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Stock-Based Compensation (Tables) | 9 Months Ended
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Sep. 30, 2023
Share-Based Payment Arrangement [Abstract] |
Share Based Compensation Expense Related to Issuance of Stock Option Awards to Employees and Non Employees Related to Employee Stock Purchase Plan | The Company recognized stock-based compensation expense related to the issuance of stock option awards to employees and non-employees and related to the Company's 2015 Employee Stock Purchase Plan, or ESPP, in the condensed consolidated statements of comprehensive loss as follows:
| Three Months Ended September 30, |  |  | Nine Months Ended September 30, |
| 2023 |  |  | 2022 |  |  | 2023 |  |  | 2022 |
Research and development | $ | 3,853 |  |  | $ | 1,820 |  |  | $ | 10,329 |  |  | $ | 4,531 |
General and administrative |  | 4,468 |  |  |  | 2,542 |  |  |  | 12,295 |  |  |  | 7,247 |
Total | $ | 8,321 |  |  | $ | 4,362 |  |  | $ | 22,624 |  |  | $ | 11,778 |
Compensation Expense by Type of Award | Compensation expense by type of award in the three and nine months ended September 30, 2023 and 2022 was as follows:
| Three Months Ended September 30, |  |  | Nine Months Ended September 30, |
| 2023 |  |  | 2022 |  |  | 2023 |  |  | 2022 |
Stock options | $ | 7,242 |  |  | $ | 3,864 |  |  | $ | 19,654 |  |  | $ | 9,977 |
Restricted Stock Units |  | 990 |  |  |  | 452 |  |  |  | 2,749 |  |  |  | 1,690 |
Employee Stock Purchase Plan |  | 89 |  |  |  | 46 |  |  |  | 221 |  |  |  | 111 |
Total | $ | 8,321 |  |  | $ | 4,362 |  |  | $ | 22,624 |  |  | $ | 11,778 |
Summary of Restricted Stock Units Activity | RSUs awarded to members of the Company's board of directors or employees vest on either (i) the one-year anniversary date of the related grant or (ii) 25% on each anniversary date of the related grant for four years. The following table summarizes the Company's RSU activity:
|  | Number of Shares |  |  | Weighted Average Grant Date Fair Value |
Unvested—December 31, 2022 |  |  | 224,788 |  |  | $ | 18.02 |
Granted (1) |  |  | 308,550 |  |  | $ | 24.94 |
Vested |  |  | (9,102 | ) |  | $ | 13.87 |
Unvested—September 30, 2023 (2) |  |  | 524,236 |  |  | $ | 22.16 |
(1) RSUs granted in 2023 and 2022 had a weighted average grant date fair value of $24.94 and $15.79, respectively. The fair values of RSUs vested in 2023 and 2022 totaled $127,000 and $656,000, respectively. (2) Unvested RSUs as of September 30, 2023, include 317,333 RSUs which were granted to members of the Company's board of directors and will not be considered issued and outstanding until the occurrence of a separation from service, death, disability, or a change in control.

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