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Equity Incentive Plans and Stock-Based Compensation (Tables)
 3 Months Ended
Mar. 31, 2021
Share-based Payment Arrangement [Abstract]  
Summary of Restricted Stock Activity
The following table summarizes restricted stock activity during the three months ended March 31, 2021:
  Number
of
Shares		Weighted-
Average
Grant Date
fair value
Outstanding and unvested, as of December 31, 2020126,522 $1.85 
Issued— $— 
Vested(30,277)$1.69 
Repurchases— $— 
Outstanding and unvested, as of March 31, 202196,245 $1.90 
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:
  Three Months Ended
March 31,
 20212020
Expected volatility
74.95% - 75.72%
72.10% - 77.50%
Risk-free interest rate
0.61% - 1.06%
0.52% - 0.82%
Expected dividend——
Expected term (in years)
5.77 - 6.08
5.38 - 6.75
Underlying common stock fair value
26.13 - 38.23
6.22
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 
Granted1,082,053 $27.11 
Exercised(105,734)$5.28 
Forfeited(41,887)$7.59 
Outstanding as of March 31, 20213,928,287 $13.30 8.97$100,437 
Exercisable as of March 31, 2021856,283 $5.24 8.31$29,194 
Vested and expected to vest as of March 31, 20213,928,287 $13.30 8.97$100,437 
Summary of Components and Classification of Stock-based Compensation Expense for Stock-based Awards
The following table presents the components and classification of stock-based compensation expense for the Company’s stock-based awards for the three months ended March 31, 2021 and 2020 (in thousands):
Three Months Ended
March 31,
20212020
Restricted stock awards$51 $57 
Stock options and ESPP2,527 368 
Total stock-based compensation expense$2,578 $425 
Research and development expenses$1,170 $164 
General and administrative expenses$1,408 $261 
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:
 Three Months Ended
March 31,
 2021
Risk-free interest rate0.07 %
Expected term of options (in years)0.5
Expected stock price volatility89.51 %
Expected dividends—