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Stock Incentive Plans (Tables)
12 Months Ended
Dec. 31, 2016
Stock incentive plans  
Summary of activity under the Company's stock option plans
The following table summarizes activity under the Company's stock incentive plans (aggregate intrinsic value in thousands):
 
Shares
Available
for Grant
 
Stock Options
Outstanding
 
Weighted
Average
Exercise Price
 
Weighted Average
Remaining
Contractual Life
(Years)
 
Aggregate
Intrinsic
Value
Balance—December 31, 2015
1,058,359

 
4,179,521

 
$
8.03

 
7.50
 
$
6,511

Additional shares authorized
1,114,416

 

 
 

 
 
 
 

Granted - stock options
(1,618,250
)
 
1,618,250

 
6.30

 
 
 
 

Granted - restricted stock units
(25,000
)
 
25,000

 
 
 
 
 
 
Canceled
358,199

 
(358,199
)
 
10.29

 
 
 
 

Exercised

 
(212,740
)
 
2.53

 
 
 
 

Balance—December 31, 2016
887,724

 
5,251,832

 
$
7.56

 
7.24
 
$
8,515

Options vested and exercisable—December 31, 2016
 

 
2,628,328

 
$
7.42

 
5.89
 
$
6,008

Options vested and expected to vest—December 31, 2016
 

 
4,962,370

 
$
7.58

 
7.16
 
$
8,254

Stock-based Compensation  
Stock incentive plans  
Summary of share-based compensation expense
The following table summarizes stock-based compensation expense related to stock options, restricted stock units and the ESPP for the years ended December 31, 2016, 2015 and 2014, and are included in the statements of operations and comprehensive loss as follows (in thousands of dollars):

 
Year Ended December 31,
 
2016
 
2015
 
2014
Cost of revenue
$
126

 
$
100

 
$
51

Research and development
1,322

 
1,178

 
790

Selling and marketing
1,594

 
1,326

 
707

General and administrative
3,336

 
2,998

 
2,000

Total stock-based compensation expense
$
6,378

 
$
5,602

 
$
3,548

Stock-based Compensation, employees  
Stock incentive plans  
Schedule of assumptions used to calculate estimated grant date fair value of stock options using the Black-Scholes option-pricing valuation model
The estimated grant-date fair value of employee stock options was calculated using the Black-Scholes option-pricing model, based on the following assumptions:

 
Year Ended December 31,
 
2016
 
2015
 
2014
Weighted-average volatility
52.49 - 56.36%
 
52.56 - 68.82%
 
70.19 - 78.54%
Weighted-average expected term (years)
5.50 - 6.27
 
5.50 - 6.08
 
5.50 - 6.08
Risk-free interest rate
1.16 - 2.09%
 
1.55 - 2.03%
 
1.66 - 2.04%
Expected dividend yield
 
 
Stock-based Compensation, non-employees  
Stock incentive plans  
Schedule of assumptions used to calculate estimated grant date fair value of stock options using the Black-Scholes option-pricing valuation model
The estimated fair value of non-employee stock options was calculated using the Black-Scholes option-pricing model, based on the following assumptions:

 
Year Ended December 31,
 
2016
 
2015
 
2014
Weighted-average volatility
52.77 - 65.85%
 
64.72 - 74.48%
 
73.20 - 74.48%
Weighted-average expected term (years)
7.80 - 8.56
 
7.92 - 10.00
 
8.75 - 10.00
Risk-free interest rate
1.39 - 2.30%
 
1.78 - 2.29%
 
2.09 - 2.20%
Expected dividend yield
 
 
ESPP  
Stock incentive plans  
Schedule of assumptions used to calculate estimated grant date fair value of stock options using the Black-Scholes option-pricing valuation model

The estimated grant date fair value of the ESPP shares was calculated using the Black-Scholes option-pricing model, based on the following assumptions:

 
Year Ended December 31,
 
2016
 
2015
Weighted-average volatility
46.38 - 75.72%
 
53.57 - 58.10%
Weighted-average expected term (years)
0.50 - 1.00
 
0.49 - 0.99
Risk-free interest rate
0.40 - 0.50%
 
0.17 - 0.33%
Expected dividend yield