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Equity Incentive Plans (Tables)
 6 Months Ended
Jun. 30, 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 six months ended June 30, 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  
Granted142,650  $16.28  
Exercises(32,256) $3.31  
Forfeitures and cancellations(452,278) $37.79  
Outstanding at June 30, 20202,697,996  $27.61  7.12$19,517  
Exercisable at June 30, 20201,365,178  $25.89  5.40$12,530  
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:
Six Months Ended
June 30,
20202019
Risk-free interest rate1.2  %2.5  %
Expected volatility91.8  %68.5  %
Expected dividend yield—  %—  %
Expected term (in years)6.256.25
Weighted average grant date fair value per share$11.97  $44.02  
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:
Three Months Ended
June 30,		Six Months Ended
June 30,
(in thousands)2020201920202019
Research and development$958  $1,649  $2,104  $2,889  
General and administrative1,783  1,994  3,612  3,621  
Total$2,741  $3,643  $5,716  $6,510