Assembly Biosciences (ASMB) 10-K 4 Mar 20 2019 FY Annual report Financial data

Document And Entity Information

Document And Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Mar. 02, 2020	Jun. 28, 2019
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Transition Report	false
Document Period End Date	Dec. 31, 2019
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	2019
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	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		32,624,725
Entity Shell Company	false
Entity Emerging Growth Company	false
Entity Small Business	true
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Public Float			$ 332.6
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 2020, to be filed within 120 days of the registrant’s fiscal year ended December 31, 2019.

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets
Cash and cash equivalents	$ 46,732	$ 41,471
Marketable securities	227,311	176,609
Accounts receivable from collaboration	3,374	2,430
Prepaid expenses and other current assets	5,363	1,992
Total current assets	282,780	222,502
Property and equipment, net	1,830	557
Operating lease right-of-use assets	11,975	0
Other assets	1,684	3,348
Indefinite-lived intangible asset	29,000	29,000
Goodwill	12,638	12,638
Total assets	339,907	268,045
Current liabilities
Accounts payable	1,731	3,693
Accrued clinical expenses	4,826	3,561
Other accrued expenses	8,286	6,118
Deferred revenue - short-term	6,411	5,100
Operating lease liabilities - short-term	3,186	0
Total current liabilities	24,440	18,472
Deferred rent	0	108
Deferred tax liabilities	2,531	3,252
Deferred revenue - long-term	30,637	35,560
Operating lease liabilities - long-term	9,082	0
Total liabilities	66,690	57,392
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, 2019 and 2018; 32,558,307 and 25,495,425 shares issued and outstanding as of December 31, 2019 and 2018, respectively	32	25
Additional paid-in capital	712,807	552,762
Accumulated other comprehensive loss	(201)	(347)
Accumulated deficit	(439,421)	(341,787)
Total stockholders' equity	273,217	210,653
Total liabilities and stockholders' equity	$ 339,907	$ 268,045

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
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	32,558,307	25,495,425
Common stock, shares outstanding	32,558,307	25,495,425

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Collaboration revenue	$ 15,963	$ 14,804	$ 9,019
Operating expenses:
Research and development	85,757	72,741	44,225
General and administrative	32,919	34,798	17,021
Total operating expenses	118,676	107,539	61,246
Loss from operations	(102,713)	(92,735)	(52,227)
Other income (expenses)
Interest and other income	4,300	3,083	983
Other income (expense), net	5		(615)
Total other income	4,305	3,083	368
Loss before income taxes	(98,408)	(89,652)	(51,859)
Income tax benefit (expense)	774	(1,099)	9,050
Net loss	(97,634)	(90,751)	(42,809)
Other comprehensive income
Unrealized gain on marketable securities, net of tax	146	45	209
Comprehensive loss	$ (97,488)	$ (90,706)	$ (42,600)
Net loss per share, basic and diluted	$ (3.72)	$ (3.98)	$ (2.41)
Weighted average common shares outstanding, basic and diluted	26,258,790	22,801,644	17,750,380

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, 2016	$ 79,878	$ 17	$ 288,689	$ (601)	$ (208,227)
Balance (in shares) at Dec. 31, 2016		17,246,754
Sale of common stock, net of underwriters' discount and costs	64,848	$ 3	64,845
Sale of common stock, net of underwriters' discount and costs (in shares)		2,541,500
Issuance of common stock upon exercise of stock options	2,393		2,393
Issuance of common stock upon exercise of stock options ( in shares )		349,720
Unrealized gain on marketable securities, net of tax	209			209
Stock-based compensation	8,601		8,601
Net loss	(42,809)				(42,809)
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	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

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities
Net loss	$ (97,634)	$ (90,751)	$ (42,809)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	494	643	219
Stock-based compensation	20,558	28,485	8,601
Net accretion and amortization of investments in marketable securities	(1,735)	(229)	0
Non-cash rent expense	4,454	0	0
Deferred income tax (benefit) expense	(774)	1,099	(9,050)
Loss on disposal of fixed assets	102	0	0
Other	(5)	0	615
Changes in operating assets and liabilities:
Accounts receivable from collaboration	(944)	(156)	(2,273)
Prepaid expenses and other current assets	(3,685)	(1,094)	(286)
Other assets	1,664	(3,008)	(84)
Accounts payable	(1,962)	1,569	(244)
Accrued clinical expenses	1,265	3,119	(242)
Other accrued expenses	2,016	382	1,628
Deferred revenue	(3,612)	(5,125)	45,785
Deferred rent	0	108	0
Operating lease liabilities	(4,269)	0	0
Net cash (used in) provided by operating activities	(84,067)	(64,958)	1,860
Cash flows from investing activities
Purchases of property and equipment	(1,554)	(340)	(865)
Purchases of marketable securities	(281,334)	(183,941)	(48,234)
Proceeds from maturities of marketable securities	203,911	48,884	33,457
Proceeds from sale of marketable securities	28,659	0	0
Net cash used in investing activities	(50,318)	(135,397)	(15,642)
Cash flows from financing activities
Proceeds from common stock and pre-funded warrants sold, net of underwriters' discount and costs	134,661	155,425	64,847
Proceeds from the issuance of common stock under ESPP	747	408	0
Proceeds from the exercise of stock options	4,238	3,960	2,393
Net cash provided by financing activities	139,646	159,793	67,240
Net increase (decrease) in cash and cash equivalents	5,261	(40,562)	53,458
Cash and cash equivalents at the beginning of the period	41,471	82,033	28,575
Cash and cash equivalents at the end of the period	46,732	41,471	82,033
Supplemental non-cash investing and financing activities
Operating lease liabilities arising from obtaining right-of-use assets	$ 15,261	$ 0	$ 0

