0001564590-21-025060 | 10-Q | XBRL Viewer

Stock Plans and Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2021

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Stock Option Activity and Related Information

A summary of the Company’s option activity and related information for the three months ended March 31, 2021 is as follows:

	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, 2020		6,696,592		$	15.70
Granted		1,112,250			4.40
Forfeited		(417,127	)		20.89
Expired		(9,200	)		30.00
Outstanding as of March 31, 2021		7,382,515		$	13.68		7.1	$	1,393
Options vested and exercisable as of March 31, 2021		4,260,738		$	14.81		5.5	$	1,110

Summary of Restricted Stock Units and Related Information

A summary of the Company’s RSUs and related information for the three months ended March 31, 2021 is as follows:

	Number of RSUs				Weighted Average Fair Value Per RSU at Grant Price
Nonvested as of December 31, 2020		886,868			$	20.35
Granted		199,875				4.33
Vested		(135,929	)			19.73
Forfeited		(144,240	)			17.49
Nonvested as of March 31, 2021		806,574		⁽¹⁾	$	16.81

⁽¹⁾	Includes 140,000 RSUs that have vested but are subject to deferred settlement, which have a weighted average remaining contractual term of 1.9 years.

Summary of Assumptions Used to Estimate Fair Values of Stock Options Grants

The fair value of the stock options granted or modified during the periods indicated was estimated using the Black-Scholes option pricing model, based on the following assumptions:

	Three Months Ended March 31,
	2021	2020
Exercise price	$4.33 - $5.79	$14.45 - $19.13
Expected volatility	81.4% - 91.2%	66.4% - 82.2%
Risk-free rate	0.50% - 1.37%	0.46% - 1.44%
Expected term (years)	5.5 - 7.0	5.5 - 7.0
Expected dividend yield	0%	0%

Schedule of Recognized Stock-Based Compensation Expense

The following table summarizes the components of total stock-based compensation expense included in the condensed consolidated statements of operations and comprehensive loss (in thousands):

	Three Months Ended March 31,
	2021				2020
Research and development	$	(2,091	)	⁽¹⁾	$	1,945
General and administrative		1,805				2,979
Total stock-based compensation expense	$	(286	)		$	4,924

⁽¹⁾	Includes the reversal of previously recognized stock-based compensation expense of $3.6 million related to forfeited awards of terminated employees, $2.7 million of which resulted from the wind-down of the Company’s Microbiome program (see Note 5).

Stock Plans and Stock-Based Compensation -Schedule of Recognized Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Compensation expense:
Stock-based compensation expense	$ (286)	$ 4,924
Research and Development
Compensation expense:
Stock-based compensation expense	(2,091)	1,945
General and Administrative
Compensation expense:
Stock-based compensation expense	$ 1,805	$ 2,979

Stock Plans and Stock-Based Compensation - Summary of Stock Option Activity and Related Information (Details) - Options $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2021 USD ($) $ / shares shares
Number of Shares
Options, Outstanding at beginning of period \| shares	6,696,592
Option, Granted \| shares	1,112,250
Option, Forfeited \| shares	(417,127)
Option, Expired \| shares	(9,200)
Option, Outstanding at end of period \| shares	7,382,515
Options vested and exercisable \| shares	4,260,738
Weighted Average Exercise Price Per Share
Options, Outstanding at beginning of period \| $ / shares	$ 15.70
Option, Granted \| $ / shares	4.40
Option, Forfeited \| $ / shares	20.89
Option, Expired \| $ / shares	30.00
Option, Outstanding at end of period \| $ / shares	13.68
Options vested and exercisable \| $ / shares	$ 14.81
Options Outstanding, Weighted Average Remaining Contractual Term (Years)	7 years 1 month 6 days
Options Exercisable, Weighted Average Remaining Contractual Term (Years)	5 years 6 months
Total Intrinsic Value
Option, Outstanding at end of period \| $	$ 1,393
Options vested and exercisable \| $	$ 1,110

Restructurings (Tables)

3 Months Ended

Mar. 31, 2021

Restructuring And Related Activities [Abstract]

Summary of Estimates of Restructuring Costs

The following table summarizes the Company’s estimates of costs to be incurred and expected to be incurred (in thousands):

	Total Restructuring Cost			Employee Severance and Related Benefits				Asset Impairment and Other Costs
Total estimated restructuring costs to be incurred	$	6,988		$	3,798			$	3,190
Restructuring costs incurred for the three months ended March 31, 2021	$	(1,052	)	$	(2,663	)	⁽¹⁾	$	1,611
Cumulative restructuring costs incurred through March 31, 2021	$	6,726		$	3,536			$	3,190

⁽¹⁾

The reversal of $2.7 million in employee severance and related benefits recognized during the three months ended March 31, 2021 reflects the reversal of previously recognized stock-based compensation expense related to forfeited awards based on the Company’s policy of recognizing stock-based awards with graded vesting schedules using an accelerated attribution method on a straight-line basis over the requisite service period for each separately vesting portion of the award and to recognize forfeitures when they occur.

Schedule of Restructuring Charges

The following table presents where the restructuring charges were recognized during the three months ended March 31, 2021 (in thousands):

Research and development	$	(1,329	)
General and administrative		277
Total	$	(1,052	)

Schedule of Accrued Restructuring Charges

The following table presents the activity in the accrued restructuring charges during the three months ended March 31, 2021 (in thousands):

	Total Restructuring Cost			Employee Severance and Related Benefits			Asset Impairment and Other Costs
Accrued balance as of December 31, 2020	$	4,164		$	4,164		$	—
Costs incurred		1,611			—			1,611
Reductions for cash payments		(4,529	)		(2,918	)		(1,611	)	⁽¹⁾
Accrued balance as of March 31, 2021	$	1,246		$	1,246		$	—

⁽¹⁾	Cash payments are presented net of proceeds received from the sale of assets of $0.9 million.

Leases - Schedule of Maturities Operating Lease Liabilities (Details) $ in Thousands	Mar. 31, 2021 USD ($)
Lease Cost [Abstract]
Nine months ending December 31, 2021	$ 2,724
Year Ending December 31, 2022	3,612
Year Ending December 31, 2023	3,460
Total	9,796
Less: present value discount	(1,218)
Operating lease liabilities	$ 8,578

Restructurings - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Restructuring Cost And Reserve [Line Items]
Restructuring costs incurred	$ (1,052,000)	$ 0
Asset Impairment and Other Costs
Restructuring Cost And Reserve [Line Items]
Restructuring costs incurred	1,611,000
Costs incurred for remaining payment obligations of leased equipment	$ 1,400,000

Summary of Significant Accounting Policies and Recent Accounting Pronouncements (Policies)

3 Months Ended

Mar. 31, 2021

Accounting Policies [Abstract]

Basis of Presentation

The accompanying unaudited interim condensed consolidated financial statements include the accounts of the Company and its subsidiaries. All intercompany balances and transactions have been eliminated in consolidation.

The accompanying unaudited interim condensed consolidated financial statements have been prepared in accordance with the accounting principles generally accepted in the United States of America (U.S. GAAP) for interim financial information and pursuant to the instructions to Form 10-Q and Rule 10-01 of Regulation S-X of the SEC. In management’s opinion, the unaudited interim condensed consolidated financial statements have been prepared on the same basis as the annual audited consolidated financial statements and include normal recurring adjustments necessary for the fair presentation of the Company’s financial position and its results of operations and comprehensive loss and its cash flows for the periods presented. These statements do not include all disclosures required by U.S. GAAP and should be read in conjunction with the Company’s audited consolidated financial statements and accompanying notes for the fiscal year ended December 31, 2020, which are contained in the 2020 Annual Report. The results for the three months ended March 31, 2021 are not necessarily indicative of results to be expected for the entire year ending December 31, 2021 or future operating periods.

Use of Estimates

The preparation of the unaudited condensed consolidated financial statements in conformity with 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.

Estimates inherent in the preparation of the accompanying unaudited condensed consolidated financial statements include estimates for the cost-based input of revenue recognition and standalone selling price estimates for allocation of transaction price to performance obligations, revenue recognition, estimates of costs incurred but not yet invoiced for clinical trial accruals, recoverability and useful lives of our long-lived assets, the estimated fair value of our indefinite-lived intangible assets, the estimated fair value of our reporting unit for purposes of evaluating goodwill impairment, provisions for income taxes, amounts receivable under collaboration agreements, measurement of operating lease liabilities, and the fair value of stock options, stock appreciation rights, and 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 these external factors could have an effect on the Company’s estimates and could cause actual results to differ materially from those estimates and assumptions.

Other Risks and Uncertainties

In March 2020, the World Health Organization declared the COVID-19 outbreak a pandemic. To date, the Company’s operations have not been significantly impacted by the COVID-19 pandemic. However, the Company cannot at this time predict the specific extent, duration, or full impact the COVID-19 pandemic will have on its business, operations, strategy, prospects and financial condition and results. The impact of the COVID-19 pandemic on the Company’s financial performance will depend on future developments, including the duration and spread of the outbreak and related governmental advisories and restrictions. These developments and the impact of the COVID-19 pandemic on the financial markets and the overall economy are highly uncertain. If the financial markets and/or the overall economy are impacted for an extended period, the Company’s results may be adversely affected.

Income Taxes

In March 2021, the American Rescue Plan (H.R. 1319) was signed into law. This legislation extends and enhances a number of current-law tax incentives for businesses, but also expands the definition of a “covered employee” as defined by Section 162(m)(1) of the Internal Revenue Code. The corporate tax provisions included within the bill are not expected to have a material impact on the Company.

