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Stock-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2021
Share-based Payment Arrangement [Abstract]  
Summary of Restricted Stock Activity
The following table summarizes restricted stock activity during the year ended December 31, 2021:
Number
of Shares		Weighted-
Average
Grant Date
fair value
Outstanding and unvested, as of December 31, 2020126,522 $1.85 
Issued— $— 
Vested(103,164)$1.78 
Repurchases(1,517)$2.12 
Outstanding and unvested, as of December 31, 202121,841 $2.16 
Summary of Black-Scholes Option Pricing Model to Estimate Stock-based Compensation Expense for Stock Option Awards
The Company used Black-Scholes option pricing model to estimate stock-based compensation expense for stock option awards with the following assumptions:
Year Ended December 31,
202120202019
Expected volatility74.83%-76.31%72.10%-77.50%74.80%-82.53%
Risk-free interest rate0.61 %-1.39 %0.27 %-0.82 %1.43%-2.59%
Expected dividend———
Expected term (in years)5.44-6.085.26-6.755-6.08
Underlying common stock fair value15.2-38.236.22-28.795.15-7.08
Summary of Option Activity Under the 2015 Plan and 2020 Plan
A summary of option activity under the 2015 Plan and the 2020 Plan is as follows:
Number
of
Options		Weighted-
Average Exercise
Price per Share		Weighted-
Average
Remaining
Contractual
Term (in Years)		Aggregate
Intrinsic
Value
Outstanding as of December 31, 20202,993,855 $8.18 8.84$43,890 
Granted1,520,391 $26.03 
Exercised(336,340)$5.21 
Forfeited(557,726)$16.4 
Outstanding as of December 31, 20213,620,180 $14.56 8.25$16,735 
Exercisable as of December 31, 20211,368,543 $9.76 7.66$9,426 
Vested and expected to vest as of December 31, 20213,620,180 $14.56 7.66$16,735 
Summary of Black-Scholes Option Pricing Model to Estimate Stock-based Compensation Expense for 2020 Employee Stock Purchase Plan The Company used Black-Scholes option pricing model to estimate stock-based compensation expense for the 2020 ESPP with the following assumptions:
Year Ended December 31,
202120202019
Risk-free interest rate
0.06% - 0.07%
0.11%—
Expected term of options (in years)0.500.58—
Expected stock price volatility
67.16% - 89.51%
83.72%—
Expected dividends———
Summary of Classification of Stock-based Compensation Expense
The following table presents the classification of stock-based compensation expense during the years ended December 31, 2021 and 2020 (in thousands):
Year Ended December 31,
202120202019
Research and development expenses$3,928 $1,719 $584 
General and administrative expenses6,509 2,176 1,245 
Total stock-based compensation expense$10,437 $3,895 $1,829