Nature of Business

Nature of Business	12 Months Ended
	Dec. 31, 2019
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 two innovative programs: a novel class of oral therapeutic candidates for the treatment of hepatitis B virus (HBV) infection and a novel class of oral live microbial biotherapeutic candidates, which are designed to treat disorders associated with the microbiome. The Company operates in one segment and, as of December 31, 2019, was headquartered in Carmel, Indiana with operations in South San Francisco, California and Groton, Connecticut. Effective January 1, 2020, the Company changed its corporate headquarters to its South San Francisco, California facility. The Company expects to continue maintaining its office in Carmel, Indiana for a period of time. The Company’s HBV-cure program is pursuing multiple drug candidates that inhibit the HBV lifecycle and block the generation of covalently closed circular DNA (cccDNA), with the aim of increasing the current low cure rates for patients with chronic HBV infection. Assembly has discovered multiple novel core inhibitors, which are small molecules that directly target and allosterically modify the HBV core (HBc) protein. The Company’s Microbiome program consists of a fully integrated platform that includes a disease-targeted strain isolation, identification, characterization and selection process, methods for strain purification and growth under current Good Manufacturing Practice (cGMP) conditions, and a licensed patented delivery system that the Company calls GEMICEL®, which is designed to allow for targeted oral delivery of live biologic and conventional therapies to the lower gastrointestinal (GI) tract. Using the Company’s microbiome platform capabilities, the Company is exploring product candidates for multiple disease indications, including ulcerative colitis (UC), Crohn’s disease and irritable bowel syndrome (IBS) with Allergan Pharmaceuticals International Limited (Allergan) in connection with its Research, Development, Collaboration, and License Agreement (the Collaboration Agreement), as well as immune-mediated and metabolic disorders and oncology, which indications the Company will pursue either internally or in collaboration with other parties. 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, an upfront payment related to the Collaboration Agreement, and research and development cost reimbursements from the Collaboration Agreement. 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 6 for recent sales of common stock). The Company cannot assure such funding will be available on reasonable terms, if at all. If the Company is unable to generate enough revenue from the Collaboration Agreement when needed or to secure additional sources of funding and receive related 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, 2019
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 revenue recognition, clinical trial accruals, recoverability and useful lives (indefinite or finite) of intangible assets, assessment of impairment of goodwill, provisions for income taxes, amounts receivable under the Collaboration Agreement, measurement of operating lease liabilities, and the fair value of stock options, stock appreciation rights, and restricted stock units (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. Cash and cash equivalents All highly liquid investments 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, 2019 and 2018, 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, 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. As of October 1, 2019 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. The Company has not recognized any goodwill impairment in any of the periods presented. 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 2019 , the Company elected to bypass the quantitative assessment and performed a qualitative impairment during the fourth quarter and concluded it is more likely than not the fair value of its IPR&D asset exceeded its carrying value and no further testing was required. 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 sheet at December 31, 2019. 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. Long-lived Assets The Company monitors the carrying value of long-lived assets, including right-of-use 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. There were no indicators of impairment of long-lived assets for any periods presented. 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, 2019 Level 1 Level 2 Level 3 Estimated Fair Value Cash equivalents Money market funds $ 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 December 31, 2018 Level 1 Level 2 Level 3 Estimated Fair Value Cash equivalents Money market fund $ 39,345 $ — $ — $ 39,345 Total cash equivalents 39,345 — — 39,345 Short-term investments U.S. and foreign corporate debt securities — 73,159 — 73,159 Asset-backed securities — 28,419 — 28,419 U.S. treasury securities — 19,895 — 19,895 U.S. and foreign commercial paper — 55,136 — 55,136 Total short-term investments — 176,609 — 176,609 Total assets measured at fair value $ 39,345 $ 176,609 $ - $ 215,954 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 Collaboration At the inception of an arrangement, we evaluate if a counterparty to a contract is a customer, if the arrangement is within the scope of revenue from contracts with customers guidance, and the term of the contract. We recognize revenue when the customer obtains control of promised goods or services in a contract for an amount that reflects the consideration we expect to receive in exchange for those goods or services. For contracts with customers, we apply the following five-step model in order to determine this amount: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations, including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) we satisfy each performance obligation. Our performance obligations under these arrangements may include licenses of intellectual property, options to license additional intellectual property, research and development services, delivery of manufactured product, and/or participation on joint steering committees. Allergan can select additional and/or back-up target indications to add to the licenses granted and also has the ability to enter into a contract manufacturing agreement with us for compound supply, and we have concluded that these rights are options. We evaluated whether such options contained material rights and concluded they 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 will be accounted for as separate contracts when they occur. We have provided standard indemnification and protection of licensed intellectual property for our customer. These provisions are part of assurance that the licenses meet the agreements, representations and are not obligations to provide goods or services. We only apply the five-step model to contracts when it is probable that we will collect the consideration we are entitled to in exchange for the goods or services we transfer to the customer. As part of the accounting for contracts with customers, we must develop assumptions that require judgment to determine the standalone selling price of each performance obligation identified in the contract. We then allocate the total transaction price to each performance obligation based on the estimated standalone selling prices of each performance obligation. For the Allergan agreement, we estimated standalone selling prices of our performance obligations using income-based valuation approach for the estimated value a licensor of the compounds would receive. We recognize 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 . For our contract with Allergan, we recognize revenues for each of our distinct performance obligations as the related research and development services are performed because Allergan consumes the benefit of research and development work simultaneously as we perform these services. Upfront License Fees If a license to our intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, we recognize 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, we utilize 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, we apply an appropriate method of measuring progress for purposes of recognizing revenue from nonrefundable, upfront license fees. We evaluate the measure of progress each reporting period and, if necessary, adjust the measure of performance and related revenue recognition. Development and Regulatory Milestone Payments Depending on facts and circumstances, we may record revenues from certain milestones in a reporting period before the milestone is achieved if we conclude that achievement of the milestone is probable and that recognition of revenue related to the milestone will not result in a significant reversal in amounts recognized in future periods. We record 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. We re-evaluate the probability of achievement of such milestones and any related constraint each reporting period. We adjust our estimate of the overall transaction price, including the amount of collaborative revenue that was recorded, if necessary. Research and Development Service Payments We are reimbursed at a certain percentage for performing research and development services based on hours worked by our employees, at a fixed contractual rate per hour, and third-party pass-through costs we incur on a quarterly basis. Research and development service payments are included in the transaction price in the reporting period that we conclude that it is probable that recording revenue in the period will not result in a significant reversal in amounts recognized in future periods. Accounts receivable are recorded when the right to the research and development service payment consideration becomes unconditional. We record the full reimbursed portion of these expenses as collaboration revenue in our consolidated statements of operations as we consider performing research and development services to be a part of our ongoing and central operations. Sales-based Milestone and Royalty Payments Our customer may be required to pay us sales-based milestone payments or royalties on future sales of commercial products. We recognize 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 our licensed intellectual property is deemed to be the predominant item to which the sales-based milestones and/or royalties relate. We receive payments from our customer based on billing schedules established in each contract. Upfront payments and fees are recorded as deferred revenue upon receipt or when due until we perform our 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. We recognize a contract asset relating to our conditional right to consideration that is not subject to a constraint. Amounts are recorded as accounts receivable when our right to consideration is unconditional. At December 31, 2019 and 2018 , the accounts receivable recorded on the consolidated balance sheet relates to the Collaboration Agreement. All accounts receivable are deemed collectible. A net contract asset or liability is presented for each contract with a customer. We do 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. 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 restricted stock units (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. We assess 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. The 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 the adoption of Accounting Standards Update (ASU) 2018-07 (ASU 2018-07) on 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. Subsequent to the adoption of ASU 2018-07, 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 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. 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 ta

Investments in Marketable Secur

Investments in Marketable Securities	12 Months Ended
	Dec. 31, 2019
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, 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 December 31, 2018 Amortized Cost Gross Unrealized Gain (1) Gross Unrealized Loss (1) Estimated Fair Value Cash equivalents Money market funds $ 39,345 $ — $ — $ 39,345 Total cash equivalents 39,345 — — 39,345 Short-term investments U.S. and foreign corporate debt securities 73,251 — (92 ) 73,159 Asset-backed securities 28,450 — (31 ) 28,419 U.S. treasury securities 19,898 — (3 ) 19,895 U.S. and foreign commercial paper 55,136 — — 55,136 Total short-term investments 176,735 - (126 ) 176,609 Total cash equivalents and investments $ 216,080 $ - $ (126 ) $ 215,954 (1) Gross unrealized gain (loss) is pre-tax. As of December 31, 2019, 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, 2019. Realized gains and losses for the years ended December 31, 2019, 2018 and 2017 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, 2019. 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, 2019
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, 2019 2018 Computer hardware and software $ — $ 194 Lab equipment 247 407 Office equipment 699 70 Leasehold improvement 2,084 790 Total property and equipment 3,030 1,461 Less: Accumulated depreciation (1,200 ) (1,057 ) Construction in progress — 153 Property and equipment, net $ 1,830 $ 557 Depreciation expense for the years ended December 31, 2019, 2018 and 2017 was approximately $0.5 million, $0.6 million, and $0.2 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, 2019
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, 2019 2018 Other accrued expenses: Accrued compensation $ 5,312 $ 5,011 Accrued restructuring charges 2,094 - Accrued professional fees and other 880 1,107 Total other accrued expenses $ 8,286 $ 6,118 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 and is expected to be fully paid in 2020. Costs related to this relocation of $0.4 million are recorded in research and development expenses and $1.7 million in general and administrative expenses in the consolidated statement of operations and comprehensive loss at December 31, 2019.

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2019
Equity [Abstract]
Stockholders' Equity	Note 6 - Stockholders’ Equity The Company is authorized to issue 5,000,000 shares of preferred stock as of December 31, 2019 and 2018, respectively. As of December 31, 2019 and 2018, 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, 2019 and 2018, 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 using a “shelf” registration statement, file No. 333-222366, which became effective January 10, 2018 (the Registration Statement). Under this shelf registration process, the Company may from time to time sell any combination of the securities described in the Registration Statement in one or more offerings up to an aggregate offering price of $250.0 million. In connection with the filing of this 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 $75.0 million through “at the market offerings” (ATM). As of December 31, 2019, $21.4 million remains available for sale under this Registration Statement. In November 2017, the Company sold to various investors an aggregate of 2,541,000 shares of common stock in a public offering at $27.25 per share, which included the exercise in full by the underwriters of their option to purchase 331,500 additional shares of common stock. The Company received aggregate net proceeds of $64.8 million from the offering and option exercise, after deducting underwriting discounts and commissions and offering expenses. 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 approximately $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 approximately $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 are exercisable immediately upon issuance at an exercise price of $0.001 per share. Per 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. In connection with the issuance and sale of the common stock and pre-funded warrants, the Company granted the pre-funded warrant holders certain registration rights with respect to the pre-funded warrants and the Warrant Shares. 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. Common Stock Warrants As of December 31, 2019, the following warrants to purchase shares of the Company’s common stock were issued and outstanding: Issue date Expiration date Exercise price Number of warrants outstanding September 10, 2010 September 10, 2020 $ 30.00 15,296 December 16, 2019 None $ 0.001 2,424,242 2,439,538 There were no warrants exercised during the years ended December 31, 2019, 2018 and 2017. 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, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	Note 7 - 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). 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. As of December 31, 2019, 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, 2019, the Company also had awards outstanding under the Assembly Biosciences, Inc. 2017 Inducement Award Plan and the 2019 Inducement Award Plan. The Company issues new shares of common stock to settle options exercised or vested restricted stock units. Stock Plan Activity Stock Options The following table summarizes the stock option activity and related information for 2019: 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, 2018 4,637,145 $ 17.21 Granted 1,948,700 15.54 Exercised (585,292 ) 7.24 Forfeited (384,700 ) 42.92 Expired (2,500 ) 49.14 Outstanding as of December 31, 2019 5,613,353 $ 15.90 7.253 $ 43,023 Exercisable as of December 31, 2019 3,096,631 $ 13.37 5.747 $ 31,391 The weighted-average grant-date fair value of options granted was $10.55, $30.84 and $17.37 during the years ended December 31, 2019, 2018 and 2017, respectively. The total intrinsic value of options exercised in 2019, 2018 and 2017 was $5.0 million, $31.1 million and $9.8 million, respectively. RSUs The following table summarizes RSU activity and related information for 2019: Number of RSU's Weighted Average Fair Value Per RSU at Grant Price Nonvested as of December 31, 2018 568,005 $ 37.18 Granted 465,428 15.19 Vested (146,833 ) 39.14 Forfeited (127,882 ) 24.38 Nonvested as of December 31, 2019 758,718 (1) $ 25.47 (1) Includes 128,334 RSUs that have vested but are subject to deferred settlement, which have a weighted average remaining contractual term of 2.4 years. The total fair value of RSUs vested and settled during 2019 and 2018 was $5.7 million and $5.3 million, respectively. The total intrinsic value of RSUs vested and settled during 2019 was $2.9 million. The total intrinsic value of RSUs vested and settled during 2018 was nominal. There were no RSUs vested and settled in 2017. As of December 31, 2019, RSUs outstanding include 45,000 granted in December 2017 and 100,000 granted in September 2019, each with performance-based conditions to executives of the Company. 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. 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. As of December 31, 2019, 319,147 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 approximately $0.4 million and $0.2 million for the years December 31, 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, 2019 2018 2017 Exercise price $9.31 - $23.04 $23.78 - $57.53 $21.81 - $44.28 Expected volatility 66.45% - 83.23% 75.6% - 86.1% 81.2% - 87.0% Risk-free interest rate 1.36% - 2.65% 2.56% - 3.04% 2.02% - 2.29% Expected term (years) 5.5 - 7.5 5.5 - 7.0 5.5 - 7.0 Expected dividend yield - - - 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, 2019 2018 2017 Research and development $ 11,376 $ 11,820 $ 5,423 General and administrative 9,182 (1) 16,665 3,178 Total stock-based compensation expense $ 20,558 $ 28,485 $ 8,601 (1) As of December 31, 2019, there was approximately $ 27.5

