	747	2,376	934
Balances as of end of year	$ 2,655	$ 6,070	$ 4,613

Leases - Additional Information

Leases - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Leases [Abstract]
Operating lease, impairment loss recognized on assets	$ 700
Operating lease, liability	10,129
Operating lease, right-of-use asset	$ 9,131	$ 11,975
Weighted-average remaining lease term - operating leases	2 years 8 months 12 days
Weighted-average discount rate - operating leases	9.20%
Operating lease cost	$ 7,083	$ 6,256	$ 4,200

Leases - Schedule of Quantitati

Leases - Schedule of Quantitative Information of Operating Leases (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Lease cost
Operating lease cost	$ 5,214	$ 4,454
Short-term lease cost	401	609
Variable lease cost	1,468	1,193
Total lease cost	7,083	6,256	$ 4,200
Operating cash flows from operating leases	4,513	4,269
Right-of-use assets exchanged for new operating lease liabilities	$ 1,302	$ 15,261	$ 0

Leases - Schedule of Maturities

Leases - Schedule of Maturities Operating Lease Liabilities (Details) $ in Thousands	Dec. 31, 2020USD ($)
Lease Cost [Abstract]
2021	$ 4,369
2022	3,905
2023	3,502
Total	11,776
Less: present value discount	(1,647)
Operating lease liabilities	$ 10,129

Employee Benefit Plan - Additio

Employee Benefit Plan - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Compensation And Retirement Disclosure [Abstract]
Defined Contribution Plan, Employer Discretionary Contribution Amount	$ 0.9	$ 0.7	$ 0.7

Selected Quarterly Financial _3

Selected Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Collaboration revenue	$ 1,037	$ 34,611	$ 39,376	$ 4,081	$ 4,767	$ 4,231	$ 3,080	$ 3,885	$ 79,105	$ 15,963	$ 14,804
Operating expenses	40,679	38,630	32,797	31,775	33,451	30,224	22,780	32,221	143,881	118,676	107,539
Interest and other income	224	670	691	1,039	859	983	1,182	1,276	2,624	4,305	3,083
Net loss	(39,418)	(3,349)	7,270	(26,655)	(27,084)	(24,995)	(18,503)	(27,052)	$ (62,152)	$ (97,634)	$ (90,751)
Unrealized gain (loss) from marketable securities, net of tax	$ (112)	$ (262)	$ 190	$ 115	$ 4	$ (18)	$ 52	$ 108
Basic net income (loss) per common share	$ (1.11)	$ (0.09)	$ 0.21	$ (0.76)
Diluted net income (loss) per common share	$ (1.11)	$ (0.09)	$ 0.19	$ (0.76)
Basic and diluted net loss per common share					$ (0.99)	$ (0.96)	$ (0.72)	$ (1.05)	$ (1.75)	$ (3.72)	$ (3.98)

Subsequent Events - Additional

Subsequent Events - Additional Information (Details) - 2020 At the Market Offerings - USD ($) $ in Millions	Feb. 25, 2021	Dec. 31, 2020
Subsequent Event [Line Items]
Number of shares sold	4,177,080	892,840
Net proceeds from the issuance of common stock through equity plans	$ 25.5	$ 5.5