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Stock-Based Compensation (Tables)
 3 Months Ended
Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Schedule of Stock Option Activity
The following table summarizes changes in stock option activity during the three months ended March 31, 2020:
 
Number
of
Shares
 
Weighted
Average
Exercise Price
 
Weighted
Average
Contractual
Term
 
Aggregate
Intrinsic
Value
 
 
 
 
 
(in years)
 
(in thousands)
Outstanding as of December 31, 2019
3,374,541

 
$
6.53

 
7.59
 
$
—

Granted
—

 
—

 
 
 
 
Exercised
—

 
—

 
 
 
 
Forfeited
(194,936
)
 
6.94

 
 
 
 
Outstanding as of March 31, 2020
3,179,605

 
$
6.51

 
7.74
 
$
93

Options vested or expected to vest as of March 31, 2020
3,179,605

 
$
6.51

 
7.74
 
$
93

Options exercisable as of March 31, 2020
1,617,860

 
$
6.46

 
7.12
 
$
9
Schedule of Assumptions Used to Determine Fair Value of the Stock Options Granted to employees and directors
The assumptions that the Company used to determine the fair value of the stock options granted to employees and directors during the three months ended March 31, 2019 were as follows, presented on a weighted average basis:
 
Three Months Ended
March 31, 2019
Expected volatility
96.65
%
Risk-free interest rate
2.45
%
Expected dividend yield
—
%
Expected life (in years)
6.06
Schedule of Stock-based Compensation Expense
The Company recorded stock-based compensation expense related to all stock-based awards and the ESPP in the following expense categories of its condensed consolidated statements of operations and comprehensive loss (in thousands):
 
Three Months
Ended March 31,
 
2020
 
2019
Research and development expenses
$
640

 
$
820

General and administrative expenses
1,029

 
909

 
$
1,669

 
$
1,729
ESPP | Unvested restricted stock units  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Schedule of Restricted Stock Unit Activity
The following table summarizes RSU activity for the three months ended March 31, 2020:
 
Number of
Shares
 
Weighted
Average Grant-
Date Fair Value
per Share
Unvested as of December 31, 2019
1,053,394

 
$
3.27

Granted
1,000,826

 
$
1.20

Vested
(273,461
)
 
$
3.64

Cancelled
(24,700
)
 
$
4.48

Unvested as of March 31, 2020
1,756,059

 
$
2.02
2015 Plan | Restricted Stock Awards  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Schedule of Restricted Common Stock Activity
The following table summarizes the Company’s restricted common stock activity under the 2015 Plan since December 31, 2019:
 
Number of
Shares
 
Weighted
Average Grant-
Date Fair Value
per Share
Unvested restricted common stock as of December 31, 2019
106,038

 
$
2.78

Vested
(37,413
)
 
$
2.86

Unvested restricted common stock as of March 31, 2020
68,625

 
$
2.74