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Stock-Based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2020
Stock-Based Compensation |
Schedule of stock-based compensation expense | Total compensation cost recognized for all stock-based compensation awards in the consolidated statements of operations was as follows (in thousands):
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ | ​ | ​ |  | ​ | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | Year Ended
​ | ​ | ​ | December 31,
​ | ​ | ​ | 2020 | ​ | 2019
Research and development | ​ | ​ | $ | 11,580 | ​ | $ | 13,538
General and administrative | ​ | ​ |  | 11,576 | ​ |  | 13,705
Total stock-based compensation expense | ​ | ​ | $ | 23,156 | ​ | $ | 27,243
​
Schedule of changes in unvested restricted stock | ​
​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ |  | Weighted
​ | ​ | ​ | ​ | Average
​ | ​ | ​ | ​ | Grant Date
​ | ​ | ​ | ​ | Fair Value
​ | ​ | Shares | ​ | Per Share
Unvested restricted stock and restricted stock unit awards as of December 31, 2019 |  | 581,408 | ​ | $ | 24.03
Issued |  | 364,549 | ​ | $ | 29.57
Vested |  | (304,638) | ​ | $ | 23.37
Forfeited | ​ | (133,869) | ​ | $ | 24.72
Unvested restricted stock and restricted stock unit awards as of December 31, 2020 |  | 507,450 | ​ | $ | 27.35
Schedule of stock option activity | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ |  | Weighted Average |  | Remaining |  | Aggregate Intrinsic
​ | ​ | Shares | ​ | Exercise Price | ​ | Contractual Life (years) | ​ | Value (in thousands)
Outstanding at December 31, 2019 |  | 4,358,291 | ​ | $ | 25.40 | ​ | 7.4 | ​ | $ | 26,060
Granted | ​ | 1,721,748 | ​ | $ | 30.44 | ​ | ​ | ​ | ​ | ​
Exercised | ​ | (964,412) | ​ | $ | 20.22 | ​ | ​ | ​ | ​ | ​
Cancelled | ​ | (1,203,370) | ​ | $ | 30.72 | ​ | ​ | ​ | ​ | ​
Outstanding at December 31, 2020 |  | 3,912,257 | ​ | $ | 27.26 | ​ | 7.9 | ​ | $ | 167,640
Exercisable at December 31, 2020 |  | 1,463,668 | ​ | $ | 24.05 | ​ | 6.8 | ​ | $ | 67,413
Schedule of assumptions used to value stock options | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | Year Ended | ​
​ | ​ | December 31, | ​
​ | ​ | 2020 | ​ | 2019 |  | 2018
Expected volatility |  | 60.0 | % | ​ | 73.8 | % | ​ | 77.5 | %
Expected option term (in years) |  | 6.25 | ​ | ​ | 6.25 | ​ | ​ | 6.25 | ​
Risk free interest rate |  | 1.5 | % | ​ | 2.0 | % | ​ | 2.9 | %
Expected dividend yield |  | — | ​ | ​ | — | ​ | ​ | — | ​

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