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Stock-Based Compensation (Tables) | 9 Months Ended
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Sep. 30, 2021
Share-based Payment Arrangement [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 September 30, |  | Nine Months Ended September 30,
| 2021 |  | 2020 |  | 2021 |  | 2020
Risk-free interest rate | 1.1 | % |  | 0.4 | % |  | 1.0 | % |  | 0.6 | %
Expected term (in years) | 6.1 |  | 6.1 |  | 6.0 |  | 6.0
Expected volatility | 95.7 | % |  | 95.1 | % |  | 97.6 | % |  | 94.9 | %
Expected dividend yield | 0 | % |  | 0 | % |  | 0 | % |  | 0 | %
Summary of Stock Option Activity | The following table summarizes the Company’s stock option activity for the nine months ended September 30, 2021:
| Number of Shares |  | Weighted Average Exercise Price |  | Weighted Average Contractual Term (Years) |  | Aggregate Intrinsic Value (in thousands)
Outstanding as of December 31, 2020 | 1,874,514 |  |  | $ | 12.94 |  |  | 8.3 |  | $ | 7 |
Granted | 336,750 |  |  | 6.98 |  |  |  |  |
Exercised | (5,860) |  |  | 7.08 |  |  |  |  |
Forfeited and Expired | (252,513) |  |  | 15.21 |  |  |  |  |
Outstanding as of September 30, 2021 | 1,952,891 |  |  | $ | 11.64 |  |  | 7.9 |  | $ | 9 |
Exercisable as of September 30, 2021 | 919,423 |  |  | $ | 15.45 |  |  | 6.7 |  | $ | — |
Vested and expected to vest as of September 30, 2021 | 1,702,001 |  |  | $ | 11.92 |  |  | 7.8 |  | $ | 5 |
Share-based Payment Arrangement, Restricted Stock and Restricted Stock Unit, Activity | Restricted Stock Units— The following table summarizes the Company's restricted stock unit activity for the nine months ended September 30, 2021:
| Number of Shares
Unvested as of December 31, 2020 | 572,460 |
Granted | 649,478 |
Vested | (205,185) |
Forfeited | (109,880) |
Unvested as of September 30, 2021 | 906,873 |
Summary of Stock-Based Compensation Expense Classification | Stock-based compensation expense was classified in the condensed consolidated statements of operations as follows:
| Three Months Ended September 30, |  | Nine Months Ended September 30,
| 2021 |  | 2020 |  | 2021 |  | 2020
Research and development expense | $ | 633 |  |  | $ | 1,006 |  |  | $ | 2,008 |  |  | $ | 1,711 |
General and administrative expense | 883 |  |  | 1,238 |  |  | 2,563 |  |  | 2,319 |
Total stock-based compensation | $ | 1,516 |  |  | $ | 2,244 |  |  | 4,571 |  |  | 4,030 |

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- Definition Tabular disclosure of cost recognized for award under share-based payment arrangement by plan. Includes, but is not limited to, related tax benefit. + 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=120381028&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=120381028&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=120381028&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=120381028&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=120381028&loc=d3e5070-113901 + Details
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- Definition Disclosure of the number and weighted-average grant date fair value for restricted stock and restricted stock units that were outstanding at the beginning and end of the year, and the number of restricted stock and restricted stock units that were granted, vested, or forfeited during the year. + References No definition available. + Details
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