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Stock-Based Compensation - Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024 | Dec. 31, 2025
Stock-Based Compensation Expense |  |  |  |  |
Stock-based compensation expense recognized | $ 9,787 | $ 7,309 | $ 35,417 | $ 13,099 |
Unrecognized stock-based compensation expense for all stock awards | $ 66,000 |  | 66,000 |  |
Period over which unamortized stock-based compensation expense to be recognized | 2 years 8 months 12 days |  |  |  |
Non-employee |  |  |  |  |
Stock-Based Compensation Expense |  |  |  |  |
Stock-based compensation expense recognized | $ 300 | 100 | 500 | 200 |
Stock options |  |  |  |  |
Stock-Based Compensation Expense |  |  |  |  |
Stock-based compensation expense recognized | 7,979 | 7,291 | 28,771 | 13,045 |
RSUs |  |  |  |  |
Stock-Based Compensation Expense |  |  |  |  |
Stock-based compensation expense recognized | 1,542 |  | 5,939 |  |
ESPP |  |  |  |  |
Stock-Based Compensation Expense |  |  |  |  |
Stock-based compensation expense recognized | 266 |  | 701 |  |
Restricted stock awards |  |  |  |  |
Stock-Based Compensation Expense |  |  |  |  |
Stock-based compensation expense recognized |  | 18 | 6 | 54 |
Restricted stock awards | Non-employee | Maximum |  |  |  |  |
Stock-Based Compensation Expense |  |  |  |  |
Stock-based compensation expense recognized | 100 | 100 | 100 | 100 |
ACELYRIN |  |  |  |  |
Stock-Based Compensation Expense |  |  |  |  |
Additional stock-based compensation | 2,100 |  | 12,900 |  |
ACELYRIN | Severance Plan |  |  |  |  |
Stock-Based Compensation Expense |  |  |  |  |
Additional stock-based compensation |  |  |  |  | $ 13,100
Research and development |  |  |  |  |
Stock-Based Compensation Expense |  |  |  |  |
Stock-based compensation expense recognized | 5,773 | 3,720 | 16,717 | 6,805 |
Research and development | ACELYRIN | Severance Plan |  |  |  |  |
Stock-Based Compensation Expense |  |  |  |  |
Additional stock-based compensation | 1,100 |  | 4,100 |  |
General and administrative |  |  |  |  |
Stock-Based Compensation Expense |  |  |  |  |
Stock-based compensation expense recognized | 4,014 | $ 3,589 | 18,700 | $ 6,294 |
General and administrative | ACELYRIN | Severance Plan |  |  |  |  |
Stock-Based Compensation Expense |  |  |  |  |
Additional stock-based compensation | $ 1,000 |  | $ 8,800 |  |

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