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Stock-Based Compensation | 9 Months Ended
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Sep. 30, 2024
Share-Based Payment Arrangement [Abstract] |
Stock-Based Compensation | Stock-Based Compensation 2021 Plan The total remaining shares of common stock authorized for issuance under the 2021 Stock Option and Incentive Plan (the "2021 Plan") increased from 6,336,068 as of December 31, 2023 to 7,339,182 as of September 30, 2024 primarily due to the automatic annual increase provision of the 2021 Plan. 2016 Plan The total remaining shares of common stock authorized for issuance under the 2016 Stock Incentive Plan as of September 30, 2024 and December 31, 2023 were 1,813,811 shares and 2,044,585 shares, respectively. 2021 Employee Stock Purchase Plan The total remaining shares of common stock authorized for issuance under the 2021 Employee Stock Purchase Plan (the "2021 ESPP") increased from 839,539 as of December 31, 2023 to 1,153,675 as of September 30, 2024 due to the automatic annual increase provision of the 2021 ESPP. Stock-Based Compensation Stock-based compensation expense recorded in the condensed consolidated statements of operations and comprehensive loss is as follows (in thousands):
| Three Months Ended September 30, |  | Nine Months Ended September 30,
| 2024 |  | 2023 |  | 2024 |  | 2023
Research and development expenses | $ | 2,132 |  | $ | 1,601 |  | $ | 5,759 |  | $ | 4,317
General and administrative expenses | 2,728 |  | 1,795 |  | 7,456 |  | 4,953
Total | $ | 4,860 |  | $ | 3,396 |  | $ | 13,215 |  | $ | 9,270
Stock Option Valuation The following table presents, on a weighted-average basis, the assumptions used in the Black-Scholes option-pricing model to determine the fair value of stock options granted during the nine months ended September 30, 2024 and 2023:
| September 30, 2024 |  | September 30, 2023
Risk‑free interest rate | 4.15 | % |  | 4.09 | %
Expected volatility | 75 | % |  | 72 | %
Expected dividend yield | — |  |  | — |
Expected term (in years) | 6.00 |  | 6.02
Early Exercise of Unvested Stock Options Shares purchased by employees pursuant to the early exercise of stock options are not deemed, for accounting purposes, to be outstanding shares until those shares vest according to their respective vesting schedules. Cash received from employee exercises of unvested options is included in current liabilities on the balance sheet. Amounts recorded are reclassified to common stock and additional paid-in capital as the shares vest. Vesting can occur in the year of exercise and thereafter. There were 4,670 and 14,745 unvested shares related to early exercises of stock options as of September 30, 2024 and December 31, 2023, respectively. As of both September 30, 2024 and December 31, 2023, the liability associated with the unvested early exercise of stock options was less than $0.1 million. Stock Options The following table summarizes the Company’s stock option activity during the nine months ended September 30, 2024:
| Number of Shares |  | Weighted‑ Average Exercise Price
Outstanding as of December 31, 2023 | 5,414,360 |  | $ | 11.42
Granted | 679,625 |  | 14.12
Exercised | (260,271) |  |  | 6.50
Forfeited | (331,629) |  |  | 14.32
Outstanding as of September 30, 2024 | 5,502,085 |  | $ | 11.81
Exercisable as of September 30, 2024(1) | 3,602,710 |  | $ | 10.55
(1)This represents the number of vested and unvested options exercisable as of September 30, 2024. The weighted-average grant-date fair value of stock options granted during the nine months ended September 30, 2024 and 2023 was $9.66 per share and $9.19 per share, respectively. As of September 30, 2024, there was $17.2 million of unrecognized compensation cost related to unvested stock options, which is expected to be recognized over a weighted-average period of 2.03 years. Restricted Stock Units During the nine months ended September 30, 2024, restricted stock units ("RSUs") were granted to employees with vesting conditions based on continued service over time. Accordingly, stock-based compensation expense for such awards is recognized using a straight-line attribution model over the vesting term of each RSU. The fair value of each RSU is based on the closing price of the Company's common stock on the date of grant. A summary of restricted stock activity during the nine months ended September 30, 2024 is as follows:
| Shares |  | Weighted‑ Average Grant‑Date Fair Value
Unvested as of December 31, 2023 | 1,268,461 |  | $ | 13.83
Granted | 1,206,985 |  | $ | 14.09
Vested | (305,470) |  | $ | 13.54
Forfeited | (155,368) |  | $ | 14.06
Unvested as of September 30, 2024 | 2,014,608 |  | $ | 14.01
As of September 30, 2024, there was $24.9 million of unrecognized stock-based compensation expense related to restricted stock that is expected to vest. These costs are expected to be recognized over a weighted-average remaining vesting period of 2.96 years. Performance Stock Units During the nine months ended September 30, 2024, the Company granted 438,500 performance stock units ("PSUs") to employees of the Company at a weighted-average grant date fair value of $14.86 per share. All 438,500 PSUs were outstanding as of September 30, 2024. The PSUs will vest upon achievement of certain clinical milestones and continued employment thereafter. Compensation expense for PSUs is recognized on a straight-line basis over the requisite service periods when the achievement of the performance condition is deemed probable. If a performance condition is not determined to be probable or is not met, no stock-based compensation expense is recognized. Forfeitures are recognized as a reduction of stock-based compensation expense in the period in which they occur. As of September 30, 2024, no PSUs were vested and no stock-based compensation expense was recognized as the performance conditions were not deemed probable.

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