Net Loss per Share

Basic net loss per common share excludes dilution and is computed by dividing net loss by the weighted average number of common shares outstanding during the period. Diluted net loss per common share reflects the potential dilution that could occur if securities or other contracts to issue common stock were exercised or converted into common stock or resulted in the issuance of common stock that then shared in the earnings of the entity unless inclusion of such shares would be anti-dilutive. Since the Company has only incurred losses, basic and diluted net loss per share is the same.

In December 2019, the Company sold 6,287,878 shares of common stock as well as pre-funded warrants to purchase up to 2,424,242 shares of common stock. The pre-funded warrants are exercisable for shares of common stock at an exercise 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.

A reconciliation of the numerators and the denominators of the basic and diluted net loss per common share computations is as follows (in thousands, except per share amounts):

	Three Months Ended March 31,
	2021			2020
Numerator:
Net loss	$	(27,200	)	$	(26,655	)
Denominator:
Weighted average common shares and pre-funded warrants outstanding - basic and diluted		39,679,734			35,079,756
Net loss per share - basic and diluted	$	(0.69	)	$	(0.76	)

Securities excluded from the computation of diluted loss per share because including them would have been antidilutive are as follows:

	March 31,
	2021		2020
Warrants to purchase common stock		—		15,296
Options to purchase common stock		7,382,515		6,370,396
Common stock subject to purchase under our ESPP		44,870		32,940
Unvested RSUs		666,574		999,926
Total		8,093,959		7,418,558

Recent Accounting Pronouncements

Adoption of Recent Accounting Pronouncements

On January 1, 2021, the Company adopted ASU 2020-10, Codification Improvements – Disclosures. This ASU improves consistency by amending the codification to include all disclosure guidance in the appropriate disclosure sections and clarifies application of various provisions in the Codification by amending and adding new headings, cross referencing to other accounting standards and refining or correcting termination. The adoption of this standard had no material impact on the Company’s condensed consolidated financial statements and related disclosures.

Recent Accounting Pronouncements

In June 2016, the Financial Accounting Standards Board (the FASB) issued ASU 2016-13, Financial Instruments – Credit Losses: Measurement of Credit Losses on Financial Instruments (ASU 2016-13), which requires that expected credit losses relating to financial assets measured on an amortized cost basis and available-for-sale debt securities be recorded through an allowance for credit losses. ASU 2016-13 limits the amount of credit losses to be recognized for available-for-sale debt securities to the amount by which carrying value exceeds fair value and also requires the reversal of previously recognized credit losses if fair value increases. In April, May and November 2019, the FASB issued additional amendments to the new guidance related to transition and clarification. In November 2019, the FASB issued ASU 2019-10, Financial Instruments – Credit Losses (Topic 326), Derivatives

and Hedging (Topic 815), and Leases (Topic 842): Effective Dates, which deferred the effective date of this standard for all entities except SEC filers that are not smaller reporting companies to fiscal years beginning after December 15, 2022, including interim periods within those fiscal years. Early adoption is permitted. The Company is currently evaluating the timing and impact of adopting this new accounting standard on its condensed consolidated financial statements and related disclosures.

In August 2020, the FASB issued ASU 2020-06, Debt—Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging—Contracts in Entity’s Own Equity (Subtopic 815-40): Accounting for Convertible Instruments and Contracts in an Entity’s Own Equity (ASU 2020-06), which simplifies the accounting for certain financial instruments with characteristics of liabilities and equity, including convertible instruments and contracts on an entity’s own equity. Specifically, ASU 2020-06 simplifies accounting for convertible instruments by removing major separation models in ASC 470-20 that require separate accounting for embedded conversion features. The ASU also removes certain settlement conditions in ASC 815-40 that are required for equity contracts to qualify for the derivative scope exception, which will permit more equity contracts to qualify for the scope exception and simplifies the diluted earnings per share (EPS) calculation in certain areas. The ASU is effective for interim and annual periods beginning after December 15, 2021, with early adoption permitted. Adoption of the ASU can either be on a modified retrospective or full retrospective basis. The Company is currently evaluating the impacts of ASU 2020-06 on its condensed consolidated financial statements and related disclosures.

- Definition

Disclosure of accounting policy for income taxes, which may include its accounting policies for recognizing and measuring deferred tax assets and liabilities and related valuation allowances, recognizing investment tax credits, operating loss carryforwards, tax credit carryforwards, and other carryforwards, methodologies for determining its effective income tax rate and the characterization of interest and penalties in the financial statements.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 45
-Paragraph 25
-URI http://asc.fasb.org/extlink&oid=120406818&loc=d3e32247-109318

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32847-109319

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 954
-SubTopic 740
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6491622&loc=d3e9504-115650

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=84158767&loc=d3e18780-107790

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 19
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32840-109319

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=116657188&loc=SL116659661-227067

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 9
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32639-109319

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 45
-Paragraph 28
-URI http://asc.fasb.org/extlink&oid=120406818&loc=d3e32280-109318

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 17
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32809-109319

+ Details

Restructurings - Summary of Estimates of Restructuring Costs (Parenthetical) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Restructuring Cost And Reserve [Line Items]
Restructuring costs incurred	$ (1,052,000)	$ 0
Employee Severance and Related Benefits
Restructuring Cost And Reserve [Line Items]
Restructuring costs incurred	$ (2,663,000)

Collaboration Agreements

3 Months Ended

Mar. 31, 2021

Collaboration Agreement [Abstract]

Collaboration Agreements

Note 8 - Collaboration Agreements

Allergan Agreement

In January 2017, the Company and Allergan Pharmaceuticals International Limited (Allergan) entered into the Research, Development, Collaboration and License Agreement (the Allergan Agreement) to develop and commercialize select microbiome gastrointestinal disease therapies. In June 2020, following its acquisition of Allergan, AbbVie Inc. (AbbVie), on behalf of Allergan, gave written notice of termination of the Allergan Agreement, which subsequently became effective on October 10, 2020. Upon termination, the licenses granted by the Company and its know-how reverted to the Company.

For the three months ended March 31, 2020, the Company recognized $4.1 million in revenue associated with the Allergan Agreement. A contract asset balance of $1.0 million was recorded as of December 31, 2020. There were no deferred revenue contract liabilities as of December 31, 2020 and no revenue, contract assets or contract liabilities recognized as of and for the three months ended March 31, 2021 due to the termination of the Allergan Agreement in 2020.

BeiGene Agreement

In July 2020, the Company and BeiGene, Ltd. (BeiGene) entered into a Collaboration Agreement (the BeiGene Agreement) to develop and commercialize the Company’s novel core inhibitor product candidates VBR, ABI-H2158 and ABI-H3733 for chronic HBV infection (the Licensed Product Candidates) in the People’s Republic of China, Hong Kong, Taiwan and Macau (the Territory). Under the agreement, the Company and BeiGene are collaborating on certain global clinical studies and both the Company and BeiGene will independently conduct other clinical studies in their own respective territories.

BeiGene agreed to pay all development and regulatory costs for the Licensed Product Candidates in the Territory up to an aggregate of $45.0 million. Development and regulatory costs for the Licensed Product Candidates for the Territory in excess of $45.0 million will be shared equally by the Company and BeiGene. If the Company conducts certain ancillary studies outside of the plan to develop these candidates in the Territory, BeiGene may elect to obtain access to the know-how and clinical data resulting for such ancillary studies and shall reimburse the Company proportionally for the Territory costs of such studies. Activities under the BeiGene Agreement will be governed by a joint steering committee (JSC) consisting of equal representatives from each party to the agreement. All decisions of the JSC are to be made by consensus with final decision-making authority granted to each party based on key areas of the collaboration for which they are responsible. During the term of the BeiGene Agreement, neither party will commercialize any competing products in the Territory. The Company will be responsible for manufacturing and supply of the candidates to be used in and outside of the Territory, although the parties may approve BeiGene to take on some or all of the commercial supply activities of the applicable Licensed Products in the Territory.

The Company is not obligated to perform pre-Phase 3 clinical study development work outside the Territory on ABI-H2158 and ABI-H3733 but must provide BeiGene pre-Phase 3 clinical study know-how and development results if and when such development efforts are completed. If, after ABI-H2158 and ABI-H3733 reach the end of Phase 2 clinical studies, the Company and BeiGene are unable to mutually agree on the terms of a Phase 3 global study, BeiGene may elect to terminate the BeiGene Agreement solely as it relates to that compound, as applicable. Such a termination would result in Assembly regaining all rights to the applicable compound in the Territory. In addition, BeiGene may terminate the BeiGene Agreement for convenience at any time upon 90 days’ advance written notice to Assembly. The BeiGene Agreement also contains customary provisions for termination by either party, including in the event of breach of the BeiGene Agreement, subject to cure.

Pursuant to the terms of the BeiGene Agreement, the Company received an upfront cash payment of $40.0 million from BeiGene for the delivery of exclusive, royalty-bearing licenses to develop and commercialize the Licensed Product Candidates in the Territory, and the Company is eligible to receive up to approximately $500.0 million in cash milestone payments, comprised of up to $113.8 million for development and regulatory milestones and up to $385.0 million in net sales milestones. In addition, the Company is eligible to receive tiered royalties at percentages ranging from the mid-teens to the low thirties of net sales.

