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Stock-Based Compensation (Tables) | 3 Months Ended
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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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- Definition 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. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition 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. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition 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. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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