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Stock-based Compensation (Tables)
 12 Months Ended
Dec. 31, 2019
Share-based Payment Arrangement [Abstract]  
Schedule of Stock-Based Compensation Expense
Total stock-based compensation expense recognized in the consolidated statements of operations and comprehensive income (loss) for the years ended December 31, 2019 and 2018 was as follows (in thousands):
 
Year Ended December 31,
 
2019
 
2018
Research and development
$
4,353

 
$
4,540

General and administrative
5,256

 
4,867

Total stock-based compensation expense
$
9,609

 
$
9,407
Schedule of Restricted Stock and Restricted Stock Units Activity
The Company has also granted RSUs to its employees under the 2017 Plan. The following table summarizes RSU activity for the year ended December 31, 2019 (in thousands, except per share amounts):
 
RSUs
 
Weighted-Average Grant Date Fair Value per Share
Unvested as of December 31, 2018
371

 
$
8.02

Issued
354

 
$
4.40

Vested
(157
)
 
$
8.02

Cancelled
(108
)
 
$
6.40

Unvested as of December 31, 2019
460

 
$
5.61

The following table summarizes RSA activity for the year ended December 31, 2019 (in thousands, except per share amounts):
 
RSAs
 
Weighted-Average Grant Date Fair Value per Share
Unvested as of December 31, 2018
7

 
$
—

Issued
—

 
$
—

Vested
(7
)
 
$
—

Repurchased
—

 
$
—

Unvested as of December 31, 2019
—

 
$
—
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions
The fair value of stock options granted to employees and directors during the years ended December 31, 2019 and 2018 was calculated on the date of grant using the following weighted-average assumptions:
 
Year Ended December 31,
 
2019
 
2018
Risk-free interest rate
2.2
%
 
2.7
%
Expected dividend yield
—
%
 
—
%
Expected term (in years)
6.0

 
6.0

Expected volatility
69.4
%
 
65.2
%
Schedule of Stock Options, Activity
The following table summarizes changes in stock option activity during the year ended December 31, 2019 (in thousands, except per share amounts):
 
Options
 
Weighted-Average Exercise Price
 
Weighted-Average Remaining Contractual Term (in years)
 
Aggregate Intrinsic Value
Outstanding at December 31, 2018
5,023

 
$
10.23

 
7.6
 
$
3,133

Granted
1,418

 
$
4.37

 
 
 
 

Exercised
(185
)
 
$
2.36

 
 
 
 

Cancelled
(521
)
 
$
13.19

 
 
 
 

Outstanding at December 31, 2019
5,735

 
$
8.76

 
7.2
 
$
18,959

Exercisable at December 31, 2019
3,586

 
$
7.41

 
6.4
 
$
13,864