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Equity (Tables)
 3 Months Ended
Mar. 31, 2018
Equity [Abstract]  
Summary of Stock Option Activity
A summary of the stock option activity under the Company’s plans for the three months ended March 31, 2018 is as follows:
 
 
 
Number of
shares
 
Weighted average
exercise price
Options outstanding at December 31, 2017
 
6,342,526

 
$
6.51

Options granted
 
1,720,500

 
$
8.06

Less:
 
 
 
 
Options exercised
 
72,500

 
$
4.14

Options canceled or expired
 
12,500

 
$
7.52

Options outstanding at March 31, 2018
 
7,978,026

 
$
6.86

Exercisable at March 31, 2018
 
2,250,840

 
$
5.63
Fair Value of Each Stock Option Award Granted
The fair value of each stock option award granted during the three months ended March 31, 2018 was estimated as of the grant date using a trinomial lattice model with the following weighted average assumptions:
 
Three Months Ended
March 31, 2018
Expected term (in years)
2.0 - 4.0
Risk-free interest rate (%)
2.4%
Expected volatility (%)
35.6% - 45.5%
Dividend yield (%)
—
Weighted average fair value/share at grant date
$2.78
Summary of Stock-Based Compensation Expense Recognized for Stock Options and Restricted Stock Included in Consolidated Statements of Operations
Stock based compensation expense recognized for stock options and restricted stock and included in the consolidated statements of operations was allocated as follows (in thousands): 
 
 
Three Months Ended March 31,
 
 
2018
 
2017
Research and development expense
 
$
18

 
$
43

General and administrative expense
 
1,606

 
1,087

Total stock based compensation expense
 
$
1,624

 
$
1,130