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Equity Incentive Plans (Tables)
 3 Months Ended
Mar. 31, 2024
Share-Based Payment Arrangement [Abstract]  
Summary of Activity Under Stock Option Plans
Stock options and RSUs activity under our 2020 Plan and 2014 Plan, which excludes options to purchase 29,638 shares granted outside of the 2020 Plan and 2014 Plan, was as follows:
Options Outstanding
Stock Options and Restricted Stock Units Activity
Options and
Restricted Stock Units Available
for Grant
Number
of
Options
Weighted-
Average
Exercise
Price Per
Share
Weighted-
Average
Remaining
Contractual
Term
(Years)
Aggregate
Intrinsic
Value
Balances — December 31, 20236,065,150 9,314,836 $25.35  7.75$348,868 
Additional shares authorized4,768,241   
Plan shares expired(1,176,888)
Options granted(1,418,472)1,418,472 $72.85  
Options exercised6,464 
(1)
(630,679)$8.74  
Options forfeited4,978 (4,978)$46.02  
Restricted stock units granted(565,682)  
Restricted stock units withheld46,279  
Restricted stock units forfeited3,625  
Balances — March 31, 20247,733,695 10,097,651 $33.05  7.98$363,125 
Vested and expected to vest — March 31, 202410,097,651 $33.05  7.98$363,125 
Exercisable at March 31, 20244,584,552 $19.11  6.92$225,684 
_______________________________________________
(1) Shares returned due to net exercises.
Nonvested Restricted Stock Shares Activity RSU activity for the three months ended March 31, 2024 was as follows:
Shares
Weighted-
Average
Grant-Date
Fair Value
Unvested at December 31, 2023753,462$38.93 
Granted565,68273.42 
Vested and released(110,580)36.41 
Cancelled(3,625)52.26 
Unvested at March 31, 20241,204,939$55.31 
Summary of Estimated Fair Value of Employee Stock Options
We estimated the fair value of employee stock options using the Black-Scholes option-pricing model for the three months ended March 31, 2024 and 2023 using the following weighted-average assumptions:
Three Months Ended March 31,
20242023
Fair Value Assumptions
Expected volatility
73.2% - 73.5%
73.6% - 74.0%
Expected dividend yield0%0%
Expected term (in years)5.4
5.3 - 5.4
Risk-free interest rate
4.0% - 4.3%
3.7% - 4.3%
Summary of Estimated Fair Value of Shares Under Employee Stock Purchase Plan
We estimated the fair value of shares under the 2020 ESPP using the Black-Scholes option-pricing model for the three months ended March 31, 2024 and 2023 using the following weighted-average assumptions:
Three Months Ended March 31,
20242023
Fair Value Assumptions
Expected volatility
38.1% - 62.7%
86.4% - 99.7%
Expected dividend yield0%0%
Expected term (in years)
0.5 - 2.0
0.5- 2.0
Risk-free interest rate
4.9% - 5.4%
4.5% - 4.7%
Summary of Stock-based Compensation Expense
We recorded total stock-based compensation expense for the three months ended March 31, 2024 and 2023 related to the 2014 Plan, the 2020 Plan and the 2020 ESPP in the condensed consolidated statements of operations and allocated the amounts as follows:
Three Months Ended March 31,
20242023
(in thousands)
Research and development$8,818 $4,527 
General and administrative8,811 5,121 
Total$17,629 $9,648