Collaboration Agreement

Collaboration Agreement	12 Months Ended
	Dec. 31, 2019
Collaboration Agreement [Abstract]
Collaboration Agreement	Note 8 - Collaboration Agreement Allergan In January 2017, the Company entered into the Collaboration Agreement with Allergan to develop and commercialize select microbiome gastrointestinal disease therapies. Pursuant to the Collaboration Agreement, the Company granted Allergan an exclusive worldwide license to certain of its intellectual property, including its intellectual property arising under the Collaboration 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 Collaboration Agreement, Allergan can select backups and additional target indications to add to the licenses granted for additional consideration and also have 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 will participate on a Joint Development Committee (JDC) and Joint Patent Committee (JPC). The Company provided to Allergan standard indemnification and protection of licensed intellectual property, which is part of assurance that the license meets the contract’s specifications and is not an obligation to provide goods or services. Allergan paid the Company an upfront non-refundable payment of $50.0 million which was received in 2017. Additionally, the Company is eligible to receive variable consideration in the form of research and development cost reimbursements, up to approximately $631.0 million related to seven development milestones and up to approximately $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 is 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 have 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 has agreed to assume all post-POC development costs. In the event any pre-POC development costs exceed $75.0 million in the aggregate, the Company may elect 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 has an option to co-promote the licensed programs in the U.S. and China, subject to certain conditions set forth in the Collaboration Agreement. Allergan may terminate the Collaboration Agreement at any time upon either 90 days’ (prior to the initiation of the first POC trial of a licensed product) or 120 days’ (after the initiation of the first POC trial of a licensed product), as applicable, advance written notice to the Company. Unless terminated early, the Collaboration Agreement has a term that ends on the earlier of the (i) the period when POC studies have been completed and no further licensed compounds are in development (ii) expiration of the last to exist valid claim covering the manufacture, use and sale of the licensed compounds. The Collaboration Agreement also contains customary provisions for termination by either party, including in the event of breach of the Collaboration Agreement, subject to cure. Upon termination for convenience, the license and know how all revert to the Company. The Company concluded that Allegan is a customer, and the contract is 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 Collaboration Agreement: 1) transfer of a licenses to intellectual property for the four initial indications, inclusive of the related technology know-how (Licenses); 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 will be accounted for as separate contracts when they occur. The Company concluded the Licenses each were considered to be functional as they have 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 is dependent on the Company to execute the Development Services, that it is only uniquely able to perform, in order for Allergan to benefit from the Licenses. As such, The Company determined that it has four performance obligations under the Collaboration Agreement associated with the transfer 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 will be performed over the duration of the contract, which began in February 2017 and ends upon completion of the Development Services which is currently estimated to occur in 202 5 . The Company is using a cost-based input method to measure proportional performance and to calculate the corresponding amount of revenue to recognize. The Company believes this is 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 measures costs incurred relative to budgeted costs to fulfill the four performance obligations. These costs consist 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 completes its performance obligations. To allocate transaction price among the four performance obligations, the Company estimated their standalone selling price (SSP) using income-based valuation approach for the estimated value a licensor of the compounds would receive considering the stage of the compounds development. The Company believes that a change in the assumptions used to determine its best estimate of selling price for the four performance obligations would not have a significant effect on the allocation of consideration received to the four performance obligations. The transaction price at the inception of the agreement, was limited to $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 are included in the transaction price in the reporting period that the Company concludes that it is probable that recording revenue in the period will not result in a significant reversal in amounts recognized. 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 at December 31, 2019. 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 license granted to Allergan. The Company reevaluates the transaction price in each reporting period and as uncertain events are resolved or other changes in circumstances occur. The Company did not incur any significant incremental costs of obtaining the Allergan contract. For the year ended December 31, 2019 and 2018, the Company recorded approximately $16.0 million and $14.8 million, respectively, in revenue associated with the Collaboration Agreement. Short-term and long-term deferred revenue contract liabilities related to the Collaboration Agreement were approximately $6.4 million and approximately $30.6 million at December 31, 2019 and approximately $5.1 million and approximately $35.6 million at December 31, 2018. On the consolidated balance sheets, contract asset balances of approximately $3.4 million and approximately $2.4 million were recorded as accounts receivable from collaboration as of December 31, 2019 and 2018, respectively. The following tables present changes in the Company’s contract liabilities and sources of collaboration revenue recognized during each period (in thousands): Balance at Beginning of Period Additions Deductions Balance at End of Period Year Ended December 31, 2019 Contract liabilities: Deferred revenue $ 40,660 $ — $ (3,612 ) $ 37,048 Year Ended December 31, 2018 Contract liabilities: Deferred revenue $ 45,785 $ — $ (5,125 ) $ 40,660 Year Ended December 31, 2019 2018 Collaboration revenue recognized in the period from Amounts included in deferred revenue at the beginning of the period 3,612 5,125 Performance obligations satisfied in previous periods — —

