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Stock-based Compensation | 3 Months Ended
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Mar. 31, 2021
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract] |
Stock-based Compensation | 9. Stock-based Compensation During the three months ended March 31, 2021, the Company recognized the following stock-based compensation expense (in thousands):
|  | Three Months Ended March 31, |
|  | 2021 |  |  | 2020 |
Research and development |  | $ | 12,845 |  |  | $ | 7,362 |
General and administrative |  |  | 9,247 |  |  |  | 6,789 |
Total |  | $ | 22,092 |  |  | $ | 14,151 |
Stock option activity The following table summarizes stock option activity for the three months ended March 31, 2021:
|  | Shares |  |  | Weighted- average exercise price per share |
Outstanding at December 31, 2020 |  |  | 8,101,980 |  |  | $ | 42.44 |
Granted |  |  | 882,558 |  |  |  | 139.05 |
Exercised |  |  | (342,471 | ) |  |  | 33.12 |
Cancelled or forfeited |  |  | (69,404 | ) |  |  | 43.31 |
Outstanding at March 31, 2021 |  |  | 8,572,663 |  |  | $ | 52.76 |
Exercisable at March 31, 2021 |  |  | 3,962,897 |  |  | $ | 31.48 |
Vested and expected to vest at March 31, 2021 |  |  | 8,572,663 |  |  | $ | 52.76 |
As of March 31, 2021, total unrecognized compensation expense related to stock options was $191.3 million, which the Company expects to recognize over a remaining weighted-average period of 3.1 years. Restricted stock activity The following table summarizes restricted stock activity for the three months ended March 31, 2021:
|  | Restricted Stock |  |  | Weighted- Average Grant Date Fair Value |
Unvested balance as of December 31, 2020 |  |  | 894,092 |  |  | $ | 70.55 |
Granted |  |  | 278,645 |  |  |  | 136.16 |
Vested |  |  | (109,355 | ) |  |  | 40.64 |
Cancelled or forfeited |  |  | (11,003 | ) |  |  | 57.94 |
Unvested balance as of March 31, 2021 |  |  | 1,052,379 |  |  | $ | 91.16 |
As of March 31, 2021, total unrecognized compensation expense related to unvested restricted common shares was $76.5 million, which the Company expects to recognize over a remaining weighted-average vesting period of 2.8 years.

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- Definition The entire disclosure for share-based payment arrangement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -URI http://asc.fasb.org/topic&trid=2228938 + Details
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