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Stock-Based Awards (Tables)
 3 Months Ended
Mar. 31, 2024
Share-Based Payment Arrangement [Abstract]  
Summary of Stock Option Activity
The following table summarizes stock option activity for the three months ended March 31, 2024:
Number of Options
Weighted-Average
Exercise Price
Balance at December 31, 202316,490,551 $27.34 
Granted
4,171,102 20.33 
Exercised
(130,214)7.85 
Forfeited
(804,563)28.50 
Balance at March 31, 202419,726,876 $25.94 
Vested and expected to vest at March 31, 2024
18,917,005 $27.02 
Exercisable at March 31, 202411,826,599 $26.70 
Summary of Assumptions Used for Estimating the Fair Value of Stock Granted
The estimated fair value of stock options granted to employees were calculated using the Black-Scholes option-pricing model using the following assumptions:

Three Months Ended March 31,
20242023
Expected term (in years)
6.08
6.08
Volatility
64.5% - 64.6%
69.1% - 69.6%
Risk-free interest rate
3.9% - 4.0%
3.6% - 4.2%
Dividend yield
——
Summary of Restricted Stock Activity
The following table summarizes restricted stock unit ("RSU") activity for the three months ended March 31, 2024:
Number of RSU sharesWeighted-Average Fair Value at Date of Grant per Share
Unvested at December 31, 20233,635,157 $35.60 
Granted1,919,193 20.23 
Vested and released(752,455)39.78 
Forfeited(504,807)28.86 
Unvested and expected to vest at March 31, 2024
4,297,088 $28.79 
Summary of Stock-Based Compensation Expense
The Company’s results of operations include expenses relating to stock-based compensation as follows (in thousands):
Three Months Ended March 31,
20242023
Research and development
$16,342 $16,784 
General and administrative
11,498 11,300 
Total
$27,840 $28,084