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Stock-Based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2021
Share-based Payment Arrangement [Abstract] |
Summary of Restricted Stock Activity | The following table summarizes restricted stock activity during the year ended December 31, 2021:
| Number of Shares |  | Weighted- Average Grant Date fair value
Outstanding and unvested, as of December 31, 2020 | 126,522 |  |  | $ | 1.85 |
Issued | — |  |  | $ | — |
Vested | (103,164) |  |  | $ | 1.78 |
Repurchases | (1,517) |  |  | $ | 2.12 |
Outstanding and unvested, as of December 31, 2021 | 21,841 |  |  | $ | 2.16 |
Summary of Black-Scholes Option Pricing Model to Estimate Stock-based Compensation Expense for Stock Option Awards | The Company used Black-Scholes option pricing model to estimate stock-based compensation expense for stock option awards with the following assumptions:
| Year Ended December 31, |
| 2021 |  | 2020 |  | 2019 |
Expected volatility | 74.83% | - | 76.31% |  | 72.10% | - | 77.50% |  | 74.80% | - | 82.53% |
Risk-free interest rate | 0.61 % | - | 1.39 % |  | 0.27 % | - | 0.82 % |  | 1.43% | - | 2.59% |
Expected dividend | — |  | — |  | — |
Expected term (in years) | 5.44 | - | 6.08 |  | 5.26 | - | 6.75 |  | 5 | - | 6.08 |
Underlying common stock fair value | 15.2 | - | 38.23 |  | 6.22 | - | 28.79 |  | 5.15 | - | 7.08 |
Summary of Option Activity Under the 2015 Plan and 2020 Plan | A summary of option activity under the 2015 Plan and the 2020 Plan is as follows:
| Number of Options |  | Weighted- Average Exercise Price per Share |  | Weighted- Average Remaining Contractual Term (in Years) |  | Aggregate Intrinsic Value
Outstanding as of December 31, 2020 | 2,993,855 |  |  | $ | 8.18 |  |  | 8.84 |  | $ | 43,890 |
Granted | 1,520,391 |  |  | $ | 26.03 |  |  |  |  |
Exercised | (336,340) |  |  | $ | 5.21 |  |  |  |  |
Forfeited | (557,726) |  |  | $ | 16.4 |  |  |  |  |
Outstanding as of December 31, 2021 | 3,620,180 |  |  | $ | 14.56 |  |  | 8.25 |  | $ | 16,735 |
Exercisable as of December 31, 2021 | 1,368,543 |  |  | $ | 9.76 |  |  | 7.66 |  | $ | 9,426 |
Vested and expected to vest as of December 31, 2021 | 3,620,180 |  |  | $ | 14.56 |  |  | 7.66 |  | $ | 16,735 |
Summary of Black-Scholes Option Pricing Model to Estimate Stock-based Compensation Expense for 2020 Employee Stock Purchase Plan | The Company used Black-Scholes option pricing model to estimate stock-based compensation expense for the 2020 ESPP with the following assumptions:
| Year Ended December 31,
| 2021 |  | 2020 |  | 2019
Risk-free interest rate | 0.06% - 0.07% |  | 0.11% |  | —
Expected term of options (in years) | 0.50 |  | 0.58 |  | —
Expected stock price volatility | 67.16% - 89.51% |  | 83.72% |  | —
Expected dividends | — |  | — |  | —
Summary of Classification of Stock-based Compensation Expense | The following table presents the classification of stock-based compensation expense during the years ended December 31, 2021 and 2020 (in thousands):
| Year Ended December 31,
| 2021 |  | 2020 |  | 2019
Research and development expenses | $ | 3,928 |  |  | $ | 1,719 |  |  | $ | 584 |
General and administrative expenses | 6,509 |  |  | 2,176 |  |  | 1,245 |
Total stock-based compensation expense | $ | 10,437 |  |  | $ | 3,895 |  |  | $ | 1,829 |

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