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Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2019

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Stock Option Valuation

The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the grant-date fair value of stock options granted to employees, directors and non- employees:

		Three Months Ended March 31,
		2019				2018
Risk-free interest rate			2.4	%			2.8	%
Expected term (in years)			5.90				5.94
Expected volatility			90.6	%			86.0	%
Expected dividend yield			0	%			0	%

Summary of Stock Option Activity

The following table summarizes the Company’s stock option activity since December 31, 2018:

		Number of Shares				Weighted Average Exercise Price				Weighted Average Contractual Term (Years)				Aggregate Intrinsic Value
Outstanding as of December 31, 2018			797,931			$	8.29				8.42			$	6,486
Assumed as part of Merger with Arsanis			271,230				62.60
Granted			136,913				17.22
Exercised			(16,483	)			6.85
Forfeited			(22,378	)			8.43
Outstanding as of March 31, 2019			1,167,213			$	21.97				7.65			$	7,168
Exercisable as of March 31, 2019			556,774			$	30.13
Vested and expected to vest as of March 31, 2019			979,466			$	48.79

Summary of Stock-Based Compensation Expense Classification

Stock-based compensation expense was classified in the consolidated statements of operations and comprehensive loss as follows:

		Three Months Ended March 31,
		2019				2018
Research and development expense		$	84			$	38
General and administrative expense			178				90
Total stock-based compensation			262				128

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Tabular disclosure of the amount of total share-based compensation cost, including the amounts attributable to each share-based compensation plan and any related tax benefits.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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Tabular disclosure for stock option plans. Includes, but is not limited to, outstanding awards at beginning and end of year, grants, exercises, forfeitures, and weighted-average grant date fair value.

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-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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Tabular disclosure of the significant assumptions used during the year to estimate the fair value of employee stock purchase plans, including, but not limited to: (a) expected term, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions.

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