Milestones and Research Agreeme

Milestones and Research Agreements	12 Months Ended
	Dec. 31, 2019
Milestones And Research Agreements [Abstract]
Milestones and Research Agreements	Note 9 - Milestones and Research 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 approximately $0.8 million, with a portion related to the first performance milestone having been paid. The Company also is obligated to pay IURTC royalty payments based on net sales of the licensed technology. The Company is also obligated 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. The Company made approximately $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 years ended December 31, 2019 or 2017. 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, 2018 and 2017.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income Taxes	Note 10 - Income Taxes There was no current income tax provision for the years ended December 31, 2019, 2018 and 2017. The Company recognized deferred income tax benefit of $0.8 million and $9.1 million for the years ended December 31, 2019 and 2017, respectively, and 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, 2019 2018 2017 Statutory federal income tax rate 21.0 % 21.0 % 34.0 % State taxes, net of federal tax benefit 5.6 7.1 2.6 Stock based compensation (3.8 ) 6.7 0.0 Research and development tax credits 5.6 2.4 2.1 Effective change in enacted tax rates — — (47.7 ) State rate change (1.4 ) 2.5 (0.2 ) Uncertain tax positions (2.1 ) (4.8 ) — Return to provision adjustments (2.5 ) 4.4 — Other (0.2 ) 0.8 (0.3 ) Change in valuation allowance (21.4 ) (41.3 ) 27.0 Income taxes provision (benefit) 0.8 % (1.2 )% 17.5 % Significant components of the Company’s deferred taxes are as follows (in thousands): As of December 31, 2019 2018 Deferred tax assets: Federal and state-operating loss carryforwards $ 81,584 $ 58,668 Stock-based compensation 12,347 14,023 Intangible assets 1,774 2,282 Deferred revenue 9,503 11,409 Operating lease liabilities 3,147 — Research and development credits 7,499 5,472 Other 702 575 Total deferred tax assets 116,556 92,429 Valuation allowance (108,577 ) (87,543 ) Deferred tax asset, net of valuation allowance $ 7,979 $ 4,886 Deferred tax liabilities: In-process research and development $ (7,439 ) $ (8,138 ) Operating lease right-of-use assets (3,071 ) — Total deferred tax liabilities (10,510 ) (8,138 ) Net deferred tax liability $ (2,531 ) $ (3,252 ) 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 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. On December 22, 2017, the Tax Act, was signed into law. Among other items, the Tax Act reduces the federal corporate tax rate to 21% from the existing applicable rate of 34%, effective January 1, 2018. As a result, in 2017 the Company recorded a decrease to its deferred tax assets of $24.7 million and to valuation allowance of $28.4 million, resulting in a net tax benefit of $3.7 million. The Tax Act also permits an indefinite carry forward of net operating losses generated in taxable years ending after December 31, 2017, subject to a utilization limitation of 80% of taxable income. Due to the change in the carryforward period for post-2017 net operating losses, the Company determined that it would be able to use the deferred tax liability associated with certain in-process research and development as a source of income in determining the realizability of its deferred tax assets. As a result, in 2017 the Company recorded a $4.9 million income tax benefit from the reduction of its valuation allowance. On December 22, 2017, Staff Accounting Bulletin No. 118, or SAB 118, was issued to address the application of GAAP in situations when a registrant does not have the necessary information available, prepared, or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effects of the Act. The Company was able to provide a reasonable estimate for the revaluation of deferred taxes. The Company completed its review in December 2018 with no material adjustments to the provisional amount previously recorded. The Company’s income tax benefit for the year ended December 31, 2017 of $9.1 million includes a tax benefit of $8.6 million related to the Tax Act. As of December 31, 2019, the Company had potentially utilizable gross federal net operating loss carryforwards of approximately $297.6 million with $182.9 million of net operating losses that carry forward indefinitely and $114.7 million of net operating losses which begin to expire in 2027. There are state net operating loss carryforwards of $309.3 million with $1.0 million carrying forward indefinitely and $308.3 million beginning to expire in 2031. In addition, the Company has federal research and development credit carryforwards of approximately $9.0 million which begin to expire in 2028 if not utilized and California research and development credit carryforwards of $5.3 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, 201 8 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, 201 8 . 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): Total Balances as of December 31, 2017 $ — Increases related to prior year tax positions 3,679 Increases related to 2018 tax positions 934 Balances as of December 31, 2018 $ 4,613 Increases related to prior year tax positions 15 Decreases related to prior year tax positions (934 ) Increases related to 2019 tax positions 2,376 Balances as of December 31, 2019 $ 6,070 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 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, 2019
Lease Cost [Abstract]
Leases	Note 11 - 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 Company also leases office space for administrative functions in Carmel, Indiana under a lease agreement that expires in August 2023. 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 and laboratory space in Groton, Connecticut under a lease that expires in March 2021. The Company’s China subsidiary leases office space and lab space in Shanghai that expires in December 2020. Additionally, the Company’s China subsidiary leases office space in Beijing under a lease agreement that expires in December 2020. 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. These equipment leases began to expire in 2017, with the final lease expiring in 2022. 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, 2019, the Company had operating lease liabilities of $12.3 million and right-of-use assets of $12.0 million, which were included in the consolidated balance sheet. The following summarizes quantitative information about the Company’s operating leases for the year ended December 31, 2019 (in thousands): Lease cost Operating lease cost $ 4,454 Short-term lease cost 609 Variable lease cost 1,193 Total lease cost $ 6,256 Operating cash flows from operating leases $ 4,268 Right-of-use assets exchanged for new operating lease liabilities $ 1,328 As of December 31, 2019, the weighted-average remaining lease term for operating leases was approximately 2.7 years and the weighted-average discount rate for operating leases was approximately 9.4%. As of December 31, 2019, the maturities of the Company’s operating lease liabilities were as follows (in thousands): 2020 4,583 2021 3,791 2022 3,438 2023 3,303 Total 15,115 Less: present value discount (2,847 ) Operating lease liabilities $ 12,268 Operating lease costs were approximately $6.3 million, $4.2 million and $2.1 million for the years ended December 31, 2019, 2018 and 2017, respectively.

Employee Benefit Plan

Employee Benefit Plan	12 Months Ended
	Dec. 31, 2019
Compensation And Retirement Disclosure [Abstract]
Employee Benefit Plan	Note 12 - 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 2019 and 2018, the Company made discretionary matching contributions of $0.7 million and $0.7 million, respectively. In 2017, the Company participated in a professional employer organization sponsored 401(k) plan. The Company did not make any discretionary matching contributions in 2017.

Selected Quarterly Financial Da

Selected Quarterly Financial Data	12 Months Ended
	Dec. 31, 2019
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Data	Note 13 - Selected Quarterly Financial Data (Unaudited) The following table contains quarterly financial information for the four quarters of 2019 and 2018 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. 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 Unrealized gain (loss) from marketable securities, net of tax $ 108 $ 52 $ (18 ) $ 4 Net loss $ (27,052 ) $ (18,503 ) $ (24,995 ) $ (27,084 ) Basic and diluted net loss per common share $ (1.05 ) $ (0.72 ) $ (0.96 ) $ (0.99 ) 2018 Quarter Ended March 31 June 30 September 30 December 31 (in thousands except for per share amounts) Collaboration revenue $ 3,565 $ 3,218 $ 4,286 $ 3,735 Operating expenses $ 20,237 $ 30,384 $ 26,861 $ 30,057 Interest and other income $ 446 $ 453 $ 1,116 $ 1,069 Unrealized gain (loss) from marketable securities, net of tax $ (23 ) $ (127 ) $ (82 ) $ 231 Net loss $ (16,249 ) $ (26,806 ) $ (21,535 ) $ (26,161 ) Basic and diluted net loss per common share $ (0.80 ) $ (1.30 ) $ (0.87 ) $ (1.03 )

