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Stock-Based Compensation
 12 Months Ended
Dec. 31, 2022
Share-Based Payment Arrangement [Abstract]  
Stock-Based Compensation Stock-Based Compensation
The total number of shares of common stock authorized for issuance under the 2020 Stock Option and Incentive Plan (the "2020 Plan") as of December 31, 2022 and December 31, 2021 was 7,449,480 shares and 5,184,455 shares, respectively. The total number of shares of common stock authorized for issuance under the 2017 Stock Inventive Plan (the "2017 Plan") as of December 31, 2022 and 2021 was 5,937,763 shares. Any authorization to issue new options under the 2017 Plan was cancelled upon the effectiveness of the 2020 Plan and no further awards will be granted under the 2017 Plan.
Stock options issued under the 2020 Plan and 2017 Plan expire ten years from the date of grant. Shares that expire, are terminated, surrendered or canceled under the 2020 Plan and 2017 Plan without having been fully exercised will be available for future awards. In addition, shares of common stock that are tendered to the Company by a participant to exercise an award are added to the number of shares of common stock available for future awards. Upon stock option exercise, the Company issues new shares and delivers them to the participant.
2020 Employee Stock Purchase Plan
The total shares authorized for issuance under the 2020 Employee Stock Purchase Plan (the "2020 ESPP") as of December 31, 2022 and 2021 was 981,306 shares and 654,204 shares, respectively. During the year ended December 31, 2022, the Company issued 141,384 shares under the 2020 ESPP. The Company did not issue any shares under the 2020 ESPP during the year ended December 31, 2021.
Restricted Stock Units
The following table summarizes the Company’s restricted stock unit activity:
SharesWeighted
Average
Grant Date
Fair Value
Unvested as of December 31, 2021440,079 $41.86 
Issued676,828 14.54 
Vested(104,803)44.89 
Forfeited(268,154)22.08 
Unvested as of December 31, 2022743,950 $23.07 
As of December 31, 2022, total unrecognized compensation cost related to unvested restricted stock units was $12.4 million, which is expected to be recognized over a weighted-average period of 2.65 years.
Stock Options
The following table summarizes the Company’s stock option activity:
Number of
Shares		Weighted
Average
Exercise Price
per Share		Weighted
Average
Remaining
Contractual
Term		Aggregate
Intrinsic Value
   (In years)(In thousands)
Outstanding as of December 31, 20216,468,501 $16.92 
Granted3,969,302 9.23 
Exercised(265,662)3.04 $850 
Cancelled or Forfeited(1,334,113)16.64 
Outstanding as of December 31, 20228,838,028 $13.93 8.26$514 
Exercisable as of December 31, 20223,315,240 $14.86 7.50$95 
Vested and expected to vest as of December 31, 20228,635,022 $13.90 8.30$497 
The aggregate intrinsic value of stock options exercised for the year ended December 31, 2021 was $19.7 million. The aggregate intrinsic value of stock options outstanding, exercisable, and vested and expected to vest is calculated as the difference between the exercise price of the underlying stock options and the estimated fair value of the Company’s common stock for those stock options that had exercise prices lower than the estimated fair value of the Company’s common stock at December 31, 2022. The aggregate intrinsic value of stock options exercised is calculated as the difference between the exercise price of the underlying stock options and the estimated fair value of the Company’s common stock on the date of exercise for those stock options that had exercise prices lower than the fair value of the Company’s common stock on the exercise date.
Valuation of Stock Options
The weighted-average assumptions that the Company used in the Black-Scholes option pricing model to determine the grant-date fair value of stock options granted to employees, members of the Board, and non-employees on the date of grant were as follows for the for the years ended December 31, 2022 and 2021:
 Year Ended December 31,
 20222021
Risk-free interest rate2.52 %0.86 %
Expected term (in years)6.076.08
Expected volatility89.20 %85.52 %
Expected dividend yield— %— %
Weighted average grant-date fair value per share$6.65 $27.58 
As of December 31, 2022, total unrecognized compensation cost related to unvested stock options was $45.9 million, which is expected to be recognized over a weighted-average period of 2.17 years.
Stock-Based Compensation
Stock-based compensation expense was allocated as follows (in thousands):
 Year Ended December 31,
 20222021
Research and development$9,972 $9,350 
General and administrative18,621 13,343 
Total stock-based compensation expense$28,593 $22,693