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Stock-Based Compensation | 9 Months Ended
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Sep. 30, 2023
Stock-Based Compensation |
Stock-Based Compensation | (8) Stock-Based Compensation ​ A summary of stock option activity for the nine months ended September 30, 2023 is as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Weighted | ​ | Weighted
​ | ​ | ​ | ​ | Average | ​ | Average
​ | ​ | ​ | ​ | Exercise | ​ | Remaining
​ | ​ | ​ | ​ | Price | ​ | Contractual
​ |  | Shares |  | Per Share |  | Term (In Years)
Options outstanding at December 31, 2022 |  | 5,085,662 | ​ | $ | 29.26 | ​ | 7.9
Granted |  | 1,553,275 | ​ | $ | 36.88 | ​ | ​
Exercised |  | (50,773) | ​ | $ | 15.89 | ​ | ​
Canceled |  | (180,580) | ​ | $ | 87.57 | ​ | ​
Options outstanding at September 30, 2023 |  | 6,407,584 | ​ | $ | 29.57 | ​ | 7.8
Options vested and expected to vest at September 30, 2023 |  | 6,271,198 | ​ | $ | 29.54 | ​ | 7.7
Options exercisable at September 30, 2023 |  | 3,088,826 | ​ | $ | 29.58 | ​ | 6.6
Shares available for grant under the 2021 Plan |  | 1,065,273 | ​ | ​ | ​ | ​ | ​
​ The weighted average grant-date fair value of stock options granted during the three and nine months ended September 30, 2023 was $25.80 and $28.38, respectively. ​ The aggregate intrinsic value of stock options vested and expected to vest at September 30, 2023 was $41.2 million. The aggregate intrinsic value of stock options exercisable at September 30, 2023 was $32.6 million. As of September 30, 2023, total compensation cost related to non-vested employee, consultant and non-employee director stock options not yet recognized was approximately $67.6 million, net of estimated forfeitures, which is expected to be recognized as expense over a weighted average period of 2.7 years. ​ Stock-based compensation expense for the three and nine months ended September 30, 2023 and 2022 was recorded as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended September 30, | ​ | Nine Months Ended September 30,
​ |  | 2023 |  | 2022 |  | 2023 |  | 2022
​ | ​ | (In thousands) | ​ | (In thousands)
Research and development | ​ | $ | 3,618 | ​ | $ | 2,342 | ​ | $ | 8,269 | ​ | $ | 5,754
General and administrative | ​ |  | 3,503 | ​ |  | 2,154 | ​ |  | 8,409 | ​ |  | 5,349
Total stock-based compensation expense | ​ | $ | 7,121 | ​ | $ | 4,496 | ​ | $ | 16,678 | ​ | $ | 11,103
​ The fair values of employee, consultant and non-employee director stock options granted during the three and nine months ended September 30, 2023 and 2022 were valued using the Black-Scholes option pricing model with the following assumptions: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended September 30, | ​ | Nine Months Ended September 30,
|  | 2023 |  | 2022 |  | 2023 |  | 2022
Expected stock price volatility |  | 92% | ​ | 91% | ​ | 92% | ​ | 90 – 97%
Expected option term |  | 6.0 Years | ​ | 6.0 Years | ​ | 6.0 Years | ​ | 6.0 Years
Risk-free interest rate |  | 4.1% | ​ | 2.9 – 3.5% | ​ | 3.5 – 4.1% | ​ | 1.7 – 3.6%
Expected dividend yield |  | None | ​ | None | ​ | None | ​ | None
​

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