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Stock-based compensation (Tables)
 12 Months Ended
Dec. 31, 2022
Share-Based Payment Arrangement [Abstract]  
Schedule of stock-based compensation expense
Stock-based compensation expense was as follows (in thousands):
Years Ended December 31,
20222021
Stock-based compensation expense:
Research and development$13,108 $7,526 
General and administrative16,908 13,986 
Total stock-based compensation expense$30,016 $21,512 
Summary of stock option activity
The following table summarizes the stock option activity under the Company’s equity awards plans for the year ended December 31, 2022:
Number of
Options		Weighted-Average
Exercise Price		Weighted-Average
Remaining
Contractual
Term (in years)		Aggregate
Intrinsic
Value
(in thousands)
Outstanding as of December 31, 20215,983,425 $22.33 8.64$75,538 
Granted2,296,800 18.93 
Exercised(181,354)4.67 
Forfeited/expired(1,105,819)23.98 
Outstanding as of December 31, 20226,993,052 $21.57 7.78$1,031 
Exercisable as of December 31, 20222,750,625 $21.69 6.86$570 
Vested and expected to vest as of December 31, 20226,993,052 $21.57 7.78$1,031 
Other information related to the option activity of the Company was as follows:
Years Ended December 31,
20222021
Weighted-average fair value of options granted$13.44 $29.54 
Intrinsic value of options exercised (in thousands)$1,796 $17,321 
Summary of significant valuation assumptions
The following table summarizes assumptions used in the Black-Scholes option pricing model to determine the fair value of stock options granted to employees:
Years Ended December 31,
20222021
Expected option life (years)
5.50 - 6.11
5.50 - 6.11
Risk-free interest rate
1.47% - 4.22%
0.59% - 1.34%
Expected volatility
81.36% - 88.75%
80.45% - 86.20%
Expected dividend yield0.00%0.00%
Summary of RSU activity
The following table summarizes the activity under the Company’s equity plans with respect to RSUs for the year ended December 31, 2022:
 SharesWeighted-Average
Grant Date
Fair Value
Outstanding as of December 31, 2021— $— 
Granted563,500 25.47 
Vested (1)
(64,383)25.01 
Forfeited(33,826)31.58 
Outstanding as of December 31, 2022465,291 $25.09 
(1) Vested RSUs include 10,287 shares retained by the Company to cover statutory minimum withholding taxes.