The BeiGene Agreement is within the scope of the collaborative arrangements guidance as both parties are active participants and are exposed to significant risks and rewards dependent on the success of commercializing the Licensed Product Candidates in the Territory but that the unit of account related to the delivery of Licensed Product Candidates is within the scope of the contract with customers guidance. The remaining units of account related to participation on the JSC and subcommittees, clinical supply and other in Territory and global development activities (the Collaboration Activities) are within the scope of the collaborative arrangements guidance. Commercial supply will be evaluated as a separate contract when the agreement is executed and a purchase order is received from BeiGene.

The Company identified the following material promises related to the contract with customers unit of account under the BeiGene Agreement: (1) the transfer of the VBR License; (2) the transfer of the ABI-H2158 License; and (3) the transfer of the ABI-H3733 License. The Company concluded each of these licenses to be functional as they have significant standalone functionality and grants BeiGene the right to use the Company’s intellectual property as it exists on the effective date of the license. The ABI-H2158 and ABI-H3733 Licenses have a continuing technology transfer obligation that is considered to be an attribute of these licenses. The agreed upon prices for the clinical and

commercial supply of the Licensed Product Candidates to BeiGene do not represent material rights, and therefore are not performance obligations, and such pricing on an aggregate basis represents the standalone selling price an entity would typically pay for such a product in that region or market. There are also no minimum purchase commitments.

The Company estimated the standalone selling price (SSP) of the Licenses using an income-based valuation approach for the estimated value a licensor of the compounds would receive considering the stage of the compound’s development. The Company believes a change in the assumptions used to determine its best estimate of SSP would not have a significant value on the allocation of consideration received.

The transaction price at the inception of the agreement was limited to the $40.0 million upfront payment. The variable consideration related to the remaining development and commercialization milestone payments has not been included in the transaction price as these were fully constrained as of March 31, 2021. 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 studies and regulatory approvals which are not within its control and uncertain at this stage. Any variable consideration related to sales-based milestones (including royalties) will be recognized when the related sales occur as they were determined to relate predominantly to the Licensed Product Candidates granted to BeiGene. The Company will reevaluate the transaction price in each reporting period as uncertain events are resolved or other changes in circumstances occur.

Upon entering into the BeiGene Agreement in July 2020, the Company recognized $31.0 million as collaboration revenue for the amount allocated to the VBR License as substantial completion of the license technology transfer had occurred. The remaining transaction price allocated to the ABI-H2158 and ABI-H3733 Licenses of $9.0 million was recorded as a long-term deferred revenue contract liability on the unaudited condensed consolidated balance sheet as of March 31, 2021 and December 31, 2020. Revenue for these performance obligations will be recognized when the Company provides pre-Phase 3 clinical study know-how and development results for these compounds to BeiGene or a termination of the BeiGene Agreement for the respective compound.

Payments to, or reimbursements from, BeiGene related to the Collaboration Activities will be accounted for as an increase to or reduction of research and development expenses when incurred or realized, respectively. During the three months ended March 31, 2021, the Company did not recognize any increase or reduction of research and development expense under the BeiGene Agreement.

The Company incurred $3.5 million in incremental costs of obtaining the BeiGene Agreement. These contract costs have been capitalized and are being recognized consistent with the pattern of recognition of revenue associated with the Licensed Product Candidates. As of March 31, 2021 and December 31, 2020, $0.8 million remains unamortized and is included in other assets on the condensed consolidated balance sheet.

Arbutus Agreement

In August 2020, the Company and Arbutus Biopharma Corporation (Arbutus) entered into a Clinical Trial Collaboration Agreement (the Arbutus Agreement) to conduct a randomized, multi-center, open-label Phase 2 clinical study to explore the safety, PK and antiviral activity of the triple combination of VBR, AB-729 and a nucleos(t)ide reverse transcriptase inhibitor (NrtI) compared to the double combinations of VBR plus NrtI and AB-729 plus NrtI. Assembly and Arbutus will share responsibility for the costs of the study equally, excluding manufacturing supply which will be the burden of each company to supply their respective drugs VBR and AB-729.

The Arbutus Agreement is within the scope of the collaborative arrangements guidance as both parties are active participants and are exposed to significant risks and rewards dependent on the success of the collaborative activity. Arbutus is not a customer as it does not obtain an output from the collaborative activities as they were not provided an exclusive license to VBR or the ability to manufacture VBR, and the Company does not consider performing such collaborative activities to be a part of its ongoing activities.

The revenue from contracts with customers guidance was considered by analogy in determining the unit of account, and the recognition and measurement of such unit of account for collaborative activities under the Arbutus Agreement and concluded there is one activity, to run an open-label Phase 2 clinical study, which is akin to performance obligation related to collaborative activities. Reimbursements and cost-sharing portions of this

performance obligation will be reflected as a reduction of research and development expense when realized in the Company’s condensed consolidated statements of operations, as the Company does not consider performing research and development services for reimbursement to be a part of its ongoing major or central operations. During the three months ended March 31, 2021, the Company recognized a reduction of research and development expenses of $0.5 million under the Arbutus Agreement.

Contract Liabilities

The following table presents changes in the Company’s contract liabilities (in thousands):

	Balance at Beginning of Period		Additions		Deductions		Balance at End of Period
Three Months Ended March 31, 2021
Contract liabilities:
Deferred revenue	$	8,987	$	—	$	—	$	8,987

	Balance at Beginning of Period		Additions		Deductions			Balance at End of Period
Three Months Ended March 31, 2020
Contract liabilities:
Deferred revenue	$	37,048	$	—	$	(1,007	)	$	36,041

Summary of Significant Accounting Policies and Recent Accounting Pronouncements - Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share (Details) - shares	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive securities excluded from computation of loss per share	8,093,959	7,418,558
Warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive securities excluded from computation of loss per share		15,296
Options
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive securities excluded from computation of loss per share	7,382,515	6,370,396
Common stock subject to purchase under ESPP
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive securities excluded from computation of loss per share	44,870	32,940
RSUs
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive securities excluded from computation of loss per share	666,574	999,926

Stock Plans and Stock-Based Compensation - Summary of Assumptions Used to Estimate Fair Values of Stock Options Grants (Details) - $ / shares	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award
Expected dividend yield	0.00%	0.00%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award
Exercise price	$ 4.33	$ 14.45
Expected volatility	81.40%	66.40%
Risk-free rate	0.50%	0.46%
Expected term (years)	5 years 6 months	5 years 6 months
Maximum
Share-based Compensation Arrangement by Share-based Payment Award
Exercise price	$ 5.79	$ 19.13
Expected volatility	91.20%	82.20%
Risk-free rate	1.37%	1.44%
Expected term (years)	7 years	7 years

Restructurings

3 Months Ended

Mar. 31, 2021

Restructuring And Related Activities [Abstract]

Restructurings

Note 5 – Restructurings

Restructuring charges relate to the Company’s decision to relocate its headquarters to South San Francisco, California, which was approved by the Board of Directors in November 2019 and effective January 1, 2020 and the wind-down of the Company’s Microbiome program, which the Company and its Board of Directors determined in December 2020 was in the Company’s best interest, enabling the Company to prioritize resources and focus on the advancement of its pipeline of novel core inhibitors for chronic HBV infection.

The following table summarizes the Company’s estimates of costs to be incurred and expected to be incurred (in thousands):

	Total Restructuring Cost			Employee Severance and Related Benefits				Asset Impairment and Other Costs
Total estimated restructuring costs to be incurred	$	6,988		$	3,798			$	3,190
Restructuring costs incurred for the three months ended March 31, 2021	$	(1,052	)	$	(2,663	)	⁽¹⁾	$	1,611
Cumulative restructuring costs incurred through March 31, 2021	$	6,726		$	3,536			$	3,190

⁽¹⁾

The following table presents where the restructuring charges were recognized during the three months ended March 31, 2021 (in thousands):

Research and development	$	(1,329	)
General and administrative		277
Total	$	(1,052	)

There were no restructuring costs incurred for the three months ended March 31, 2020.

The following table presents the activity in the accrued restructuring charges during the three months ended March 31, 2021 (in thousands):

	Total Restructuring Cost			Employee Severance and Related Benefits			Asset Impairment and Other Costs
Accrued balance as of December 31, 2020	$	4,164		$	4,164		$	—
Costs incurred		1,611			—			1,611
Reductions for cash payments		(4,529	)		(2,918	)		(1,611	)	⁽¹⁾
Accrued balance as of March 31, 2021	$	1,246		$	1,246		$	—

⁽¹⁾	Cash payments are presented net of proceeds received from the sale of assets of $0.9 million.

The asset impairment and other costs includes $1.4 million for the remaining payment obligations of leased equipment the Company purchased and sold to third parties.

The Company expects the accrued restructuring liability to be fully paid in 2021.

