Stock-Based Compensation (Tables)
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12 Months Ended |
Dec. 31, 2020 |
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| Fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees, subsequent to the adoption of ASU 2018-07 |
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| Schedule of stock option activity |
| | | | | | | | | | | | | | | Weighted | | Weighted | | | | | | | | average | | average | | Aggregate | | | Number of | | exercise | | contractual | | intrinsic | | | shares | | price | | term | | value | | | | | | | | (in years) | | (in thousands) | Outstanding as of December 31, 2019 | | 15,103,907 | | $ | 10.41 | | 8.22 | | $ | 36,006 | Granted | | 4,289,471 | | | 7.27 | | | | | | Exercised | | (1,037,406) | | | 1.44 | | | | | | Forfeited | | (2,395,648) | | | 10.25 | | | | | | Outstanding as of December 31, 2020 | | 15,960,324 | | $ | 10.17 | | 7.38 | | $ | 15,018 | Vested and expected to vest as of December 31, 2020 | | 15,960,324 | | $ | 10.17 | | 7.38 | | $ | 15,018 | Options exercisable as of December 31, 2020 | | 7,051,088 | | $ | 8.12 | | 6.21 | | $ | 11,798 |
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| Schedule of restricted common stock award activity |
| | | | | | | | | | Weighted average | | | | | grant‑date | | | Shares | | fair value | Unvested restricted common stock as of December 31, 2019 | | 99,314 | | $ | 0.526 | Issued | | — | | | — | Vested | | (99,314) | | | 0.526 | Forfeited | | — | | | — | Unvested restricted common stock as of December 31, 2020 | | — | | $ | — |
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| Schedule of restricted stock unit activity |
| | | | | | | | | | Weighted average | | | | | grant‑date | | | Shares | | fair value | Unvested restricted common stock as of December 31, 2019 | | 277,200 | | $ | 10.660 | Issued | | — | | | — | Vested | | — | | | — | Forfeited | | (25,200) | | | 10.660 | Unvested restricted common stock as of December 31, 2020 | | 252,000 | | $ | 10.660 |
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| Schedule of allocation of share based compensation |
The Company recorded stock-based compensation expense in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands): | | | | | | | | | | | | Year ended December 31, | | | 2020 | | 2019 | | 2018 | Research and development expenses | | $ | 8,023 | | $ | 9,011 | | $ | 3,787 | General and administrative expenses | | | 25,642 | | | 32,260 | | | 23,741 | | | $ | 33,665 | | $ | 41,271 | | $ | 27,528 |
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| Employees, directors and non-employees |
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| Fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees, subsequent to the adoption of ASU 2018-07 |
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| Schedule of assumptions used to determine the fair value of stock awards granted |
| | | | | | | | | | | | Year ended December 31, | | | 2020 | | 2019 | | 2018 | Risk-free interest rate | | 1.10 | % | | 2.07 | % | | 2.71 | % | Expected volatility | | 69.4 | % | | 76.9 | % | | 74.0 | % | Expected dividend yield | | — | | | — | | | — | | Expected term (in years) | | 6.05 | | | 6.06 | | | 6.21 | |
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| Market-Based Stock Options |
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| Fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees, subsequent to the adoption of ASU 2018-07 |
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| Schedule of assumptions used to determine the fair value of stock awards granted |
| | | | | | Year ended December 31, | | | | 2018 | | Risk-free interest rate | | 3.15 | % | Expected volatility | | 69.0 | % | Expected dividend yield | | — | | Derived service period (in years) | | 2.30 | |
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