Summary of Significant Accoun_2

Summary of Significant Accounting Policies and Recent Accounting Pronouncements (Policies)	12 Months Ended
	Dec. 31, 2019
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 revenue recognition, clinical trial accruals, recoverability and useful lives (indefinite or finite) of intangible assets, assessment of impairment of goodwill, provisions for income taxes, amounts receivable under the Collaboration Agreement, measurement of operating lease liabilities, and the fair value of stock options, stock appreciation rights, and restricted stock units (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.
Cash and cash equivalents	Cash and cash equivalents All highly liquid investments 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, 2019 and 2018, 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, 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. As of October 1, 2019 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. The Company has not recognized any goodwill impairment in any of the periods presented. 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 2019 , the Company elected to bypass the quantitative assessment and performed a qualitative impairment during the fourth quarter and concluded it is more likely than not the fair value of its IPR&D asset exceeded its carrying value and no further testing was required. 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 sheet at December 31, 2019. 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.
Long-lived Assets	Long-lived Assets The Company monitors the carrying value of long-lived assets, including right-of-use 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. There were no indicators of impairment of long-lived assets for any periods presented.
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, 2019 Level 1 Level 2 Level 3 Estimated Fair Value Cash equivalents Money market funds $ 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 December 31, 2018 Level 1 Level 2 Level 3 Estimated Fair Value Cash equivalents Money market fund $ 39,345 $ — $ — $ 39,345 Total cash equivalents 39,345 — — 39,345 Short-term investments U.S. and foreign corporate debt securities — 73,159 — 73,159 Asset-backed securities — 28,419 — 28,419 U.S. treasury securities — 19,895 — 19,895 U.S. and foreign commercial paper — 55,136 — 55,136 Total short-term investments — 176,609 — 176,609 Total assets measured at fair value $ 39,345 $ 176,609 $ - $ 215,954 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 Collaboration	Revenue Recognition and Accounts Receivable from Collaboration At the inception of an arrangement, we evaluate if a counterparty to a contract is a customer, if the arrangement is within the scope of revenue from contracts with customers guidance, and the term of the contract. We recognize revenue when the customer obtains control of promised goods or services in a contract for an amount that reflects the consideration we expect to receive in exchange for those goods or services. For contracts with customers, we apply the following five-step model in order to determine this amount: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations, including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) we satisfy each performance obligation. Our performance obligations under these arrangements may include licenses of intellectual property, options to license additional intellectual property, research and development services, delivery of manufactured product, and/or participation on joint steering committees. Allergan can select additional and/or back-up target indications to add to the licenses granted and also has the ability to enter into a contract manufacturing agreement with us for compound supply, and we have concluded that these rights are options. We evaluated whether such options contained material rights and concluded they 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 will be accounted for as separate contracts when they occur. We have provided standard indemnification and protection of licensed intellectual property for our customer. These provisions are part of assurance that the licenses meet the agreements, representations and are not obligations to provide goods or services. We only apply the five-step model to contracts when it is probable that we will collect the consideration we are entitled to in exchange for the goods or services we transfer to the customer. As part of the accounting for contracts with customers, we must develop assumptions that require judgment to determine the standalone selling price of each performance obligation identified in the contract. We then allocate the total transaction price to each performance obligation based on the estimated standalone selling prices of each performance obligation. For the Allergan agreement, we estimated standalone selling prices of our performance obligations using income-based valuation approach for the estimated value a licensor of the compounds would receive. We recognize 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 . For our contract with Allergan, we recognize revenues for each of our distinct performance obligations as the related research and development services are performed because Allergan consumes the benefit of research and development work simultaneously as we perform these services. Upfront License Fees If a license to our intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, we recognize 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, we utilize 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, we apply an appropriate method of measuring progress for purposes of recognizing revenue from nonrefundable, upfront license fees. We evaluate the measure of progress each reporting period and, if necessary, adjust the measure of performance and related revenue recognition. Development and Regulatory Milestone Payments Depending on facts and circumstances, we may record revenues from certain milestones in a reporting period before the milestone is achieved if we conclude that achievement of the milestone is probable and that recognition of revenue related to the milestone will not result in a significant reversal in amounts recognized in future periods. We record 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. We re-evaluate the probability of achievement of such milestones and any related constraint each reporting period. We adjust our estimate of the overall transaction price, including the amount of collaborative revenue that was recorded, if necessary. Research and Development Service Payments We are reimbursed at a certain percentage for performing research and development services based on hours worked by our employees, at a fixed contractual rate per hour, and third-party pass-through costs we incur on a quarterly basis. Research and development service payments are included in the transaction price in the reporting period that we conclude that it is probable that recording revenue in the period will not result in a significant reversal in amounts recognized in future periods. Accounts receivable are recorded when the right to the research and development service payment consideration becomes unconditional. We record the full reimbursed portion of these expenses as collaboration revenue in our consolidated statements of operations as we consider performing research and development services to be a part of our ongoing and central operations. Sales-based Milestone and Royalty Payments Our customer may be required to pay us sales-based milestone payments or royalties on future sales of commercial products. We recognize 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 our licensed intellectual property is deemed to be the predominant item to which the sales-based milestones and/or royalties relate. We receive payments from our customer based on billing schedules established in each contract. Upfront payments and fees are recorded as deferred revenue upon receipt or when due until we perform our 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. We recognize a contract asset relating to our conditional right to consideration that is not subject to a constraint. Amounts are recorded as accounts receivable when our right to consideration is unconditional. At December 31, 2019 and 2018 , the accounts receivable recorded on the consolidated balance sheet relates to the Collaboration Agreement. All accounts receivable are deemed collectible. A net contract asset or liability is presented for each contract with a customer. We do 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.
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 restricted stock units (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. We assess 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. The 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 the adoption of Accounting Standards Update (ASU) 2018-07 (ASU 2018-07) on 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. Subsequent to the adoption of ASU 2018-07, 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.
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.
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. The following table presents the potential common stock equivalents that were excluded from the computation of diluted loss from per share of common stock for the periods presented because including them would have been antidilutive: Year Ended December 31, 2019 2018 2017 Warrants to purchase common stock 15,296 15,296 15,296 Options to purchase common stock 5,613,353 4,637,145 4,551,819 Common stock subject to purchase under our ESPP 11,342 21,483 — Unvested RSUs 630,384 568,005 120,000 Total 6,270,375 5,241,929 4,687,115 In December 2019, the Company sold 6,287,878 shares of common stock as well as 2,424,242 pre-funded warrants (see Note 6). 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.
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, significant delays can occur if the qualification of a new supplier is required. If delivery of material from the Company’s suppliers were interrupted for any reason, the Company may be unable to supply any of its product candidates for clinical trials.
Adoption of Recent Accounting Pronouncements	Adoption of Recent Accounting Pronouncements On January 1, 2018, the Company adopted ASU No. 2014-09, Revenue from Contracts with Customers In February 2016, the Financial Accounting Standards Board (FASB) issued ASU 2016-02, Leases The Company adopted ASU 2016-02 on January 1, 2019 using the modified retrospective approach and elected the package of practical expedients permitted under transition guidance, which allowed the Company to carry forward its historical assessments of: (1) whether contracts are or contain leases, (2) lease classification and (3) initial direct costs. The Company did not elect the use-of-hindsight practical expedient, which would require the Company to reassess the lease term of its leases based on all facts and circumstances through the effective date, and the Company did not elect the practical expedient pertaining to land easements as this is not applicable to the Company’s current contract portfolio. The Company elected the post-transition practical expedient to not separate lease components from nonlease components for all existing lease classes. The Company also elected a policy of not recording leases on its consolidated balance sheets when the leases have a term of 12 months or less and the Company is not reasonably certain to elect an option to purchase the leased asset. The adoption of this standard resulted in the recognition of right of use (ROU) assets and lease liabilities of $13.8 million and $14.0 million, respectively, and the derecognition of the deferred rent balance of $0.1 million as of January 1, 2019. The adoption of the standard had no impact on the Company’s consolidated statements of operations and comprehensive loss or to its cash flows from or used in operating, financing, or investing activities on its consolidated statements of cash flows. No cumulative-effect adjustment within accumulated deficit was required to be recorded as a result of adopting this standard. On January 1, 2019, the Company adopted ASU 2018-02, Income Statement - Reporting Comprehensive Income, (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income, On January 1, 2019, the Company adopted ASU 2018-07, Compensation-Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting Compensation – Stock Compensation 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 guidance related to transition and clarification. Financial Instruments—Credit Losses (Topic 326), Derivatives and Hedging (Topic 815), and Leases (Topic 842): Effective Dates In January 2017, the FASB issued ASU 2017-04, Intangibles-Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement, In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 recognized under Topic 606 is precluded if the collaborative arrangement participant is not a customer. The standard is effective for interim and annual periods beginning after December 15, 2019, with early adoption permitted, including adoption in any interim period for public business entities for periods in which financial statements have not been issued. Amendments in the standard should be applied retrospectively to the date of initial application of Topic 606, but entities may elect to apply the amendments in Topic 808 retrospectively either to all contracts or only to contracts that are not completed at the date of initial application of Topic 606, and should disclose the election. An entity may also elect to apply the practical expedient for contract modifications that is permitted for entities using the modified retrospective transition method in Topic 606. The Company is still finalizing its analysis and evaluating the impact adopting this new accounting standard will have on its consolidated financial statements and related disclosures. In December 2019, the FASB issued ASU No. 2019-12, Simplifying the Accounting for Income Taxes

Summary of Significant Accoun_3

Summary of Significant Accounting Policies and Recent Accounting Pronouncements (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 Level 1 Level 2 Level 3 Estimated Fair Value Cash equivalents Money market funds $ 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 December 31, 2018 Level 1 Level 2 Level 3 Estimated Fair Value Cash equivalents Money market fund $ 39,345 $ — $ — $ 39,345 Total cash equivalents 39,345 — — 39,345 Short-term investments U.S. and foreign corporate debt securities — 73,159 — 73,159 Asset-backed securities — 28,419 — 28,419 U.S. treasury securities — 19,895 — 19,895 U.S. and foreign commercial paper — 55,136 — 55,136 Total short-term investments — 176,609 — 176,609 Total assets measured at fair value $ 39,345 $ 176,609 $ - $ 215,954
Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share	The following table presents the potential common stock equivalents that were excluded from the computation of diluted loss from per share of common stock for the periods presented because including them would have been antidilutive: Year Ended December 31, 2019 2018 2017 Warrants to purchase common stock 15,296 15,296 15,296 Options to purchase common stock 5,613,353 4,637,145 4,551,819 Common stock subject to purchase under our ESPP 11,342 21,483 — Unvested RSUs 630,384 568,005 120,000 Total 6,270,375 5,241,929 4,687,115

Investments in Marketable Sec_2

Investments in Marketable Securities (Tables)	12 Months Ended
	Dec. 31, 2019
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, 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 December 31, 2018 Amortized Cost Gross Unrealized Gain (1) Gross Unrealized Loss (1) Estimated Fair Value Cash equivalents Money market funds $ 39,345 $ — $ — $ 39,345 Total cash equivalents 39,345 — — 39,345 Short-term investments U.S. and foreign corporate debt securities 73,251 — (92 ) 73,159 Asset-backed securities 28,450 — (31 ) 28,419 U.S. treasury securities 19,898 — (3 ) 19,895 U.S. and foreign commercial paper 55,136 — — 55,136 Total short-term investments 176,735 - (126 ) 176,609 Total cash equivalents and investments $ 216,080 $ - $ (126 ) $ 215,954 (1) Gross unrealized gain (loss) is pre-tax.

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2019
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment, Net	Property and equipment consist of the following (in thousands): As of December 31, 2019 2018 Computer hardware and software $ — $ 194 Lab equipment 247 407 Office equipment 699 70 Leasehold improvement 2,084 790 Total property and equipment 3,030 1,461 Less: Accumulated depreciation (1,200 ) (1,057 ) Construction in progress — 153 Property and equipment, net $ 1,830 $ 557

Other Accrued Expenses (Tables)

Other Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2019
Payables And Accruals [Abstract]
Schedule of Other Accrued Expenses	Other accrued expenses consist of the following (in thousands): As of December 31, 2019 2018 Other accrued expenses: Accrued compensation $ 5,312 $ 5,011 Accrued restructuring charges 2,094 - Accrued professional fees and other 880 1,107 Total other accrued expenses $ 8,286 $ 6,118

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2019
Equity [Abstract]
Summary of Warrants to Purchase Shares of Company's Common Stock	As of December 31, 2019, the following warrants to purchase shares of the Company’s common stock were issued and outstanding: Issue date Expiration date Exercise price Number of warrants outstanding September 10, 2010 September 10, 2020 $ 30.00 15,296 December 16, 2019 None $ 0.001 2,424,242 2,439,538

