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Stock-Based Compensation (Tables)
6 Months Ended
Jun. 30, 2023
Share-Based Payment Arrangement [Abstract]  
Schedule of Total Employee, Director and Non-Employee Stock-Based Compensation Expense Recognized
Total employee, director and non-employee stock-based compensation expense recognized for the three and six months ended June 30, 2023 and 2022 are as follows (in thousands):
Three Months Ended
June 30,
Six Months Ended
June 30,
2023202220232022
General and administrative $4,471 $4,350 $8,747 $8,024 
Research and development 9,092 8,253 17,415 15,384 
$13,563 $12,603 $26,162 $23,408 
Three Months Ended
June 30,
Six Months Ended
June 30,
2023202220232022
Stock options$6,842 $7,512 $13,825 $14,345 
ESPP341 290 663 591 
RSUs6,380 4,801 11,674 8,472 
$13,563 $12,603 $26,162 $23,408 
Summary of Stock Option Activity
The following table summarizes option activity under our stock plans and related information:
Number of
Shares Subject
to Outstanding
Options
Weighted
Average
Exercise
Price
(Per
Share)
Weighted
Average
Remaining
Contractual
Term
(in years)
Aggregate
Intrinsic
Value
(in thousands)
Balance at December 31, 202210,082,642$29.12 6.30$27,141 
Options granted1,941,412$30.65 
Options forfeited(269,747)$33.95 
Options exercised(179,391)$8.92 
Balance at June 30, 202311,574,916$29.58 6.41$20,561 
Exercisable7,492,446$28.17 5.04$20,504 
Schedule of Weighted Average Assumptions Used for Estimation of Fair Value of Stock Options We estimated the fair value of each stock option using the Black-Scholes option-pricing model based on the date of grant of such stock option with the following weighted average assumptions for the three and six months ended June 30, 2023 and 2022:
OptionsOptions
Three Months Ended
June 30,
Six Months Ended
June 30,
2023202220232022
Expected term (years)6.56.36.16.4
Expected volatility51.2 %53.1 %50.5 %53.0 %
Risk-free interest rate3.69 %3.13 %4.17 %1.93 %
Expected dividend yield— %— %— %— %
Schedule of Weighted Average Assumptions Used for Estimation of Fair Value of ESPP
ESPPESPP
Three Months Ended
June 30,
Six Months Ended
June 30,
2023202220232022
Expected term (years)
0.5 - 2.0
0.5 - 2.0
0.5 - 2.0
0.5 -2.0
Expected volatility
38.2% - 55.7%
43.2% - 55.7%
38.2% - 55.7%
43.2% - 55.7%
Risk-free interest rate
0.13% - 5.39%
0.13% - 2.82%
0.13% - 5.39%
0.13% - 2.82%
Expected dividend yield— %— %— %— %
Summary of Restricted Stock Unit Activity
The following table summarizes the RSU activity for the six-month period ended June 30, 2023:
Restricted
Stock
Units
Weighted
Average Grant
Date Fair Value
(Per unit)
Unvested RSUs at December 31, 20221,232,551$32.41 
Granted944,72630.82 
Vested(367,647)32.73 
Forfeited(75,672)31.93 
Unvested RSUs at June 30, 20231,733,958$31.50