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Equity Incentive Plans (Tables)
 9 Months Ended
Sep. 30, 2024
Share-Based Payment Arrangement [Abstract]  
Schedule 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(1,819,750)1,819,750 $73.42  
Options exercised17,807 
(1)
(1,404,703)$14.84  
Options forfeited294,370 (294,370)$42.67  
Restricted stock units granted(824,077)  
Restricted stock units withheld138,567  
Restricted stock units forfeited73,838  
Balances — September 30, 20247,537,258 9,435,513 $35.64  7.63$741,952 
Vested and expected to vest — September 30, 20249,435,513 $35.64  7.63$741,952 
Exercisable at September 30, 20244,863,669 $23.00  6.75$443,927 
_______________________________________________
(1) Shares returned due to net exercises.
Schedule of Nonvested Restricted Stock Shares Activity RSU activity for the nine months ended September 30, 2024 was as follows:
Shares
Weighted-
Average
Grant-Date
Fair Value
Unvested at December 31, 2023753,462$38.93 
Granted824,077$79.32 
Vested and released(350,755)$47.87 
Cancelled(73,838)$56.85 
Unvested at September 30, 20241,152,946$63.93 
Schedule 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 and nine months ended September 30, 2024 and 2023 using the following assumptions:
Three Months Ended September 30,Nine Months Ended September 30,
2024202320242023
Fair Value Assumptions
Expected volatility
69.3% - 69.6%
71.3% - 72.1%
69.3% - 71.1%
71.3% - 74.0%
Expected dividend yield0%0%0%0%
Expected term (in years)
5.3 - 5.4
5.4
5.3 - 5.4
5.3 - 5.4
Risk-free interest rate
3.5% - 4.4%
4.2% - 4.4%
3.5% - 4.5%
3.5% - 4.4%
Schedule 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 and nine months ended September 30, 2024 and 2023 using the following assumptions:
Three Months Ended September 30,Nine Months Ended September 30,
2024202320242023
Fair Value Assumptions
Expected volatility
38.8% - 57.8%
74.0% - 79.6%
38.1% - 62.7%
74.0% - 99.7%
Expected dividend yield0%0%0%0%
Expected term (in years)
0.5 - 2.0
0.5- 2.0
0.5 - 2.0
0.5 - 2.0
Risk-free interest rate
4.8% - 5.4%
4.2% - 5.4%
4.8% - 5.4%
4.2% - 5.4%
Schedule of Stock-based Compensation Expense
We recorded total stock-based compensation expense for the three and nine months ended September 30, 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 September 30,Nine Months Ended September 30,
2024202320242023
(in thousands)(in thousands)
Research and development$10,860 $6,335 $30,533 $16,773 
General and administrative10,405 6,885 29,919 18,639 
Total$21,265 $13,220 $60,452 $35,412