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STOCKHOLDERS' EQUITY (Tables)
 9 Months Ended
Sep. 30, 2020
Equity [Abstract]  
Summary of Fair Value Assumptions for Stock Options
The fair value of each option issued to employees was estimated on the date of grant using the Black-Scholes option pricing model with the following assumptions:
Three Months Ended
September 30,		Nine Months Ended
September 30,
2020201920202019
Risk-free interest rate0.38 %
1.43% - 1.95%
1.12 %
1.43% - 2.50%
Expected option life (in years)6.06
0.34 - 6.25
5.93
0.25 - 6.25
Expected dividend yield0 %0 %0 %0 %
Expected volatility 75.5 %73.4 %73.6 %73.4 %
Summary of Options Plan
The following table summarizes the Company’s stock option activity for the nine months ended September 30, 2020:
Number of SharesWeighted
Average
Exercise
Price		Weighted
Average
Remaining
Life
(in Years)		Intrinsic
Value
(in
thousands)
Outstanding as of January 1, 20205,176,944 $7.35 
Granted801,890 4.44 
Exercised(4,071)1.12 
Canceled(812,963)8.93 
Outstanding as of September 30, 20205,161,800 $6.65 7.52$1,772 
Exercisable as of September 30, 20202,835,131 $6.98 6.71$1,151 
Vested or expected to vest as of September 30, 20205,136,800 $6.65 7.52$1,772 
Share-based Payment Arrangement, Restricted Stock Unit, Activity The following table summarizes the Company's restricted stock unit activity for the nine months ended September 30, 2020:
Number of SharesWeighted Average Grant Date Fair Value per Share
Outstanding as of January 1, 202066,801 $3.40 
Granted336,166 4.61 
Vested— — 
Forfeited(108,371)3.68 
Outstanding as of September 30, 2020294,596 $4.67 
Summary of Stock Based Compensation Expense
Stock-based compensation expense was classified in the statements of operations and comprehensive loss as follows (in thousands):
Three Months Ended
September 30,		Nine Months Ended
September 30,
2020201920202019
Research and development
$532$659$2,129$1,727
General and administrative
8401,0192,7982,595
Total stock compensation expense$1,372$1,678$4,927$4,322