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Stockholders' Equity (Tables)
3 Months Ended
Mar. 31, 2019
Equity [Abstract]  
Schedule of Stock-based Compensation Expense Stock-based compensation expense was allocated as follows:
(in thousands)
Three Months Ended March 31,
2019
 
2018
Research and development
$
2,079

 
$
1,923

General and administrative
2,080

 
2,235

Total stock-based compensation expense
$
4,159

 
$
4,158

Summary of Common Stock Equivalents Excluded from Computation of Diluted Net Income (Loss) Per Share Common stock equivalents that were excluded from the computation of diluted net loss per share are presented as below:
 
As of March 31,
 
2019
 
2018
Outstanding common stock options
4,372,676

 
3,691,797

Outstanding common stock warrants
34,113

 
34,113

Unvested restricted stock awards
768,770

 
686,523

Shares of common stock expected to be purchased on June 30 under the 2014 ESPP
35,619

 
16,895