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Stock-Based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2020
Share-based Payment Arrangement [Abstract] |
Stock-Based Compensation Expense | The following table shows total stock-based compensation expense recognized in the accompanying Consolidated Statements of Operations (in thousands):
| Year Ended December 31,
| 2020 |  | 2019 |  | 2018
Research and development | $ | 13,523 |  |  | $ | 10,135 |  |  | $ | 8,249 |
General and administrative | 12,185 |  |  | 9,195 |  |  | 6,428 |
Total stock-based compensation expense | $ | 25,708 |  |  | $ | 19,330 |  |  | $ | 14,677 |
Assumptions Used for Estimating Fair Value of Employee Stock Options | The assumptions used for estimating the fair value of the employee stock options were as follows:
| Year Ended December 31,
| 2020 |  | 2019 |  | 2018
Risk-free interest rate | 0.34-0.61% |  | 1.68-2.25% |  | 2.53-2.96%
Expected term (in years) | 5.51-5.57 |  | 5.50-5.62 |  | 5.59-5.61
Expected dividend yield of stock | — |  |  | — |  |  | — |
Expected volatility | 77.61-80.32% |  | 76.46-78.39% |  | 72.33-75.49%
Weighted-Average Assumptions Used for Estimating Fair Value of ESPP Purchase Rights | The assumptions used for estimating the fair value of the ESPP purchase rights are as follows:
| Year Ended December 31,
| 2020 |  | 2019 |  | 2018
Risk-free interest rate | 1.53-2.80% |  | 1.53-2.42% |  | 2.16-2.80%
Expected term (in years) | 0.5-2.0 |  | 0.5-2.0 |  | 0.5-2.0
Expected dividend yield of stock | — |  |  | — |  |  | — |
Expected volatility | 51.02-91.96% |  | 51.02-91.96% |  | 73.21-83.25%

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