Fair Value Measurements and Investments in Marketable Securities - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Investments in marketable securities:
Long-term marketable securities	$ 0
Fair value assets transfers between level 1, level 2 or level 3	$ 0	$ 0
Short Term Investments
Investments in marketable securities:
Available for sale securities maturity term	less than one year

Restructurings - Summary of Estimates of Restructuring Costs (Details) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Restructuring Cost And Reserve [Line Items]
Total estimated restructuring costs to be incurred	$ 6,988,000
Restructuring costs incurred	(1,052,000)	$ 0
Cumulative restructuring costs incurred through March 31, 2021	6,726,000
Employee Severance and Related Benefits
Restructuring Cost And Reserve [Line Items]
Total estimated restructuring costs to be incurred	3,798,000
Restructuring costs incurred	(2,663,000)
Cumulative restructuring costs incurred through March 31, 2021	3,536,000
Asset Impairment and Other Costs
Restructuring Cost And Reserve [Line Items]
Total estimated restructuring costs to be incurred	3,190,000
Restructuring costs incurred	1,611,000
Cumulative restructuring costs incurred through March 31, 2021	$ 3,190,000

Summary of Significant Accounting Policies and Recent Accounting Pronouncements - Additional Information (Details) - $ / shares	1 Months Ended	3 Months Ended
	Dec. 31, 2019	Mar. 31, 2021
ASU 2020-10
Significant Accounting Policies [Line Items]
Change in Accounting Principle, Accounting Standards Update, Adoption Date		Jan. 01, 2021
Change in Accounting Principle, Accounting Standards Update, Adopted [true false]		true
Change in Accounting Principle, Accounting Standards Update, Immaterial Effect [true false]		true
Common Stock
Significant Accounting Policies [Line Items]
Number of shares sold		5,991,858
Public Offering
Significant Accounting Policies [Line Items]
Number of pre-funded warrants sold to purchase of common stock	2,424,242
Pre-funded warrants exercisable price per share	$ 0.001
Public Offering \| Common Stock
Significant Accounting Policies [Line Items]
Number of shares sold	6,287,878

Other Accrued Expenses

3 Months Ended

Mar. 31, 2021

Payables And Accruals [Abstract]

Other Accrued Expenses

Note 4 – Other Accrued Expenses

Other accrued expenses consist of the following (in thousands):

	March 31,		December 31,
	2021		2020
Accrued expenses:
Accrued compensation	$	1,842	$	7,016
Accrued restructuring charges		1,246		4,164
Accrued professional fees and other		571		807
Total accrued expenses	$	3,659	$	11,987

Stockholders' Equity (Tables)

3 Months Ended

Mar. 31, 2021

Equity [Abstract]

Summary of Warrants to Purchase Shares of Company's Common Stock

The following warrants to purchase shares of the Company’s common stock were issued and outstanding:

Issue date

Expiration

date

Exercise Price

per Share

Number of warrants outstanding

December 16, 2019

None

0.001

2,424,242

Restructurings - Schedule of Accrued Restructuring Charges (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021 USD ($)
Restructuring Cost And Reserve [Line Items]
Accrued balance as of December 31, 2020	$ 4,164
Costs incurred	1,611
Reductions for cash payments	(4,529)
Accrued balance as of March 31, 2021	1,246
Employee Severance and Related Benefits
Restructuring Cost And Reserve [Line Items]
Accrued balance as of December 31, 2020	4,164
Reductions for cash payments	(2,918)
Accrued balance as of March 31, 2021	1,246
Asset Impairment and Other Costs
Restructuring Cost And Reserve [Line Items]
Costs incurred	1,611
Reductions for cash payments	$ (1,611)

CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Current assets
Cash and cash equivalents	$ 78,253	$ 59,444
Marketable securities	136,631	156,969
Accounts receivable from collaborations	203	1,230
Prepaid expenses and other current assets	6,547	6,850
Total current assets	221,634	224,493
Property and equipment, net	1,493	1,600
Operating lease right-of-use (ROU) assets	8,104	9,131
Other assets	6,034	6,392
Indefinite-lived intangible asset	29,000	29,000
Goodwill	12,638	12,638
Total assets	278,903	283,254
Current liabilities
Accounts payable	3,562	4,598
Accrued clinical expenses	4,342	4,444
Other accrued expenses	3,659	11,987
Operating lease liabilities - short-term	2,934	3,404
Total current liabilities	14,497	24,433
Deferred tax liabilities	2,531	2,531
Deferred revenue	8,987	8,987
Operating lease liabilities - long-term	5,644	6,725
Total liabilities	31,659	42,676
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 March 31, 2021 and December 31, 2020; 40,104,684 and 34,026,680 shares issued and outstanding as of March 31, 2021 and December 31, 2020, respectively	40	34
Additional paid-in capital	776,248	742,387
Accumulated other comprehensive loss	(271)	(270)
Accumulated deficit	(528,773)	(501,573)
Total stockholders' equity	247,244	240,578
Total liabilities and stockholders' equity	$ 278,903	$ 283,254

Restructurings - Schedule of Accrued Restructuring Charges (Parenthetical) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Restructuring And Related Activities [Abstract]
Proceeds received from the sale of assets	$ 857	$ 0

Other Accrued Expenses - Schedule of Other Accrued Expenses (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Accrued expenses:
Accrued compensation	$ 1,842	$ 7,016
Accrued restructuring charges	1,246	4,164
Accrued professional fees and other	571	807
Total accrued expenses	$ 3,659	$ 11,987

Leases - Additional Information (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Leases [Abstract]
Operating lease, liability	$ 8,578
Operating lease, ROU assets	$ 8,104	$ 9,131
Weighted-average remaining lease term - operating leases	2 years 8 months 12 days	2 years 8 months 12 days
Weighted-average discount rate - operating leases	9.70%	9.20%

Other Accrued Expenses (Tables)

3 Months Ended

Mar. 31, 2021

Payables And Accruals [Abstract]

Schedule of Other Accrued Expenses

Other accrued expenses consist of the following (in thousands):

	March 31,		December 31,
	2021		2020
Accrued expenses:
Accrued compensation	$	1,842	$	7,016
Accrued restructuring charges		1,246		4,164
Accrued professional fees and other		571		807
Total accrued expenses	$	3,659	$	11,987

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Cash flows from operating activities
Net loss	$ (27,200)	$ (26,655)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	107	115
Stock-based compensation	(286)	4,924
Net accretion and amortization of investments in marketable debt securities	92	(135)
Non-cash rent expense	1,135	1,151
Loss on disposal of fixed assets	1,611	0
Changes in operating assets and liabilities:
Accounts receivable from collaboration	1,027	319
Prepaid expenses and other current assets	303	795
Other assets	358	23
Accounts payable	(1,036)	373
Accrued clinical expenses	(102)	(193)
Other accrued expenses	(8,312)	(3,808)
Deferred revenue	0	(1,007)
Operating lease liabilities	(1,041)	(1,120)
Net cash used in operating activities	(33,344)	(25,218)
Cash flows from investing activities
Purchases of property and equipment	(3,078)	(65)
Proceeds from sale of property and equipment	857	0
Purchases of marketable securities	(49,213)	(44,242)
Proceeds from maturities of marketable securities	66,450	55,000
Proceeds from sale of marketable securities	3,000	10,000
Net cash provided by investing activities	18,016	20,693
Cash flows from financing activities
Proceeds from the exercise of stock options	0	119
Proceeds from sale of common stock, net of issuance costs	34,137	0
Net cash provided by financing activities	34,137	119
Net increase (decrease) in cash and cash equivalents	18,809	(4,406)
Cash and cash equivalents at the beginning of the period	59,444	46,732
Cash and cash equivalents at the end of the period	78,253	42,326
Supplemental non-cash investing and financing activities
Operating lease liabilities arising from obtaining ROU assets	42	362
Remeasurement of lease liabilities arising from modification of ROU assets	$ (788)	$ 0

Stock Plans and Stock-Based Compensation - Additional Information (Details) - USD ($)	1 Months Ended	3 Months Ended
	Dec. 31, 2017	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Jun. 30, 2020	May 31, 2019	May 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award
Shares reserved for issuance (in shares)					4,600,000	3,000,000	1,900,000
Weighted-average grant-date fair value of options granted		$ 3.15	$ 9.74
Total intrinsic value of options exercised			$ 200,000
Options exercised		0
Stock-based compensation expense		$ (286,000)	$ 4,924,000
Unrecognized stock-based compensation expense		$ 18,600,000
Unrecognized stock-based compensation expense expected to be recognized, term		1 year 9 months 18 days
2018 ESPP
Share-based Compensation Arrangement by Share-based Payment Award
Number of common stock purchased by employees		0	0
RSUs
Share-based Compensation Arrangement by Share-based Payment Award
Total fair value of awards vested and settled		$ 2,500,000	$ 2,400,000
Total intrinsic value of awards vested and settled		$ 600,000	$ 1,500,000
Granted		199,875
Forfeited		144,240
Outstanding awards		806,574		886,868
RSUs \| Chief Executive Officer
Share-based Compensation Arrangement by Share-based Payment Award
Granted	100,000
Forfeited		25,000
Outstanding awards		75,000
Total fair value of awards		$ 800,000
Stock-based compensation expense		$ 0
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award
Shares reserved for issuance (in shares)							400,000

CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Mar. 31, 2021	Dec. 31, 2020
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	40,104,684	34,026,680
Common stock, shares outstanding	40,104,684	34,026,680

Document And Entity Information - shares	3 Months Ended
Document And Entity Information - shares	Mar. 31, 2021	May 03, 2021
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Transition Report	false
Document Period End Date	Mar. 31, 2021
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	2021
Document Fiscal Period Focus	Q1
Entity Central Index Key	0001426800
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Trading Symbol	ASMB
Security Exchange Name	NASDAQ
Title of 12(b) Security	Common Stock, par value $0.001
Entity Common Stock, Shares Outstanding		40,158,197
Entity Shell Company	false
Entity Emerging Growth Company	false
Entity Small Business	false

Collaboration Agreements - Schedule of Changes in Contract Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Collaboration Agreement [Abstract]
Balance at Beginning of Period	$ 8,987	$ 37,048
Additions	0	0
Deductions	0	(1,007)
Balance at End of Period	$ 8,987	$ 36,041

Stock Plans and Stock-Based Compensation

3 Months Ended

Mar. 31, 2021

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock Plans and Stock-Based Compensation

Note 7 – Stock Plans and Stock-Based Compensation

Equity Incentive Plans

In May 2018, the Company’s stockholders approved (1) the Assembly Biosciences, Inc. 2018 Stock Incentive Plan (the 2018 Plan) pursuant to which the Company reserved 1,900,000 shares of its common stock for issuance in connection with equity incentive awards and (2) the Assembly Biosciences Inc. Employee Stock Purchase Plan (the 2018 ESPP) pursuant to which the Company reserved 400,000 shares of its common stock for issuance in connection with purchases by employees pursuant to this plan.

