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Equity Incentive Plans (Tables)
 12 Months Ended
Dec. 31, 2020
Share-based Payment Arrangement [Abstract]  
Summary of Activity Related to Stock Option Awards A summary of the activity related to stock option awards during the year ended December 31, 2020 is as follows:
Shares
Subject to
Options
Weighted-Average
Exercise
Price per
Share
Weighted-Average
Remaining
Contractual
Term (in years)
Aggregate
Intrinsic
Value (in
thousands)
Outstanding at January 1, 20203,039,880 $29.40 
Granted539,825 $19.13 
Exercises(100,493)$4.94 
Forfeitures and cancellations(558,512)$38.13 
Outstanding at December 31, 2020
2,920,700 $26.67 7.29$17,434 
Exercisable at December 31, 20201,606,744 $26.56 5.93$12,326 
Summary of Weighted Average Assumptions in Stock Option Valuations
The estimated fair values of stock option awards granted to employees were determined on the date of grant using the Black-Scholes option valuation model with the following weighted-average assumptions:
Year Ended
December 31,
202020192018
Risk-free interest rate0.5 %2.0 %2.7 %
Expected volatility90.2 %77.98 %67.7 %
Expected dividend yield— %— %— %
Expected term (in years)6.256.256.25
Weighted average grant date fair value per share$14.51$21.99$61.41
Summary of Non-cash Stock-based Compensation Expense
Total non-cash stock-based compensation expense for all stock awards that was recognized in the consolidated statements of operations and comprehensive loss is as follows:
Year Ended
December 31,
(in thousands)202020192018
Research and development$4,122 $5,564 $3,371 
General and administrative7,377 6,847 6,590 
Total
$11,499 $12,411 $9,961