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Equity Incentive Plans (Tables)
 12 Months Ended
Dec. 31, 2022
Share-Based Payment Arrangement [Abstract]  
Summary of Fair Value of Stock Option Award Granted
The following assumptions were used to estimate the fair value of stock option awards granted to employees under the Company’s equity incentive plans and the shares purchasable under the ESPP during the periods presented:
Year Ended December 31,
202220212020
Employee Stock Options
Expected term (in years)
4.4 - 6.1
4.6 - 6.1
4.6 - 6.1
Risk-free interest rate
1.31% - 4.35%
0.65% - 1.30%
0.22% - 1.67%
Volatility
71.10% - 89.87%
78.80% - 84.79%
84.38% - 87.23%
Dividend yield———
Employee Stock Purchase Plan
Expected term (in years)
0.49 - 2.00
0.49 - 2.00
0.49 - 2.00
Risk-free interest rate
0.60% -3.51%
0.06% - 0.20%
0.12% - 0.95%
Volatility
64.98% - 121.43%
61.29% - 87.13%
85.50% - 99.13%
Dividend yield———
Summary of Stock Option Activity
The following table summarizes stock option activity for the years ended December 31, 2022, 2021 and 2020:
Shares Subject to
Options Outstanding		Weighted-
Average
Remaining
Contractual
Life
(Years)		Aggregate
Intrinsic Value
SharesWeighted-
Average
Exercise
Price
(in thousands)
Outstanding as of December 31, 20198,538,060 $13.67 9.0$35,385 
Options granted2,712,372 $13.78 
Options exercised(134,803)$3.96 
Options forfeited/cancelled(1,714,547)$15.99 
Outstanding as of December 31, 20209,401,082 $13.42 8.1$10,182 
Options granted3,159,126 $9.82 
Options exercised(325,494)$6.19 
Options forfeited/cancelled(2,800,054)$14.14 
Outstanding as of December 31, 20219,434,660 $12.24 7.4$15,822 
Options granted9,271,272 $6.75 
Options exercised(270,707)$6.41 
Options forfeited/cancelled(948,060)$15.69 
Outstanding as of December 31, 202217,487,165 $9.24 8.1$47 
Options vested and expected to vest as of December 31, 2022
17,487,165 $9.24 8.1$47 
Options exercisable as of December 31, 2022
6,433,686 $12.19 6.4$— 
Summary of Restricted Stock Activity
The summary of the Company’s restricted stock activity during the years ended December 31, 2022, 2021 and 2020 is as follows: 
Number of
Restricted
Stock Units
Outstanding		Weighted-
Average
Grant Date
Fair Value
Nonvested at December 31, 20194,648,526 $3.98 
Granted2,003,900 10.96 
Vested(2,557,377)4.00 
Forfeited / cancelled(764,228)8.30 
Nonvested at December 31, 20203,330,821 $7.16 
Granted1,349,885 10.23 
Vested(1,391,476)5.86 
Forfeited / cancelled(728,011)10.03 
Nonvested at December 31, 20212,561,219 $8.67 
Granted572,901 11.94 
Vested(1,592,588)7.78 
Forfeited / cancelled(191,497)10.68 
Nonvested at December 31, 20221,350,035 $10.83 
Stock-Based Compensation Expense Reported in Condensed Consolidated Statements of Operations
Stock-based compensation expense has been reported in the Company’s consolidated statements of operations and comprehensive loss as follows (in thousands):
Year Ended December 31,
202220212020
Research and development$24,415 $18,943 $18,997 
General and administrative18,138 13,065 19,751 
Total stock-based compensation expense$42,553 $32,008 $38,748