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2019
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 2019: 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, 2018 4,637,145 $ 17.21 Granted 1,948,700 15.54 Exercised (585,292 ) 7.24 Forfeited (384,700 ) 42.92 Expired (2,500 ) 49.14 Outstanding as of December 31, 2019 5,613,353 $ 15.90 7.253 $ 43,023 Exercisable as of December 31, 2019 3,096,631 $ 13.37 5.747 $ 31,391
Summary of Restricted Stock Units and Related Information	The following table summarizes RSU activity and related information for 2019: Number of RSU's Weighted Average Fair Value Per RSU at Grant Price Nonvested as of December 31, 2018 568,005 $ 37.18 Granted 465,428 15.19 Vested (146,833 ) 39.14 Forfeited (127,882 ) 24.38 Nonvested as of December 31, 2019 758,718 (1) $ 25.47 (1) Includes 128,334 RSUs that have vested but are subject to deferred settlement, which have a weighted average remaining contractual term of 2.4 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, 2019 2018 2017 Exercise price $9.31 - $23.04 $23.78 - $57.53 $21.81 - $44.28 Expected volatility 66.45% - 83.23% 75.6% - 86.1% 81.2% - 87.0% Risk-free interest rate 1.36% - 2.65% 2.56% - 3.04% 2.02% - 2.29% Expected term (years) 5.5 - 7.5 5.5 - 7.0 5.5 - 7.0 Expected dividend yield - - -
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, 2019 2018 2017 Research and development $ 11,376 $ 11,820 $ 5,423 General and administrative 9,182 (1) 16,665 3,178 Total stock-based compensation expense $ 20,558 $ 28,485 $ 8,601 (1)

Collaboration Agreement (Tables

Collaboration Agreement (Tables)	12 Months Ended
	Dec. 31, 2019
Collaboration Agreement [Abstract]
Schedule of Changes in Contract Liabilities and Sources of Collaboration Revenue Recognized	The following tables present changes in the Company’s contract liabilities and sources of collaboration revenue recognized during each period (in thousands): Balance at Beginning of Period Additions Deductions Balance at End of Period Year Ended December 31, 2019 Contract liabilities: Deferred revenue $ 40,660 $ — $ (3,612 ) $ 37,048 Year Ended December 31, 2018 Contract liabilities: Deferred revenue $ 45,785 $ — $ (5,125 ) $ 40,660
Schedule of Collaboration Revenue Recognized with the Period	Year Ended December 31, 2019 2018 Collaboration revenue recognized in the period from Amounts included in deferred revenue at the beginning of the period 3,612 5,125 Performance obligations satisfied in previous periods — —

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 2018 2017 Statutory federal income tax rate 21.0 % 21.0 % 34.0 % State taxes, net of federal tax benefit 5.6 7.1 2.6 Stock based compensation (3.8 ) 6.7 0.0 Research and development tax credits 5.6 2.4 2.1 Effective change in enacted tax rates — — (47.7 ) State rate change (1.4 ) 2.5 (0.2 ) Uncertain tax positions (2.1 ) (4.8 ) — Return to provision adjustments (2.5 ) 4.4 — Other (0.2 ) 0.8 (0.3 ) Change in valuation allowance (21.4 ) (41.3 ) 27.0 Income taxes provision (benefit) 0.8 % (1.2 )% 17.5 %
Components of Deferred Taxes	Significant components of the Company’s deferred taxes are as follows (in thousands): As of December 31, 2019 2018 Deferred tax assets: Federal and state-operating loss carryforwards $ 81,584 $ 58,668 Stock-based compensation 12,347 14,023 Intangible assets 1,774 2,282 Deferred revenue 9,503 11,409 Operating lease liabilities 3,147 — Research and development credits 7,499 5,472 Other 702 575 Total deferred tax assets 116,556 92,429 Valuation allowance (108,577 ) (87,543 ) Deferred tax asset, net of valuation allowance $ 7,979 $ 4,886 Deferred tax liabilities: In-process research and development $ (7,439 ) $ (8,138 ) Operating lease right-of-use assets (3,071 ) — Total deferred tax liabilities (10,510 ) (8,138 ) Net deferred tax liability $ (2,531 ) $ (3,252 )
Activity Related to Gross Unrecognized Tax Benefits	The following table summarizes activity related to the Company’s gross unrecognized tax benefits (in thousands): Total Balances as of December 31, 2017 $ — Increases related to prior year tax positions 3,679 Increases related to 2018 tax positions 934 Balances as of December 31, 2018 $ 4,613 Increases related to prior year tax positions 15 Decreases related to prior year tax positions (934 ) Increases related to 2019 tax positions 2,376 Balances as of December 31, 2019 $ 6,070

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2019
Lease Cost [Abstract]
Schedule of Quantitative Information of Operating Leases	The following summarizes quantitative information about the Company’s operating leases for the year ended December 31, 2019 (in thousands): Lease cost Operating lease cost $ 4,454 Short-term lease cost 609 Variable lease cost 1,193 Total lease cost $ 6,256 Operating cash flows from operating leases $ 4,268 Right-of-use assets exchanged for new operating lease liabilities $ 1,328
Schedule of Maturities Operating Lease Liabilities	As of December 31, 2019, the maturities of the Company’s operating lease liabilities were as follows (in thousands): 2020 4,583 2021 3,791 2022 3,438 2023 3,303 Total 15,115 Less: present value discount (2,847 ) Operating lease liabilities $ 12,268

Selected Quarterly Financial _2

Selected Quarterly Financial Data (Tables)	12 Months Ended
	Dec. 31, 2019
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Information	The following table contains quarterly financial information for the four quarters of 2019 and 2018 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. 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 Unrealized gain (loss) from marketable securities, net of tax $ 108 $ 52 $ (18 ) $ 4 Net loss $ (27,052 ) $ (18,503 ) $ (24,995 ) $ (27,084 ) Basic and diluted net loss per common share $ (1.05 ) $ (0.72 ) $ (0.96 ) $ (0.99 ) 2018 Quarter Ended March 31 June 30 September 30 December 31 (in thousands except for per share amounts) Collaboration revenue $ 3,565 $ 3,218 $ 4,286 $ 3,735 Operating expenses $ 20,237 $ 30,384 $ 26,861 $ 30,057 Interest and other income $ 446 $ 453 $ 1,116 $ 1,069 Unrealized gain (loss) from marketable securities, net of tax $ (23 ) $ (127 ) $ (82 ) $ 231 Net loss $ (16,249 ) $ (26,806 ) $ (21,535 ) $ (26,161 ) Basic and diluted net loss per common share $ (0.80 ) $ (1.30 ) $ (0.87 ) $ (1.03 )

Nature of Business - Additional

Nature of Business - Additional Information (Details)	12 Months Ended
	Dec. 31, 2019segmentproduct
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, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019
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
Lessee, operating lease, term of contract						12 months
Operating lease, right-of-use asset	11,975,000		11,975,000	$ 0		$ 13,800,000
Operating lease, liability	$ 12,268,000		$ 12,268,000			14,000,000
Deferred rent credit						$ 100,000
Statutory federal tax rate (as percent)			21.00%	21.00%	34.00%
Common Stock
Number of shares sold	6,287,878			4,600,000	2,541,500
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, 2019	Dec. 31, 2018
Short-term investments
Short-term investments	$ 227,311	$ 176,609
Recurring
Cash equivalents
Total cash equivalents	42,578	39,345
Short-term investments
Short-term investments	227,311	176,609
Total assets measured at fair value	269,889	215,954
Recurring | U.S and foreign corporate debt securities
Cash equivalents
Total cash equivalents	4,999
Short-term investments
Short-term investments	72,486	73,159
Recurring | U.S. and foreign commercial paper
Cash equivalents
Total cash equivalents	4,484
Short-term investments
Short-term investments	76,086	55,136
Recurring | Asset-backed securities
Short-term investments
Short-term investments	34,025	28,419
Recurring | U.S. treasury securities
Short-term investments
Short-term investments	44,714	19,895
Recurring | Money market fund
Cash equivalents
Total cash equivalents	33,095	39,345
Recurring | Level 1
Cash equivalents
Total cash equivalents	33,095	39,345
Short-term investments
Short-term investments	0	0
Total assets measured at fair value	33,095	39,345
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 | U.S. and foreign commercial paper
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	33,095	39,345
Recurring | Level 2
Cash equivalents
Total cash equivalents	9,483	0
Short-term investments
Short-term investments	227,311	176,609
Total assets measured at fair value	236,794	176,609
Recurring | Level 2 | U.S and foreign corporate debt securities
Cash equivalents
Total cash equivalents	4,999
Short-term investments
Short-term investments	72,486	73,159
Recurring | Level 2 | U.S. and foreign commercial paper
Cash equivalents
Total cash equivalents	4,484
Short-term investments
Short-term investments	76,086	55,136
Recurring | Level 2 | Asset-backed securities
Short-term investments
Short-term investments	34,025	28,419
Recurring | Level 2 | U.S. treasury securities
Short-term investments
Short-term investments	44,714	19,895
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 corporate debt securities
Cash equivalents
Total cash equivalents	0
Short-term investments
Short-term investments	0	0
Recurring | Level 3 | U.S. and foreign commercial paper
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, 2019	Dec. 31, 2018	Dec. 31, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive securities excluded from computation of earnings (loss)	6,270,375	5,241,929	4,687,115
Warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive securities excluded from computation of earnings (loss)	15,296	15,296	15,296
Options
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive securities excluded from computation of earnings (loss)	5,613,353	4,637,145	4,551,819
Common stock subject to purchase under ESPP
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive securities excluded from computation of earnings (loss)	11,342	21,483
RSUs
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive securities excluded from computation of earnings (loss)	630,384	568,005	120,000