In May 2019, the Company’s stockholders approved an amendment to the 2018 Plan that increased the aggregate number of shares of common stock reserved under the 2018 Plan to 3,000,000.

In June 2020, the Company’s stockholders approved an amendment to the 2018 Plan that increased the aggregate number of shares of common stock reserved under the 2018 Plan to 4,600,000.

As of March 31, 2021, the Company had awards outstanding under the following shareholder-approved plans: the 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 March 31, 2021, the Company also had awards outstanding under the Assembly Biosciences, Inc. 2017 Inducement Award Plan (the 2017 Plan), the Assembly Biosciences, Inc. 2019 Inducement Award Plan (the 2019 Plan) and the Assembly Biosciences, Inc. 2020 Inducement Award Plan (the 2020 Plan).

The Company issues new shares of common stock to settle options exercised and vested RSUs. The Company also issues new shares of common stock in connection with purchases of shares of common stock by eligible employees under the Company’s 2018 ESPP.

Stock Plan Activity

Stock Options

A summary of the Company’s option activity and related information for the three months ended March 31, 2021 is as follows:

	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, 2020		6,696,592		$	15.70
Granted		1,112,250			4.40
Forfeited		(417,127	)		20.89
Expired		(9,200	)		30.00
Outstanding as of March 31, 2021		7,382,515		$	13.68		7.1	$	1,393
Options vested and exercisable as of March 31, 2021		4,260,738		$	14.81		5.5	$	1,110

The weighted-average grant-date fair value of options granted was $3.15 and $9.74 during the three months ended March 31, 2021 and 2020, respectively. The total intrinsic value of options exercised during the three months ended March 31, 2020 was $0.2 million. There were no options exercised during the three months ended March 31, 2021.

RSUs

A summary of the Company’s RSUs and related information for the three months ended March 31, 2021 is as follows:

	Number of RSUs				Weighted Average Fair Value Per RSU at Grant Price
Nonvested as of December 31, 2020		886,868			$	20.35
Granted		199,875				4.33
Vested		(135,929	)			19.73
Forfeited		(144,240	)			17.49
Nonvested as of March 31, 2021		806,574		⁽¹⁾	$	16.81

⁽¹⁾	Includes 140,000 RSUs that have vested but are subject to deferred settlement, which have a weighted average remaining contractual term of 1.9 years.

The total fair value of RSUs vested and settled during the three months ended March 31, 2021 and 2020 was $2.5 million and $2.4 million, respectively. The total intrinsic value of RSUs vested and settled during the three months ended March 31, 2021 and 2020 was $0.6 million and $1.5 million, respectively.

In September 2019, the Company granted 100,000 RSUs with performance-based vesting conditions to its chief executive officer. On March 31, 2021, 25,000 of these awards were forfeited back to the Company due to the time period to complete one of the performance conditions expiring. The outstanding 75,000 awards with an aggregate fair value of $0.8 million vest upon performance conditions not yet deemed probable. Accordingly, no stock-based compensation expense has been recognized as of March 31, 2021.

ESPP

There were no shares purchased under the 2018 ESPP during the three months ended March 31, 2021 or 2020.

Valuation Assumptions

The fair value of the stock options granted or modified during the periods indicated was estimated using the Black-Scholes option pricing model, based on the following assumptions:

	Three Months Ended March 31,
	2021	2020
Exercise price	$4.33 - $5.79	$14.45 - $19.13
Expected volatility	81.4% - 91.2%	66.4% - 82.2%
Risk-free rate	0.50% - 1.37%	0.46% - 1.44%
Expected term (years)	5.5 - 7.0	5.5 - 7.0
Expected dividend yield	0%	0%

The fair value of RSUs granted is determined based on the price of the Company’s common stock on the date of grant.

The fair value of ESPP purchase rights were not material for any period presented.

Stock-Based Compensation Expense

The following table summarizes the components of total stock-based compensation expense included in the condensed consolidated statements of operations and comprehensive loss (in thousands):

	Three Months Ended March 31,
	2021				2020
Research and development	$	(2,091	)	⁽¹⁾	$	1,945
General and administrative		1,805				2,979
Total stock-based compensation expense	$	(286	)		$	4,924

⁽¹⁾	Includes the reversal of previously recognized stock-based compensation expense of $3.6 million related to forfeited awards of terminated employees, $2.7 million of which resulted from the wind-down of the Company’s Microbiome program (see Note 5).

As of March 31, 2021, there was $18.6 million of total unrecognized stock-based compensation related to outstanding equity awards, which is expected to be recognized over a weighted average remaining amortization period of 1.8 years.

Fair Value Measurements and Investments in Marketable Securities - Schedule of Investments in Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Investments in marketable securities:
Amortized Cost	$ 78,253	$ 59,444
Fair Value	136,631	156,969
Amortized Cost	203,131	211,017
Gross Unrealized Gain	2	7
Gross Unrealized Loss	(8)	(4)
Fair Value	203,125	211,020
Cash equivalents
Investments in marketable securities:
Amortized Cost	66,494	54,051
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Fair Value	66,494	54,051
Cash equivalents \| Money market fund
Investments in marketable securities:
Amortized Cost	66,494	47,553
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Fair Value	66,494	47,553
Short-term investments
Investments in marketable securities:
Amortized Cost	136,637	156,966
Gross Unrealized Gain	2	7
Gross Unrealized Loss	(8)	(4)
Fair Value	136,631	156,969
U.S and foreign corporate debt securities \| Short-term investments
Investments in marketable securities:
Amortized Cost	22,954	16,939
Gross Unrealized Gain	0	3
Gross Unrealized Loss	(6)	(3)
Fair Value	22,948	16,939
U.S. and foreign commercial paper \| Cash equivalents
Investments in marketable securities:
Amortized Cost		6,498
Gross Unrealized Gain		0
Gross Unrealized Loss		0
Fair Value		6,498
U.S. and foreign commercial paper \| Short-term investments
Investments in marketable securities:
Amortized Cost	96,921	103,356
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Fair Value	96,921	103,356
Asset-backed securities \| Short-term investments
Investments in marketable securities:
Amortized Cost	16,762	12,674
Gross Unrealized Gain	2	2
Gross Unrealized Loss	(2)	(1)
Fair Value	$ 16,762	12,675
U.S. treasury securities \| Short-term investments
Investments in marketable securities:
Amortized Cost		23,997
Gross Unrealized Gain		2
Gross Unrealized Loss		0
Fair Value		$ 23,999

Leases

3 Months Ended

Mar. 31, 2021

Lease Cost [Abstract]

Leases

Note 10 - Leases

The Company leases office and laboratory space in South San Francisco, California under a sub-sublease that expires in December 2023. The sub-sublease contains scheduled rent increases over the lease term. The Company also leases office space in Carmel, Indiana under a lease agreement that expires in August 2023. In February 2021, the Company subleased substantially all of the office space under the Carmel, Indiana lease. The Company also leases office and laboratory space in Groton, Connecticut that supported the Microbiome program under a lease that expires in June 2021. Due to the wind-down of the Microbiome program, the lease will not be renewed. The Company’s China subsidiary leased office space and lab space in Shanghai, which the Company let expire in March 2021 and December 2020, respectively. Additionally, the Company’s China subsidiary leases office space in Beijing under a lease agreement that expires in December 2021. Certain lease contracts contain renewal clauses that the Company assesses on a case-by-case basis. The Company also leases certain laboratory equipment accounted for as operating leases expiring at various dates, with the final lease expiring in 2023. In February 2021, the Company purchased substantially all of the leased equipment used for the Microbiome program from its leasing agency and sold them to third parties (see Note 5).

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 March 31, 2021, the Company had operating lease liabilities of $8.6 million and ROU assets of $8.1 million.

The following summarizes quantitative information about the Company’s operating leases (in thousands):

	Three Months Ended March 31,
	2021			2020
Lease cost
Operating lease cost	$	1,135		$	1,151
Short-term lease cost		103			100
Variable lease cost		343			284
Sublease income		(27	)		—
Total lease cost	$	1,554		$	1,535

	Three Months Ended March 31,
	2021		2020
Operating cash flows from operating leases	$	1,041	$	1,120
ROU assets exchanged for new operating lease liabilities	$	42	$	362

As of both March 31, 2021 and December 31, 2020, the weighted-average remaining lease term for operating leases was 2.7 years. As of March 31, 2021 and December 31, 2020, the weighted-average discount rate for operating leases was 9.7% and 9.2%, respectively.

