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SHARE-BASED COMPENSATION (Tables)
 12 Months Ended
Dec. 31, 2019
Share-based Payment Arrangement [Abstract]  
Stock Option Activity
A summary of Common Stock option activity is as follows:
 
Number of Options
(in thousands)
 
Weighted Average Exercise Price
 
Weighted Average Remaining Contractual Term
(years)
 
Aggregate Intrinsic Value
(in thousands)
Outstanding at December 31, 2017
2,863

 
$
4.85

 
 
 
 
Granted
1,108

 
$
7.37

 
 
 
 
Exercised
(280
)
 
$
0.65

 
 
 
 
Forfeited or expired
(164
)
 
$
9.52

 
 
 
 
Outstanding at December 31, 2018
3,527

 
$
5.76

 
 
 
 
Granted
1,078

 
$
2.89

 
 
 
 
Exercised
(143
)
 
$
0.60

 
 
 
 
Forfeited or expired
(965
)
 
$
5.39

 
 
 
 
Outstanding at December 31, 2019
3,497

 
$
5.19

 
5.91
 
$
—

Vested or expected to vest at December 31, 2019
3,497

 
$
5.19

 
5.91
 
$
—

Exercisable as of December 31, 2019
2,388

 
$
5.06

 
4.68
 
$
—

Vested as of December 31, 2019
2,388

 
$
5.06

 
4.68
 
$
—
Fair Value Assumptions for Stock Options
The fair value was determined by the Black-Scholes option pricing model using the following weighted-average assumptions:
 
Year Ended
December 31,
 
2019
 
2018
Expected term, in years
6.35

 
6.35

Expected volatility
98.8
%
 
85.2
%
Risk-free interest rate
2.5
%
 
2.6
%
Expected dividend yield
—
%
 
—
%
Weighted-average exercise price
$
2.97

 
$
7.37
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 purchase periods active during each respective period is included in the consolidated statements of operations and comprehensive loss as follows:
 
Year Ended
December 31,
 
2019
 
2018
 
(in thousands)
Research and development
$
1,677

 
$
1,286

General and administrative
2,293

 
2,393

Total share-based compensation expense
$
3,970

 
$
3,679