Investments in Marketable Sec_3

Investments in Marketable Securities - Schedule of Investments in Marketable Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Investments in marketable securities:
Amortized Cost	$ 46,732	$ 41,471
Estimated Fair Value	227,311	176,609
Amortized Cost	269,817	216,080
Gross Unrealized Gain	79	0
Gross Unrealized Loss	(7)	(126)
Estimated Fair Value	269,889	215,954
Cash equivalents
Investments in marketable securities:
Amortized Cost	42,579	39,345
Gross Unrealized Gain	0	0
Gross Unrealized Loss	(1)	0
Estimated Fair Value	42,578	39,345
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	4,484
Gross Unrealized Gain	0
Gross Unrealized Loss	0
Estimated Fair Value	4,484
Cash equivalents | Money market fund
Investments in marketable securities:
Amortized Cost	33,095	39,345
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Estimated Fair Value	33,095	39,345
Short-term investments
Investments in marketable securities:
Amortized Cost	227,238	176,735
Gross Unrealized Gain	79	0
Gross Unrealized Loss	(6)	(126)
Estimated Fair Value	227,311	176,609
Short-term investments | U.S and foreign corporate debt securities
Investments in marketable securities:
Amortized Cost	72,452	73,251
Gross Unrealized Gain	38	0
Gross Unrealized Loss	(4)	(92)
Estimated Fair Value	72,486	73,159
Short-term investments | Asset-backed securities
Investments in marketable securities:
Amortized Cost	34,008	28,450
Gross Unrealized Gain	17	0
Gross Unrealized Loss	0	(31)
Estimated Fair Value	34,025	28,419
Short-term investments | U.S. and foreign commercial paper
Investments in marketable securities:
Amortized Cost	76,086	55,136
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Estimated Fair Value	76,086	55,136
Short-term investments | U.S. treasury securities
Investments in marketable securities:
Amortized Cost	44,692	19,898
Gross Unrealized Gain	24	0
Gross Unrealized Loss	(2)	(3)
Estimated Fair Value	$ 44,714	$ 19,895

Investments in Marketable Sec_4

Investments in Marketable Securities - Additional Information (Details)	Dec. 31, 2019USD ($)
Investments Debt And Equity Securities [Abstract]
Long-term marketable securities	$ 0

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Property and Equipment
Total property and equipment	$ 3,030	$ 1,461
Less: Accumulated depreciation	(1,200)	(1,057)
Property and equipment, net	1,830	557
Computer hardware and software
Property and Equipment
Total property and equipment		194
Lab equipment
Property and Equipment
Total property and equipment	247	407
Office equipment
Property and Equipment
Total property and equipment	699	70
Leasehold improvement
Property and Equipment
Total property and equipment	$ 2,084	790
Construction in progress
Property and Equipment
Total property and equipment		$ 153

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Depreciation	$ 0.5	$ 0.6	$ 0.2

Other Accrued Expenses - Schedu

Other Accrued Expenses - Schedule of Other Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Other accrued expenses:
Accrued compensation	$ 5,312	$ 5,011
Accrued restructuring charges	2,094
Accrued professional fees and other	880	1,107
Total other accrued expenses	$ 8,286	$ 6,118

Other Accrued Expenses - Additi

Other Accrued Expenses - Additional Information (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
Accrued Expenses
Accrued restructuring charges	$ 2,094
Accrued restructuring charges expected to be paid year	2020
Research And Development
Accrued Expenses
Relocation expense	$ 400
General And Administrative
Accrued Expenses
Relocation expense	$ 1,700

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended			12 Months Ended
	Dec. 31, 2019	Jul. 31, 2018	Nov. 30, 2017	Dec. 31, 2019	Dec. 31, 2018	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						$ 250,000
Common stock remains available for sale						21,400
Net proceeds raised from common stock issued in a public offering				$ 134,661	$ 155,425	$ 64,847
Warrants exercisable description				Per 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	$ 37,500
Number of warrants exercised				0	0	0
Warrant to Purchase of Common Stock					1,613
Common Stock
Subsidiary Sale Of Stock [Line Items]
Number of shares sold	6,287,878				4,600,000	2,541,500
Net proceeds raised from common stock issued in a public offering			$ 64,800
Common Stock | Investors
Subsidiary Sale Of Stock [Line Items]
Number of shares sold			2,541,000
Sale of stock, price per share			$ 27.25
Maximum
Subsidiary Sale Of Stock [Line Items]
Ownership percentage to exercise warrant	19.99%
At the Market Offerings | Maximum
Subsidiary Sale Of Stock [Line Items]
Initial public offering value of shares authorized						$ 75,000
Underwriters
Subsidiary Sale Of Stock [Line Items]
Number of shares sold	1,136,363	600,000
Underwriters | Common Stock
Subsidiary Sale Of Stock [Line Items]
Number of shares sold			331,500
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	$ 134,700	$ 155,400
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

Stockholders' Equity - Summary

Stockholders' Equity - Summary of Warrants to Purchase Shares of Company's Common Stock (Details)	12 Months Ended
	Dec. 31, 2019$ / sharesshares
Subsidiary Sale Of Stock [Line Items]
Number of warrants outstanding	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 | $ / shares	$ 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 | $ / shares	$ 0.001
Number of warrants outstanding	2,424,242

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended					3 Months Ended	12 Months Ended
	Nov. 30, 2019	Sep. 30, 2019	May 31, 2019	Nov. 30, 2018	Dec. 31, 2017	Jun. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	May 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award
Shares reserved for issuance (in shares)			3,000,000							1,900,000
Weighted-average grant-date fair value of options granted							$ 10.55	$ 30.84	$ 17.37
Total intrinsic value of options exercised							$ 5,000	$ 31,100	$ 9,800
Stock-based compensation expense							20,558	28,485	8,601
Stock-based compensation							20,558	28,485	$ 8,601
Unrecognized stock-based compensation expense							$ 27,500
Unrecognized stock-based compensation expense expected to be recognized, term							1 year 8 months 12 days
2018 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award
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	22,566		36,804	21,483
Common stock, capital shares reserved for future issuance							319,147
Stock-based compensation							$ 400	200
Former Officer
Share-based Compensation Arrangement by Share-based Payment Award
Stock-based compensation expense							(3,600)
RSUs
Share-based Compensation Arrangement by Share-based Payment Award
Total fair value of awards vested and settled							5,700	$ 5,300
Total intrinsic value of awards vested and settled							$ 2,900
Number of awards vested and settled							146,833		0
Granted							465,428
Forfeited							127,882
RSUs | Executive Officer
Share-based Compensation Arrangement by Share-based Payment Award
Granted		100,000			45,000
RSUs | Former Officer
Share-based Compensation Arrangement by Share-based Payment Award
Forfeited						100,000
Grant date fair value						$ 2,400
Stock-based compensation expense						$ (500)

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity and Related Information (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Number of Shares
Options, Outstanding at beginning of period	4,637,145
Options, Granted	1,948,700
Options, Exercised	(585,292)
Options, Forfeited	(384,700)
Options, Expired	(2,500)
Options, Outstanding at end of period	5,613,353	4,637,145
Options, Exercisable	3,096,631
Weighted Average Exercise Price Per Share
Options, Outstanding at beginning of period	$ 17.21
Options, Granted	15.54
Options, Exercised	7.24
Options, Forfeited	42.92
Options, Expired	49.14
Options, Outstanding at end of period	15.90	$ 17.21
Options, Exercisable	$ 13.37
Options Outstanding, Weighted Average Remaining Contractual Term (Years)	7 years 3 months 1 day
Options Exercisable, Weighted Average Remaining Contractual Term (Years)	5 years 8 months 29 days
Total Intrinsic Value
Options, Exercised	$ 5,000	$ 31,100	$ 9,800
Options, Outstanding at end of period	43,023
Options, Exercisable	$ 31,391

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Restricted Stock Units and Related Information (Details) - RSUs - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2017
Number of Shares
Beginning of period	568,005
Granted	465,428
Vested	(146,833)	0
Forfeited	(127,882)
Ending of period	758,718
Weighted Average Fair Value Per RSU at Grant Price
Beginning of period	$ 37.18
Granted	15.19
Vested	39.14
Forfeited	24.38
Ending of period	$ 25.47

Stock-Based Compensation - Su_3

Stock-Based Compensation - Summary of Restricted Stock Units and Related Information (Parenthetical) (Details) - RSUs	12 Months Ended
	Dec. 31, 2019shares
Share-based Compensation Arrangement by Share-based Payment Award
Vested and subject to deferred settlement	128,334
Weighted average remaining contractual term subject to deferred settlement	2 years 4 months 24 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, 2019	Dec. 31, 2018	Dec. 31, 2017
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	$ 9.31	$ 23.78	$ 21.81
Expected volatility	66.50%	75.60%	81.20%
Risk-free interest rate	1.36%	2.56%	2.02%
Expected term (years)	5 years 6 months	5 years 6 months	5 years 6 months
Maximum
Share-based Compensation Arrangement by Share-based Payment Award
Exercise price	$ 23.04	$ 57.53	$ 44.28
Expected volatility	83.20%	86.10%	87.00%
Risk-free interest rate	2.65%	3.04%	2.29%
Expected term (years)	7 years 6 months	7 years	7 years

