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Stock-based Compensation (Tables)
 12 Months Ended
Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Schedule of Stock-Based Compensation Expense
Total stock-based compensation expense recognized in the consolidated statements of operations for the years ended December 31, 2018 and 2017 was as follows (in thousands):
 
Year Ended December 31,
 
2018
 
2017
Research and development
$
4,540

 
$
2,840

General and administrative
4,867

 
1,935

Total stock-based compensation expense
$
9,407

 
$
4,775
Schedule of Restricted Stock and Restricted Stock Units Activity
The following table summarizes RSA activity for the year ended December 31, 2018 (in thousands, except per share amounts):
 
RSAs
 
Weighted-Average Grant Date Fair Value per Share
Unvested as of December 31, 2017
16

 
$
—

Issued
—

 
$
—

Vested
(9
)
 
$
—

Repurchased
—

 
$
—

Unvested as of December 31, 2018
7

 
$
—

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, 2018 (in thousands, except per share amounts):
 
RSUs
 
Weighted-Average Grant Date Fair Value per Share
Unvested as of December 31, 2017
—

 
$
—

Issued
388

 
$
8.02

Vested
—

 
$
—

Cancelled
(17
)
 
$
8.02

Unvested as of December 31, 2018
371

 
$
8.02
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, 2018 and 2017 was calculated on the date of grant using the following weighted-average assumptions:
 
Year Ended December 31,
 
2018
 
2017
Risk-free interest rate
2.7
%
 
2.1
%
Expected dividend yield
—
%
 
—
%
Expected term (in years)
6.0

 
6.1

Expected volatility
65.2
%
 
70.1
%
Schedule of Stock Options, Activity
The following table summarizes changes in stock option activity during the year ended December 31, 2018 (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, 2017
4,868

 
$
6.28

 
7.9
 
$
35,178

Granted
1,581

 
$
21.15

 
 
 
 

Exercised
(683
)
 
$
2.26

 
 
 
 

Cancelled
(743
)
 
$
14.95

 
 
 
 

Outstanding at December 31, 2018
5,023

 
$
10.23

 
7.6
 
$
3,133

Exercisable at December 31, 2018
2,733

 
$
5.91

 
6.8
 
$
3,014