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Stock-Based Compensation (Tables)
9 Months Ended
Sep. 30, 2023
Share-Based Payment Arrangement [Abstract]  
Summary of Stock Option Activity Under Plan The following summarizes option activity under the 2018 Plan:
Outstanding Options
Number
of
Options
Weighted-
Average
Exercise
Price
Weighted-
Average
Remaining
Contract
Term
Aggregate Intrinsic Value
(in years)(in thousands)
Balance, December 31, 202217,569,575 $12.90 7.73$6,658 
Granted9,300,276 5.13 
Exercised(782,538)2.47 2,283 
Forfeited(4,097,851)10.85 
Balance, September 30, 202321,989,462 $10.37 7.61$673 
Exercisable, September 30, 202313,651,398 $12.33 6.95$673 
Vested and expected to vest, September 30, 202321,989,462 $10.37 7.61$— 
Schedule Stock Option Valuation Assumptions The fair value of employee, consultant and director stock option awards was estimated at the date of grant using a Black-Scholes option-pricing model with the following assumptions:
Nine Months Ended September 30,
20232022
Expected term in years
5.27 - 6.08
5.25 - 6.08
Expected volatility
73.18% - 73.85%
70.82% - 73.39%
Expected risk-free interest rate
3.45% - 4.30%
1.61% - 3.49%
Expected dividend
0%
0%
Schedule of Restricted Stock Units Activity Under Plan The following summarizes restricted stock unit activity under the 2018 Plan:
Outstanding Restricted Stock Units
Restricted
Stock Units
Weighted-
Average Grant Date Fair
Value per
Share
Weighted Average Remaining Vesting LifeAggregate Intrinsic Value
(in years)(in thousands)
Unvested December 31, 20225,493,406 $16.86 1.54$34,554 
Granted11,215,162 4.70 2.0
Vested(1,545,458)18.49 
Forfeited(2,993,273)9.35 
Unvested September 30, 202312,169,837 $7.29 2.14$38,578 
Vested and expected to vest, September 30, 202312,169,837 $7.29 2.14$38,578 
Schedule of Stock-Based Compensation Expense Total stock-based compensation expense related to stock options, restricted stock units, employee stock purchase plan and vesting of the founders’ common stock was as follows (in thousands):
Three Months Ended September 30,Nine Months Ended September 30,
2023202220232022
Research and development$6,733 $11,016 $22,849 $35,068 
General and administrative8,621 10,132 27,869 31,286 
Total stock-based compensation$15,354 $21,148 $50,718 $66,354