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Equity Incentive Plans (Tables)
 6 Months Ended
Jun. 30, 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(1,642,952)1,642,952 $72.27  
Options exercised15,288 
(1)
(876,113)$10.16  
Options forfeited219,279 (219,279)$43.10  
Restricted stock units granted(700,547)  
Restricted stock units withheld54,235  
Restricted stock units forfeited40,160  
Balances — June 30, 20247,641,966 9,862,396 $34.12  7.77$408,223 
Vested and expected to vest — June 30, 20249,862,396 $34.12  7.77$408,223 
Exercisable at June 30, 20244,897,258 $21.41  6.82$264,933 
_______________________________________________
(1) Shares returned due to net exercises.
Nonvested Restricted Stock Shares Activity RSU activity for the six months ended June 30, 2024 was as follows:
Shares
Weighted-
Average
Grant-Date
Fair Value
Unvested at December 31, 2023753,462$38.93 
Granted700,547$72.97 
Vested and released(145,528)$38.71 
Cancelled(40,160)$58.85 
Unvested at June 30, 20241,268,321$57.13 
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 and six months ended June 30, 2024 and 2023 using the following assumptions:
Three Months Ended June 30,Six Months Ended June 30,
2024202320242023
Fair Value Assumptions
Expected volatility
69.8% - 70.5%
72.6% - 73.2%
69.8% - 71.1%
72.6% - 74.0%
Expected dividend yield0%0%0%0%
Expected term (in years)
5.4
5.4
5.4
5.3 - 5.4
Risk-free interest rate
4.3% - 4.5%
3.5% - 4.0%
4.0% - 4.5%
3.5% - 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 and six months ended June 30, 2024 and 2023 using the following assumptions:
Three Months Ended June 30,Six Months Ended June 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%
Summary of Stock-based Compensation Expense
We recorded total stock-based compensation expense for the three and six months ended June 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 June 30,Six Months Ended June 30,
2024202320242023
(in thousands)(in thousands)
Research and development$10,855 $5,911 $19,673 $10,438 
General and administrative10,703 6,633 $19,514 $11,754 
Total$21,558 $12,544 $39,187 $22,192