As of March 31, 2021, the maturities of the Company’s operating lease liabilities were as follows (in thousands):

Nine months ending December 31, 2021	$	2,724
Year Ending December 31, 2022		3,612
Year Ending December 31, 2023		3,460
Total		9,796
Less: present value discount		(1,218	)
Operating lease liabilities	$	8,578

Milestones and Research Agreements - Additional Information (Details) - USD ($)	1 Months Ended	3 Months Ended
	Nov. 30, 2020	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Milestone and Research Agreements:
Potential milestone payment		$ 800,000
Performance milestone payment		0
Accrued expenses		3,659,000	$ 11,987,000
Microbiome Targeted Colonic Delivery Program
Milestone and Research Agreements:
Accrued expenses		0		$ 0
Door Agreement
Milestone and Research Agreements:
License agreement term	2 years
Up-front payment	$ 1,800,000
Research and development funding charges		$ 400,000
Door Agreement \| Maximum
Milestone and Research Agreements:
Success-based milestones	35,000,000.0
Exercise and annual fees	2,000,000.0
Door Agreement \| Minimum [Member]
Milestone and Research Agreements:
Exercise and annual fees	$ 100,000

Nature of Business - Additional Information (Details)	3 Months Ended
Nature of Business - Additional Information (Details)	Mar. 31, 2021 segment product
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Operating segments (in segments) \| segment	1
FDA approved products (in products) \| product	0

Fair Value Measurements and Investments in Marketable Securities (Tables)

3 Months Ended

Mar. 31, 2021

Investments Debt And Equity Securities [Abstract]

Schedule of Investments in Marketable Securities

Investments in marketable securities consisted of the following (in thousands):

	March 31, 2021
	Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss			Fair Value
Cash equivalents
Money market funds	$	66,494	$	—	$	—		$	66,494
Total cash equivalents		66,494		—		—			66,494
Short-term investments
U.S. and foreign corporate debt securities		22,954		—		(6	)		22,948
Asset-backed securities		16,762		2		(2	)		16,762
U.S. and foreign commercial paper		96,921		—		—			96,921
Total short-term investments		136,637		2		(8	)		136,631
Total cash equivalents and investments	$	203,131	$	2	$	(8	)	$	203,125

	December 31, 2020
	Amortized Cost		Gross Unrealized Gain ⁽¹⁾		Gross Unrealized Loss ⁽¹⁾			Fair Value
Cash equivalents
Money market funds	$	47,553	$	—	$	—		$	47,553
U.S. and foreign commercial paper		6,498		—		—			6,498
Total cash equivalents		54,051		—		—			54,051
Short-term investments
U.S. and foreign corporate debt securities		16,939		3		(3	)		16,939
Asset-backed securities		12,674		2		(1	)		12,675
U.S. treasury securities		23,997		2		—			23,999
U.S. and foreign commercial paper		103,356		—		—			103,356
Total short-term investments		156,966		7		(4	)		156,969
Total cash equivalents and investments	$	211,017	$	7	$	(4	)	$	211,020

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 (in thousands):

	March 31, 2021
	Level 1		Level 2		Level 3		Fair Value
Cash equivalents
Money market fund	$	66,494	$	—	$	—	$	66,494
Total cash equivalents		66,494		—		—		66,494
Short-term investments
U.S. and foreign corporate debt securities		—		22,948		—		22,948
Asset-backed securities		—		16,762		—		16,762
U.S. and foreign commercial paper		—		96,921		—		96,921
Total short-term investments		—		136,631		—		136,631
Total assets measured at fair value	$	66,494	$	136,631	$	—	$	203,125

	December 31, 2020
	Level 1		Level 2		Level 3		Fair Value
Cash equivalents
Money market fund	$	47,553	$	—	$	—	$	47,553
U.S. and foreign commercial paper		—		6,498		—		6,498
Total cash equivalents		47,553		6,498		—		54,051
Short-term investments
U.S. and foreign corporate debt securities		—		16,939		—		16,939
Asset-backed securities		—		12,675		—		12,675
U.S. treasury securities		—		23,999		—		23,999
U.S. and foreign commercial paper		—		103,356		—		103,356
Total short-term investments		—		156,969		—		156,969
Total assets measured at fair value	$	47,553	$	163,467	$	—	$	211,020

- Definition

Tabular disclosure of financial instruments measured at fair value, including those classified in shareholders' equity measured on a recurring or nonrecurring basis. Disclosures include, but are not limited to, fair value measurements recorded and the reasons for the measurements, level within the fair value hierarchy in which the fair value measurements are categorized and transfers between levels 1 and 2. Nonrecurring fair value measurements are those that are required or permitted in the statement of financial position in particular circumstances.

+ References + Details

Leases (Tables)

3 Months Ended

Mar. 31, 2021

Lease Cost [Abstract]

Schedule of Quantitative Information of Operating Leases

The following summarizes quantitative information about the Company’s operating leases (in thousands):

	Three Months Ended March 31,
	2021			2020
Lease cost
Operating lease cost	$	1,135		$	1,151
Short-term lease cost		103			100
Variable lease cost		343			284
Sublease income		(27	)		—
Total lease cost	$	1,554		$	1,535

	Three Months Ended March 31,
	2021		2020
Operating cash flows from operating leases	$	1,041	$	1,120
ROU assets exchanged for new operating lease liabilities	$	42	$	362

Schedule of Maturities Operating Lease Liabilities

As of March 31, 2021, the maturities of the Company’s operating lease liabilities were as follows (in thousands):

Nine months ending December 31, 2021	$	2,724
Year Ending December 31, 2022		3,612
Year Ending December 31, 2023		3,460
Total		9,796
Less: present value discount		(1,218	)
Operating lease liabilities	$	8,578

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Income Statement [Abstract]
Collaboration revenue		$ 4,081
Operating expenses:
Research and development	$ 18,554	23,046
General and administrative	8,704	8,729
Total operating expenses	27,258	31,775
Loss from operations	(27,258)	(27,694)
Other income:
Interest and other income, net	58	1,039
Total other income	58	1,039
Net loss	(27,200)	(26,655)
Other comprehensive (loss) income
Unrealized (loss) gain on marketable securities	(1)	115
Comprehensive loss	$ (27,201)	$ (26,540)
Net loss per share, basic and diluted	$ (0.69)	$ (0.76)
Weighted average common shares outstanding, basic and diluted	39,679,734	35,079,756

Nature of Business

3 Months Ended

Mar. 31, 2021

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Nature of Business

Note 1 - Nature of Business

Overview

Assembly Biosciences, Inc. (together with its subsidiaries, Assembly or the Company), incorporated in Delaware in October 2005, is a clinical-stage biotechnology company advancing a novel class of oral therapeutic candidates for the treatment of chronic hepatitis B virus (HBV) infection. The Company operates in one segment and is headquartered in South San Francisco, California, with operations in California and China. Prior to the Company’s wind-down of its Microbiome program on January 31, 2021, the Company also had operations in Connecticut.

The Company’s research and development programs are pursuing multiple drug candidates that inhibit the HBV replication cycle and block the generation of covalently closed circular DNA (cccDNA), with the aim of discovering and developing finite and curative therapies for patients with HBV. Assembly has discovered several novel core inhibitors, which are small molecules that directly target and allosterically modify the HBV core (HBc) protein in a way that affects assembly and stability of HBV nucleocapsids.

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, proceeds from the exercise of warrants and stock options, issuance of debt, and upfront payments related to collaboration agreements. The Company has incurred losses from operations since inception and expects to continue to incur substantial losses for the next several years as it continues its product development efforts. Management believes the Company currently has sufficient funds to meet its operating requirements for at least the next twelve months following the date these unaudited condensed consolidated interim 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. Market volatility resulting from the global novel coronavirus disease (COVID-19) pandemic or other factors could also adversely impact the Company’s ability to access capital when and as needed.

If the Company is unable to generate enough revenue from its collaborations, secure additional sources of funding or receive full and timely collections of amounts due, it may be necessary to significantly reduce the current rate of spending through reductions in staff and delaying, scaling back, or stopping certain research and development programs, including more costly clinical trials.

Stock Plans and Stock-Based Compensation -Schedule of Stock-Based Compensation Expense (Parenthetical) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Compensation expense:
Stock-based compensation expense	$ (286)	$ 4,924
Terminated Employees
Compensation expense:
Stock-based compensation expense	(3,600)
Wind-down of Microbiome Program
Compensation expense:
Stock-based compensation expense	$ (2,700)

Stockholders' Equity - Summary of Warrants to Purchase Shares of Company's Common Stock (Details) - Warrants Issued on December 16, 2019	3 Months Ended
	Mar. 31, 2021 $ / shares shares
Subsidiary Sale Of Stock [Line Items]
Issue date	Dec. 16, 2019
Exercise Price per Share \| $ / shares	$ 0.001
Number of warrants outstanding \| shares	2,424,242

Leases - Schedule of Quantitative Information of Operating Leases (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Lease cost
Operating lease cost	$ 1,135	$ 1,151
Short-term lease cost	103	100
Variable lease cost	343	284
Sublease income	(27)
Total lease cost	1,554	1,535
Operating cash flows from operating leases	1,041	1,120
ROU assets exchanged for new operating lease liabilities	$ 42	$ 362

Stockholders' Equity

3 Months Ended

Mar. 31, 2021

Equity [Abstract]

Stockholders' Equity

Note 6 – Stockholders’ Equity

The Company was authorized to issue 5,000,000 shares of preferred stock as of March 31, 2021 and December 31, 2020, respectively. As of March 31, 2021 and December 31, 2020, no shares of preferred stock were issued and outstanding. The Company was authorized to issue 100,000,000 shares of common stock as of March 31, 2021 and December 31, 2020, respectively.

