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Stock-Based Awards | 9 Months Ended
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Sep. 30, 2021
Share-based Payment Arrangement [Abstract] |
Stock-Based Awards | Stock-Based Awards The Company has issued stock-based awards from various equity incentive and stock purchase plans, as more fully described in Note 10, "Stock-Based Awards" to the consolidated financial statements in the Company's 2020 Annual Report on Form 10-K. Stock Option Activity The following table summarizes stock option activity for the nine months ended September 30, 2021:
| Number of Options |  | Weighted-Average Exercise Price
Balance at December 31, 2020 | 12,959,926 |  |  | $ | 16.31 |
Granted | 1,944,717 |  |  | 73.31 |
Exercised | (831,207) |  |  | 14.27 |
Forfeited | (259,060) |  |  | 34.89 |
Balance at September 30, 2021 | 13,814,376 |  |  | $ | 24.11 |
Vested and expected to vest at September 30, 2021 | 12,078,689 |  |  | $ | 27.48 |
Exercisable at September 30, 2021 | 7,856,046 |  |  | $ | 15.22 |
The estimated fair value of stock options granted to employees were calculated using the Black-Scholes option-pricing model using the following assumptions:
| Nine Months Ended September 30,
| 2021 |  | 2020
Expected term (in years) | 5.50 - 6.08 |  | 5.50 - 6.08
Volatility | 62.2% - 63.7% |  | 65.2% - 67.1%
Risk-free interest rate | 0.5% - 1.2% |  | 0.3% - 1.7%
Dividend yield | — |  | —
Restricted Stock Activity The following table summarizes restricted stock unit ("RSU") activity for the nine months ended September 30, 2021:
| Number of RSU shares |  | Weighted-Average Fair Value at Date of Grant per Share
Unvested at December 31, 2020 | 2,301,679 |  |  | $ | 27.08 |
Granted | 1,046,259 |  |  | 69.50 |
Vested and released | (599,258) |  |  | 25.63 |
Forfeited | (126,827) |  |  | 60.92 |
Unvested at September 30, 2021 | 2,621,853 |  |  | $ | 43.83 |
Expected to vest at September 30, 2021 | 2,621,853 |  |  | $ | 43.83 |
Stock-Based Compensation Expense The Company’s results of operations include expenses relating to stock-based compensation as follows (in thousands):
| Three Months Ended September 30, |  | Nine Months Ended September 30,
| 2021 |  | 2020 |  | 2021 |  | 2020
Research and development | $ | 12,174 |  |  | $ | 7,781 |  |  | $ | 36,997 |  |  | $ | 20,981 |
General and administrative | 8,824 |  |  | 5,471 |  |  | 26,165 |  |  | 15,540 |
Total | $ | 20,998 |  |  | $ | 13,252 |  |  | $ | 63,162 |  |  | $ | 36,521 |

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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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