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Recognized Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Compensation expense:
Stock-based compensation expense	$ 20,558	$ 28,485	$ 8,601
Research and Development
Compensation expense:
Stock-based compensation expense	11,376	11,820	5,423
General and administrative
Compensation expense:
Stock-based compensation expense	$ 9,182	$ 16,665	$ 3,178

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, 2019	Dec. 31, 2018	Dec. 31, 2017
Compensation expense:
Stock-based compensation expense	$ 20,558	$ 28,485	$ 8,601
Former Officer
Compensation expense:
Stock-based compensation expense	$ (3,600)

Collaboration Agreement - Addit

Collaboration Agreement - Additional Information (Details) $ in Thousands	Feb. 10, 2017USD ($)	Dec. 31, 2019USD ($)	Sep. 30, 2019USD ($)	Jun. 30, 2019USD ($)	Mar. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Sep. 30, 2018USD ($)	Jun. 30, 2018USD ($)	Mar. 31, 2018USD ($)	Dec. 31, 2019USD ($)Milestone	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Collaboration Agreements:
Collaboration revenue		$ 4,767	$ 4,231	$ 3,080	$ 3,885	$ 3,735	$ 4,286	$ 3,218	$ 3,565	$ 15,963	$ 14,804	$ 9,019
Contract with customer, liability, current		6,411				5,100				6,411	5,100
Contract with customer, liability, noncurrent		30,637				35,560				30,637	35,560
Accounts receivable from collaboration		3,374				2,430				3,374	2,430
Allergan
Collaboration Agreements:
Agreed to share maximum amount development cost	$ 75,000
Premium percentage on agreed contribution development cost amount	25.00%
Collaborative Arrangement
Collaboration Agreements:
Collaboration revenue	$ 50,000									12,500
Cost of reimbursable expenses										16,000	14,800
Contract with customer, liability, current		6,400				5,100				6,400	5,100
Contract with customer, liability, noncurrent		$ 30,600				$ 35,600				30,600	$ 35,600
Collaborative Arrangement | Milestone Scenario One
Collaboration Agreements:
Potential milestone payments										$ 631,000
Number of milestones | Milestone										7
Collaborative Arrangement | Milestone Scenario Two
Collaboration Agreements:
Potential milestone payments										$ 2,140,000
Number of milestones | Milestone										12

Collaboration Agreement - Sched

Collaboration Agreement - Schedule of Changes in Contract Liabilities and Sources of Collaboration Revenue Recognized (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Collaboration Agreement [Abstract]
Balance at Beginning of Period	$ 40,660	$ 45,785
Additions	0	0
Deductions	(3,612)	(5,125)
Balance at End of Period	$ 37,048	$ 40,660

Collaboration Agreement - Sch_2

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

Milestones and Research Agree_2

Milestones and Research Agreements - Additional Information (Details)	12 Months Ended
	Dec. 31, 2019USD ($)Milestone	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Milestone and Research Agreements:
Potential milestone payment	$ 800,000
Payment of milestone	0	$ 100,000	$ 0
Microbiome Targeted Colonic Delivery Program
Milestone and Research Agreements:
Potential milestone payment	$ 400,000
Accrued expenses		$ 0	$ 0
Number of milestones | Milestone	2

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Taxes [Line Items]
Current income tax provision	$ 0	$ 0	$ 0
Deferred income tax expense (benefit)	$ (774,000)	$ 1,099,000	$ (9,050,000)
Statutory federal tax rate	21.00%	21.00%	34.00%
Increase (decrease) in deferred income taxes			$ 24,700,000
Valuation allowance, deferred tax asset, increase (decrease), amount			28,400,000
Increase (decrease) in income taxes			3,700,000
Operating loss carryforwards,limitations on use	The Tax Act also permits an indefinite carry forward of net operating losses generated in taxable years ending after December 31, 2017, subject to a utilization limitation of 80% of taxable income.
Deferred tax assets, net of valuation allowance, current			4,900,000
Tax benefit related to Tax Act			$ 8,600,000
Deferred tax assets, operating loss carryforwards, federal	$ 297,600,000
Deferred tax assets, operating loss carryforwards, state and local	309,300,000
Deferred tax assets, tax credit carryforwards, research	7,499,000	$ 5,472,000
Domestic Tax Authority
Income Taxes [Line Items]
Deferred tax assets, operating loss carryforwards, not subject to expiration	182,900,000
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	$ 9,000,000
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	$ 1,000,000
Deferred tax assets, operating loss carryforwards, subject to expiration	$ 308,300,000
Operating loss carry forwards expiration period	2031
California Franchise Tax Board
Income Taxes [Line Items]
Deferred tax assets, tax credit carryforwards, research	$ 5,300,000

Income Taxes - Effective Income

Income Taxes - Effective Income Tax Rate of Provision for Income Taxes (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Statutory federal tax rate (as percent)	21.00%	21.00%	34.00%
State taxes, net of federal tax benefit	5.60%	7.10%	2.60%
Stock based compensation	(3.80%)	6.70%	0.00%
Research and development tax credits	5.60%	2.40%	2.10%
Effective change in enacted tax rates	0.00%	0.00%	(47.70%)
State rate change	(1.40%)	2.50%	(0.20%)
Uncertain tax positions	(2.10%)	(4.80%)	0.00%
Return to provision adjustments	(2.50%)	4.40%	0.00%
Other	(0.20%)	0.80%	(0.30%)
Change in valuation allowance	(21.40%)	(41.30%)	27.00%
Income taxes provision (benefit)	0.80%	(1.20%)	17.50%

Income Taxes - Components of De

Income Taxes - Components of Deferred Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Federal and state-operating loss carryforwards	$ 81,584	$ 58,668
Stock-based compensation	12,347	14,023
Intangible assets	1,774	2,282
Deferred revenue	9,503	11,409
Operating lease liabilities	3,147
Research and development credits	7,499	5,472
Other	702	575
Total deferred tax assets	116,556	92,429
Valuation allowance	(108,577)	(87,543)
Deferred tax asset, net of valuation allowance	7,979	4,886
Deferred tax liabilities:
In-process research and development	(7,439)	(8,138)
Operating lease right-of-use assets	(3,071)
Total deferred tax liabilities	(10,510)	(8,138)
Net deferred tax liability	$ (2,531)	$ (3,252)

Income Taxes - Activity Related

Income Taxes - Activity Related to Gross Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Unrecognized tax benefits, Beginning Balance	$ 4,613	$ 0
Increases related to prior year tax positions	15	3,679
Decreases related to prior year tax positions	(934)
Increases related to tax positions	2,376	934
Unrecognized tax benefits, Ending Balance	$ 6,070	$ 4,613

Leases - Additional Information

Leases - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019
Leases [Abstract]
Operating lease, liability	$ 12,268			$ 14,000
Operating lease, right-of-use asset	$ 11,975	$ 0		$ 13,800
Weighted-average remaining lease term - operating leases	2 years 8 months 12 days
Weighted-average discount rate - operating leases	9.40%
Operating lease cost	$ 6,256	$ 4,200	$ 2,100

Leases - Schedule of Quantitati

Leases - Schedule of Quantitative Information of Operating Leases (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Lease cost
Operating lease cost	$ 4,454
Short-term lease cost	609
Variable lease cost	1,193
Total lease cost	6,256	$ 4,200	$ 2,100
Operating cash flows from operating leases	4,268
Right-of-use assets exchanged for new operating lease liabilities	$ 1,328

Leases - Schedule of Maturities

Leases - Schedule of Maturities Operating Lease Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Jan. 01, 2019
Lease Cost [Abstract]
2020	$ 4,583
2021	3,791
2022	3,438
2023	3,303
Total	15,115
Less: present value discount	(2,847)
Operating lease liabilities	$ 12,268	$ 14,000

Employee Benefit Plan - Additio

Employee Benefit Plan - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Compensation And Retirement Disclosure [Abstract]
Defined Contribution Plan, Employer Discretionary Contribution Amount	$ 700,000	$ 700,000	$ 0

Selected Quarterly Financial _3

Selected Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Collaboration revenue	$ 4,767	$ 4,231	$ 3,080	$ 3,885	$ 3,735	$ 4,286	$ 3,218	$ 3,565	$ 15,963	$ 14,804	$ 9,019
Operating expenses	33,451	30,224	22,780	32,221	30,057	26,861	30,384	20,237	118,676	107,539	61,246
Interest and other income	859	983	1,182	1,276	1,069	1,116	453	446	4,300	3,083	983
Unrealized gain (loss) from marketable securities, net of tax	4	(18)	52	108	231	(82)	(127)	(23)
Net loss	$ (27,084)	$ (24,995)	$ (18,503)	$ (27,052)	$ (26,161)	$ (21,535)	$ (26,806)	$ (16,249)	$ (97,634)	$ (90,751)	$ (42,809)
Basic and diluted net loss per common share	$ (0.99)	$ (0.96)	$ (0.72)	$ (1.05)	$ (1.03)	$ (0.87)	$ (1.30)	$ (0.80)	$ (3.72)	$ (3.98)	$ (2.41)