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Share-Based Compensation (Tables)
6 Months Ended
Jun. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Stock Option Activity
A summary of Common Stock option activity is as follows:
 
Number of Options
(in thousands)
 
Weighted Average Exercise Price
Outstanding at December 31, 2017
2,863

 
$
4.85

Granted
1,008

 
$
7.56

Exercised
(216
)
 
$
0.65

Forfeited or canceled
(117
)
 
$
10.18

Outstanding at June 30, 2018
3,538

 
$
5.70

Fair Value Assumptions for Stock Options
The fair value was determined by the Black-Scholes option pricing model using the following weighted-average assumptions:
 
Six Months Ended
June 30,
 
2018
 
2017
Expected term, in years
6.33

 
6.36

Expected volatility
84.5
%
 
84.1
%
Risk-free interest rate
2.6
%
 
2.1
%
Expected dividend yield
%
 
%
Weighted-average exercise price
$
7.56

 
$
11.46

Allocation of Share-based Compensation Expense
Share-based compensation related to all equity awards issued pursuant to the 2008 Plan and 2016 Plan and for estimated shares to be issued under the ESPP for the current purchase period is included in the condensed consolidated statements of operations and comprehensive loss as follows:
 
 
Six Months Ended
June 30,
 
 
2018
 
2017
 
(in thousands)
Research and development
 
$
595

 
$
380

General and administrative
 
1,179

 
634

Total share-based compensation expense
 
$
1,774

 
$
1,014