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Equity Incentive Plans and Stock-Based Compensation (Tables)
 6 Months Ended
Jun. 30, 2021
Share-based Payment Arrangement [Abstract]  
Summary of Restricted Stock Activity
The following table summarizes restricted stock activity during the six months ended months ended June 30, 2021:
  Number
of
Shares		Weighted-
Average
Grant Date
fair value
Outstanding and unvested, as of December 31, 2020126,522 $1.85 
Issued— $— 
Vested(60,413)$1.69 
Repurchases(63)$2.15 
Outstanding and unvested, as of June 30, 202166,046 $1.99 
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:
  Six Months Ended June 30,
 20212020
Expected volatility
74.95% - 76.31%
72.10% - 77.50%
Risk-free interest rate
0.61% - 1.07%
0.52% - 0.82%
Expected dividend——
Expected term (in years)
5.44 - 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 
Granted1,283,741 $27.46 
Exercised(181,173)$5.13 
Forfeited / expired(253,835)$14.59 
Outstanding as of June 30, 20213,842,588 $14.34 8.78$57,309 
Exercisable as of June 30, 20211,000,620 $6.64 8.14$22,496 
Vested and expected to vest as of June 30, 20213,842,588 $14.34 8.78$57,309 
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 and six months ended June 30, 2021 and 2020 (in thousands):
Three Months Ended
June 30,		Six Months Ended
June 30,
2021202020212020
Restricted stock awards$51 $51 $102 $109 
Stock options and ESPP2,229 735 4,756 1,102 
Total stock-based compensation expense$2,280 $786 $4,858 $1,211 
Research and development expenses$868 $287 $2,038 $451 
General and administrative expenses$1,412 $499 $2,820 $760 
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:
 Six Months Ended
June 30,
 2021
Risk-free interest rate0.07 %
Expected term of options (in years)0.5
Expected stock price volatility89.51 %
Expected dividends—