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Stockholders' Equity
 3 Months Ended
Mar. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Stockholders' Equity
Warrants 

As of March 31, 2018, the following warrants for common stock were issued and outstanding:
Issue date
 
Expiration date
 
Exercise price
 
Number of warrants outstanding
January 11, 2017
 
None
 
$
0.001

 
7,258,263

November 20, 2017
 
None
 
$
0.001

 
4,137,999

 
 
 
 
 
 
11,396,262



During the three months ended March 31, 2018 and 2017, no warrants were exercised.
Stockholders' Equity

Share-based Compensation

Total share-based compensation expense by statement of operations classification is presented below (in thousands):
 
Three Months Ended March 31,
 
2018
 
2017
Research and development expense
$
1,493

 
$
982

General and administrative expense
2,167

 
784

 
$
3,660

 
$
1,766


    
During the three months ended March 31, 2018, 402,948 shares were issued pursuant to stock option exercises, generating net proceeds of $5.9 million. During the three months ended March 31, 2017, no shares were issued pursuant to stock option exercises.