Sale of Common Stock

In August 2020, the Company filed a shelf registration statement on Form S-3 with the SEC, File No. 333-248469, that became effective on September 4, 2020 (the 2020 Registration Statement). The Company may from time to time sell any combination of the securities described in the 2020 Registration Statement in one or more offerings up to an aggregate offering price of $300.0 million. In connection with the filing of the 2020 Registration Statement, the Company entered into a sales agreement under which the Company may offer and sell shares of its common stock having an aggregate offering price of up to $100.0 million through “at-the-market” offerings (2020 ATM), which shares are included in the $300.0 million of securities registered pursuant to the 2020 Registration Statement. During the three months ended March 31, 2021, the Company sold 5,991,858 shares of common stock under the 2020 ATM, for which the Company received net proceeds of $34.1 million, after deducting commissions, fees and expenses.

Common Stock Warrants

The following warrants to purchase shares of the Company’s common stock were issued and outstanding:

Issue date

Expiration

date

Exercise Price

per Share

Number of warrants outstanding

December 16, 2019

None

0.001

2,424,242

There were no warrants exercised during the three months ended March 31, 2021 or 2020.

Summary of Significant Accounting Policies and Recent Accounting Pronouncements

3 Months Ended

Mar. 31, 2021

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

Use of Estimates

Other Risks and Uncertainties

Income Taxes

Net Loss per Share

A reconciliation of the numerators and the denominators of the basic and diluted net loss per common share computations is as follows (in thousands, except per share amounts):

	Three Months Ended March 31,
	2021			2020
Numerator:
Net loss	$	(27,200	)	$	(26,655	)
Denominator:
Weighted average common shares and pre-funded warrants outstanding - basic and diluted		39,679,734			35,079,756
Net loss per share - basic and diluted	$	(0.69	)	$	(0.76	)

Securities excluded from the computation of diluted loss per share because including them would have been antidilutive are as follows:

	March 31,
	2021		2020
Warrants to purchase common stock		—		15,296
Options to purchase common stock		7,382,515		6,370,396
Common stock subject to purchase under our ESPP		44,870		32,940
Unvested RSUs		666,574		999,926
Total		8,093,959		7,418,558

Adoption of Recent Accounting Pronouncements

Recent Accounting Pronouncements

Milestones and Research Agreements

3 Months Ended

Mar. 31, 2021

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 $0.8 million, with a portion related to the first performance milestone having been paid. The Company is obligated to pay IURTC royalty payments based on net sales of the licensed technology as well as a portion of any sublicensing revenue Assembly receives. The Company is also required to pay diligence maintenance fees each year to the extent that the royalty, sublicensing, and milestone payments to IURTC are less than such fees for that year. No amounts were paid in the three months ended March 31, 2021. Amounts paid in the three months ended March 31, 2020 were insignificant.

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 was solely responsible for all research and development activities with respect to any product it developed under the license.

The Company was obligated to pay Therabiome clinical and regulatory milestones for each product or therapy advanced from the platform for U.S. regulatory milestones. In addition, the Company was obligated to pay Therabiome lesser amounts for foreign regulatory milestones, which varied by country and region. The Company was also required to pay Therabiome royalties on annual net sales of a product in the low to mid-single digit percentages plus, once annual net sales exceeded certain thresholds, a one-time cash payment upon reaching such thresholds.

Therabiome was obligated to pay the Company royalties on annual net sales of any product Therabiome was 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.

No amounts were accrued for this agreement as of and for the three months ended March 31, 2021 or 2020. In connection with the wind-down of the Microbiome program, the License and Collaboration Agreement with Therabiome was terminated in January 2021, and the termination became effective on April 21, 2021.

Door Agreement

In November 2020, the Company and Door Pharmaceuticals, LLC (Door) entered into an exclusive, two-year Collaboration Agreement and Sublicense Agreement (collectively, the Door Agreement) focused on the development of a novel class of HBV inhibitors. Under the terms of the agreement, Door will build upon its previous efforts to lead and conduct new discovery research, which the Company will fund. In return for an up-front payment of $1.8 million, success-based milestones up to $35.0 million, exercise and annual fees ranging from $0.1 million to $2.0 million and royalties in the low to mid-single digits, the Company will be granted an exclusive option to license compounds arising from the collaboration and will be responsible for the continued development and commercialization of optioned compounds. For the three months ended March 31, 2021, the Company incurred $0.4 million of research and development funding.

Under the consolidation accounting standard, the Company determined that Door is a variable interest entity. The Company does not have the power to direct the activities that most significantly affect the economic performance of Door and as such the Company is not the primary beneficiary and consolidation is not required. As of March 31, 2021, the Company has not provided financial or other support to Door that was not contractually required.

Fair Value Measurements and Investments in Marketable Securities

3 Months Ended

Mar. 31, 2021

Investments Debt And Equity Securities [Abstract]

Fair Value Measurements and Investments in Marketable Securities

Note 3 – Fair Value Measurements and Investments in Marketable Securities

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, accrued expenses, lease liability-short term and deferred revenue-short term.

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 that 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.

Investments in marketable securities consisted of the following (in thousands):

	March 31, 2021
	Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss			Fair Value
Cash equivalents
Money market funds	$	66,494	$	—	$	—		$	66,494
Total cash equivalents		66,494		—		—			66,494
Short-term investments
U.S. and foreign corporate debt securities		22,954		—		(6	)		22,948
Asset-backed securities		16,762		2		(2	)		16,762
U.S. and foreign commercial paper		96,921		—		—			96,921
Total short-term investments		136,637		2		(8	)		136,631
Total cash equivalents and investments	$	203,131	$	2	$	(8	)	$	203,125

	December 31, 2020
	Amortized Cost		Gross Unrealized Gain ⁽¹⁾		Gross Unrealized Loss ⁽¹⁾			Fair Value
Cash equivalents
Money market funds	$	47,553	$	—	$	—		$	47,553
U.S. and foreign commercial paper		6,498		—		—			6,498
Total cash equivalents		54,051		—		—			54,051
Short-term investments
U.S. and foreign corporate debt securities		16,939		3		(3	)		16,939
Asset-backed securities		12,674		2		(1	)		12,675
U.S. treasury securities		23,997		2		—			23,999
U.S. and foreign commercial paper		103,356		—		—			103,356
Total short-term investments		156,966		7		(4	)		156,969
Total cash equivalents and investments	$	211,017	$	7	$	(4	)	$	211,020

The contractual term to maturity of short-term marketable securities held by the Company as of March 31, 2021 is less than one year. There were no long-term marketable securities held by the Company as of March 31, 2021.

Realized gains and losses for the three months ended March 31, 2021 and 2020 were not significant. None of the Company’s investments have been in a continuous unrealized loss position for more than 12 months as of March 31, 2021.

The following tables present the fair value of the Company’s financial assets measured at fair value on a recurring basis (in thousands):

	March 31, 2021
	Level 1		Level 2		Level 3		Fair Value
Cash equivalents
Money market fund	$	66,494	$	—	$	—	$	66,494
Total cash equivalents		66,494		—		—		66,494
Short-term investments
U.S. and foreign corporate debt securities		—		22,948		—		22,948
Asset-backed securities		—		16,762		—		16,762
U.S. and foreign commercial paper		—		96,921		—		96,921
Total short-term investments		—		136,631		—		136,631
Total assets measured at fair value	$	66,494	$	136,631	$	—	$	203,125

	December 31, 2020
	Level 1		Level 2		Level 3		Fair Value
Cash equivalents
Money market fund	$	47,553	$	—	$	—	$	47,553
U.S. and foreign commercial paper		—		6,498		—		6,498
Total cash equivalents		47,553		6,498		—		54,051
Short-term investments
U.S. and foreign corporate debt securities		—		16,939		—		16,939
Asset-backed securities		—		12,675		—		12,675
U.S. treasury securities		—		23,999		—		23,999
U.S. and foreign commercial paper		—		103,356		—		103,356
Total short-term investments		—		156,969		—		156,969
Total assets measured at fair value	$	47,553	$	163,467	$	—	$	211,020

The Company estimates the fair value of its investments in marketable securities 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 were no transfers between Level 1, Level 2 or Level 3 during the periods presented.

