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Stock-Based Compensation | 3 Months Ended
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Mar. 31, 2024
Stock-Based Compensation |
Stock-Based Compensation | (8) Stock-Based Compensation ​ A summary of stock option activity for the three months ended March 31, 2024 is as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Weighted | ​ | Weighted
​ | ​ | ​ | ​ | Average | ​ | Average
​ | ​ | ​ | ​ | Exercise | ​ | Remaining
​ | ​ | ​ | ​ | Price | ​ | Contractual
​ |  | Shares |  | Per Share |  | Term (In Years)
Options outstanding at December 31, 2023 |  | 6,378,924 | ​ | $ | 29.69 | ​ | 7.5
Granted |  | 5,000 | ​ | $ | 37.08 | ​ | ​
Exercised |  | (222,619) | ​ | $ | 16.27 | ​ | ​
Canceled |  | (2,568) | ​ | $ | 83.21 | ​ | ​
Options outstanding at March 31, 2024 |  | 6,158,737 | ​ | $ | 30.16 | ​ | 7.4
Options vested and expected to vest at March 31, 2024 |  | 6,065,176 | ​ | $ | 30.14 | ​ | 7.3
Options exercisable at March 31, 2024 |  | 3,284,980 | ​ | $ | 29.45 | ​ | 6.3
Shares available for grant under the 2021 Plan |  | 1,011,071 | ​ | ​ | ​ | ​ | ​
​ The weighted average grant-date fair value of stock options granted during the three months ended March 31, 2024 was $28.64. ​ The aggregate intrinsic value of stock options vested and expected to vest at March 31, 2024 was $104.1 million. The aggregate intrinsic value of stock options exercisable at March 31, 2024 was $74.3 million. As of March 31, 2024, total compensation cost related to non-vested employee, consultant and non-employee director stock options not yet recognized was approximately $55.3 million, net of estimated forfeitures, which is expected to be recognized as expense over a weighted average period of 2.4 years. ​ Stock-based compensation expense for the three months ended March 31, 2024 and 2023 was recorded as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended March 31,
​ |  | 2024 |  | 2023
​ | ​ | (In thousands)
Research and development | ​ | $ | 3,693 | ​ | $ | 2,162
General and administrative | ​ |  | 3,509 | ​ |  | 2,178
Total stock-based compensation expense | ​ | $ | 7,202 | ​ | $ | 4,340
​ The fair values of employee, consultant and non-employee director stock options granted during the three months ended March 31, 2024 and 2023 were valued using the Black-Scholes option pricing model with the following assumptions: ​
​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended March 31,
|  | 2024 |  | 2023
Expected stock price volatility |  | 92% | ​ | 92%
Expected option term |  | 6.0 Years | ​ | 6.0 Years
Risk-free interest rate |  | 4.0 – 4.3% | ​ | 3.7 – 4.0%
Expected dividend yield |  | None | ​ | None
​

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