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Stock-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2019
Stock-Based Compensation  
Schedule of Stock Option Activity
The following table summarizes changes in stock option activity during the year ended December 31, 2019:
 
Number
of
Shares
 
Weighted
Average
Exercise Price
 
Weighted
Average
Contractual
Term
 
Aggregate
Intrinsic
Value
 
 
 
 
(in years)
 
(in thousands)
Outstanding as of December 31, 2018
2,548,073

 
$
7.11

 
8.68
 
$
1,965

Granted
1,334,028

 
5.69

 
 
 
 
Exercised
(44,132
)
 
2.62

 
 
 
 
Forfeited
(463,428
)
 
7.63

 
 
 
 
Outstanding as of December 31, 2019
3,374,541

 
$
6.53

 
7.59
 
$
270

Options vested or expected to vest as of December 31, 2019
3,374,541

 
$
6.53

 
7.57
 
$
270

Options exercisable as of December 31, 2019
1,540,865

 
$
6.52

 
6.84
 
$
—
Schedule of Assumptions Used to Determine Fair Value of Stock Options Granted
The assumptions that the Company used to determine the fair value of the stock options granted to employees and directors were as follows, presented on a weighted average basis:
 
Year Ended December 31,
 
2019
 
2018
Expected volatility
93.72
%
 
101.10
%
Risk-free interest rate
2.31
%
 
2.69
%
Expected dividend yield
0.00
%
 
0.00
%
Expected life (in years)
6.01

 
6.00
Schedule of Stock-based Compensation Expense
The Company recorded stock-based compensation expense related to all stock-based awards and the ESPP in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands):
 
Year Ended December 31,
 
2019
 
2018
Research and development expenses
$
3,589

 
$
3,840

General and administrative expenses
3,751

 
2,389

 
$
7,340

 
$
6,229
2018 Plan | Unvested restricted stock units  
Stock-Based Compensation  
Schedule of Nonvested Restricted Stock Units Activity
The following table summarizes RSU activity for the year ended December 31, 2019:
 
Number of
Shares
 
Weighted
Average Grant-
Date Fair Value
per Share
Unvested as of December 31, 2018
—

 
$
—

Granted
1,172,339

 
$
3.47

Vested
—

 
$
—

Cancelled
(118,945
)
 
$
5.15

Unvested as of December 31, 2019
1,053,394

 
$
3.27
2018 Plan | Restricted Common Stock  
Stock-Based Compensation  
Schedule of Restricted Common Stock Activity
The following table summarizes the Company’s restricted common stock activity since December 31, 2018:
 
Number of Shares
 
Weighted Average Grant-Date Fair Value per Share
Unvested restricted common stock as of December 31, 2018
383,964

 
$
1.97

Granted
25,000

 
$
4.64

Canceled
(44,210
)
 
$
1.25

Vested
(258,716
)
 
$
2.02

Unvested restricted common stock as of December 31, 2019
106,038

 
$
2.78
Outside of 2015 Plan | Founder and Collaborator Awards  
Stock-Based Compensation  
Schedule of Restricted Common Stock Activity
A summary of the changes in the Company's unvested restricted common stock awards granted to founders and collaborators outside of the Company's 2015 Plan or 2018 Plan since December 31, 2018 is as follows:
 
Number of
Shares
 
Weighted
Average Grant-
Date Fair Value
Unvested restricted common stock as of December 31, 2018
350,625

 
$
1.29

Vested
(170,625
)
 
$
1.29

Unvested restricted common stock as of December 31, 2019
180,000

 
$
1.29