Stockholders' Equity - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Aug. 31, 2020
Subsidiary Sale Of Stock [Line Items]
Preferred stock, shares authorized	5,000,000		5,000,000
Common stock, shares authorized	100,000,000		100,000,000
Preferred stock, shares issued	0		0
Preferred stock, shares outstanding	0		0
Initial public offering value of shares authorized				$ 300.0
Number of warrants exercised	0	0
2020 At the Market Offerings
Subsidiary Sale Of Stock [Line Items]
Number of shares sold	5,991,858
Net proceeds from the issuance of common stock through equity plans	$ 34.1
2020 At the Market Offerings \| Maximum
Subsidiary Sale Of Stock [Line Items]
Initial public offering value of shares authorized				$ 100.0

Collaboration Agreements - Additional Information (Details) - USD ($)		3 Months Ended		12 Months Ended
	Jul. 17, 2020	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Collaboration Agreements:
Collaboration revenue			$ 4,081,000
Accounts receivable from collaborations		$ 203,000		$ 1,230,000
Contract liability/deferred revenue		8,987,000	36,041,000	8,987,000	$ 37,048,000
Contract with customer, liability, noncurrent		8,987,000		8,987,000
Research and development expense		18,554,000	23,046,000
Allergan Agreement
Collaboration Agreements:
Collaboration revenue		0	$ 4,100,000
Accounts receivable from collaborations		0		1,000,000.0
Contract liability/deferred revenue		0		0
BeiGene Agreement
Collaboration Agreements:
Collaboration revenue	$ 31,000,000.0
Upfront cash payment received		40,000,000.0
Agreed to pay maximum amount development and regulatory costs		45,000,000.0
Contract with customer, liability, noncurrent		9,000,000.0		9,000,000.0
Research and development expense		0
Incremental costs		3,500,000
BeiGene Agreement \| Other Assets
Collaboration Agreements:
Incremental costs		800,000		$ 800,000
BeiGene Agreement \| Maximum
Collaboration Agreements:
Potential milestone payments		500,000,000.0
BeiGene Agreement \| Development and Regulatory \| Maximum
Collaboration Agreements:
Potential milestone payments		113,800,000
BeiGene Agreement \| Sales Milestone Payments \| Maximum
Collaboration Agreements:
Potential milestone payments		385,000,000.0
Bei Gene Limited
Collaboration Agreements:
Contract liability/deferred revenue	$ 40,000,000.0
Arbutus Agreement
Collaboration Agreements:
Research and development expense		$ 500,000

CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Balance at Dec. 31, 2019	$ 273,217	$ 32	$ 712,807	$ (201)	$ (439,421)
Balance (in shares) at Dec. 31, 2019		32,558,307
Issuance of common stock upon exercise of stock options	119		119
Issuance of common stock upon exercise of stock options ( in shares )		16,834
Issuance of shares of common stock for settlement of restricted stock units (RSUs) (in shares)		49,584
Unrealized (loss) gain on marketable securities	115			115
Stock-based compensation	4,972		4,972
Net loss	(26,655)				(26,655)
Balance at Mar. 31, 2020	251,768	$ 32	717,898	(86)	(466,076)
Balance (in shares) at Mar. 31, 2020		32,624,725
Balance at Dec. 31, 2020	240,578	$ 34	742,387	(270)	(501,573)
Balance (in shares) at Dec. 31, 2020		34,026,680
Issuance of common stock under at-the-market (ATM) equity offering program, net of issuance costs	$ 34,137	$ 6	34,131
Issuance of common stock under at-the-market (ATM) equity offering program, net of issuance costs (in shares)		5,991,858
Issuance of common stock upon exercise of stock options ( in shares )	0
Issuance of shares of common stock for settlement of restricted stock units (RSUs) (in shares)		86,146
Unrealized (loss) gain on marketable securities	$ (1)			(1)
Stock-based compensation	(270)		(270)
Net loss	(27,200)				(27,200)
Balance at Mar. 31, 2021	$ 247,244	$ 40	$ 776,248	$ (271)	$ (528,773)
Balance (in shares) at Mar. 31, 2021		40,104,684

Summary of Significant Accounting Policies and Recent Accounting Pronouncements - Schedule of Earnings Per Share, Basic and Diluted (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Numerator:
Net loss	$ (27,200)	$ (26,655)
Denominator:
Weighted average common shares and pre-funded warrants outstanding - basic and diluted	39,679,734	35,079,756
Net loss per share, basic and diluted	$ (0.69)	$ (0.76)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 1A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121641772&loc=SL7669619-108580

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 31
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8924-108599

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 11
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22694-107794

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(22))
-URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 11
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22694-107794

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8933-108599

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=109222650&loc=SL51721683-107760

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22595-107794

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 1B
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121641772&loc=SL7669625-108580

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8736-108599

Reference 11: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(18))
-URI http://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263

Reference 12: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(20))
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

Reference 13: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585

Reference 14: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)(2)
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22499-107794

Reference 15: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 9
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22663-107794

Reference 16: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 28
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14064-108612

Reference 17: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 30
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8906-108599

Reference 18: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22583-107794

Reference 19: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(g)(1)(ii))
-URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690

Reference 20: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 60B
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121326447&loc=SL5780133-109256

Reference 21: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571

Reference 22: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8933-108599

Reference 23: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22658-107794

+ Details

Collaboration Agreements (Tables)

3 Months Ended

Mar. 31, 2021

Collaboration Agreement [Abstract]

Schedule of Changes in Contract Liabilities

The following table presents changes in the Company’s contract liabilities (in thousands):

	Balance at Beginning of Period		Additions		Deductions		Balance at End of Period
Three Months Ended March 31, 2021
Contract liabilities:
Deferred revenue	$	8,987	$	—	$	—	$	8,987

	Balance at Beginning of Period		Additions		Deductions			Balance at End of Period
Three Months Ended March 31, 2020
Contract liabilities:
Deferred revenue	$	37,048	$	—	$	(1,007	)	$	36,041

Stock Plans and Stock-Based Compensation - Summary of Restricted Stock Units and Related Information (Details) - RSUs	3 Months Ended
	Mar. 31, 2021 $ / shares shares
Number of Shares
Beginning of period \| shares	886,868
Granted \| shares	199,875
Vested \| shares	(135,929)
Forfeited \| shares	(144,240)
Ending of period \| shares	806,574
Weighted Average Fair Value Per RSU at Grant Price
Beginning of period \| $ / shares	$ 20.35
Granted \| $ / shares	4.33
Vested \| $ / shares	19.73
Forfeited \| $ / shares	17.49
Ending of period \| $ / shares	$ 16.81

Stock Plans and Stock-Based Compensation - Summary of Restricted Stock Units and Related Information (Parenthetical) (Details) - RSUs	3 Months Ended
	Mar. 31, 2021 shares
Share-based Compensation Arrangement by Share-based Payment Award
Vested and subject to deferred settlement	140,000
Weighted average remaining contractual term subject to deferred settlement	1 year 10 months 24 days

Summary of Significant Accounting Policies and Recent Accounting Pronouncements (Tables)

3 Months Ended

Mar. 31, 2021

Accounting Policies [Abstract]

Schedule of Earnings Per Share, Basic and Diluted

A reconciliation of the numerators and the denominators of the basic and diluted net loss per common share computations is as follows (in thousands, except per share amounts):

	Three Months Ended March 31,
	2021			2020
Numerator:
Net loss	$	(27,200	)	$	(26,655	)
Denominator:
Weighted average common shares and pre-funded warrants outstanding - basic and diluted		39,679,734			35,079,756
Net loss per share - basic and diluted	$	(0.69	)	$	(0.76	)

Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share

Securities excluded from the computation of diluted loss per share because including them would have been antidilutive are as follows:

	March 31,
	2021		2020
Warrants to purchase common stock		—		15,296
Options to purchase common stock		7,382,515		6,370,396
Common stock subject to purchase under our ESPP		44,870		32,940
Unvested RSUs		666,574		999,926
Total		8,093,959		7,418,558

Fair Value Measurements and Investments in Marketable Securities - Schedule of Fair Value of Financial Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Short-term investments
Short-term investments	$ 136,631	$ 156,969
Recurring
Cash equivalents
Cash equivalents	66,494	54,051
Short-term investments
Short-term investments	136,631	156,969
Total assets measured at fair value	203,125	211,020
Recurring \| U.S and foreign corporate debt securities
Short-term investments
Short-term investments	22,948	16,939
Recurring \| U.S. and foreign commercial paper
Cash equivalents
Cash equivalents		6,498
Short-term investments
Short-term investments	96,921	103,356
Recurring \| Asset-backed securities
Short-term investments
Short-term investments	16,762	12,675
Recurring \| U.S. treasury securities
Short-term investments
Short-term investments		23,999
Recurring \| Money market fund
Cash equivalents
Cash equivalents	66,494	47,553
Recurring \| Level 1
Cash equivalents
Cash equivalents	66,494	47,553
Short-term investments
Short-term investments	0	0
Total assets measured at fair value	66,494	47,553
Recurring \| Level 1 \| U.S and foreign corporate debt securities
Short-term investments
Short-term investments	0	0
Recurring \| Level 1 \| U.S. and foreign commercial paper
Cash equivalents
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
Recurring \| Level 1 \| Money market fund
Cash equivalents
Cash equivalents	66,494	47,553
Recurring \| Level 2
Cash equivalents
Cash equivalents	0	6,498
Short-term investments
Short-term investments	136,631	156,969
Total assets measured at fair value	136,631	163,467
Recurring \| Level 2 \| U.S and foreign corporate debt securities
Short-term investments
Short-term investments	22,948	16,939
Recurring \| Level 2 \| U.S. and foreign commercial paper
Cash equivalents
Cash equivalents		6,498
Short-term investments
Short-term investments	96,921	103,356
Recurring \| Level 2 \| Asset-backed securities
Short-term investments
Short-term investments	16,762	12,675
Recurring \| Level 2 \| U.S. treasury securities
Short-term investments
Short-term investments		23,999
Recurring \| Level 2 \| Money market fund
Cash equivalents
Cash equivalents	0	0
Recurring \| Level 3
Cash equivalents
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
Short-term investments
Short-term investments	0	0
Recurring \| Level 3 \| U.S. and foreign commercial paper
Cash equivalents
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
Recurring \| Level 3 \| Money market fund
Cash equivalents
Cash equivalents	$ 0	$ 0

Restructurings - Schedule of Restructuring Charges (Details) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Restructuring Cost And Reserve [Line Items]
Restructuring charges	$ (1,052,000)	$ 0
Research and Development
Restructuring Cost And Reserve [Line Items]
Restructuring charges	(1,329,000)
General and Administrative
Restructuring Cost And Reserve [Line Items]
Restructuring charges	$ 277,000