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Equity Plans and Stock-Based Compensation (Tables)
9 Months Ended
Sep. 30, 2016
Share Based Compensation [Abstract]  
Option Activity under Stock-Based Compensation Plans

Option activity under our stock-based compensation plans during the nine months ended September 30, 2016 was as follows (in thousands except per share amounts):

 

 

 

Shares Underlying  Outstanding Options

(in thousands)

 

 

Weighted-Average Exercise

Price Per Share

 

 

Weighted-Average Remaining Contractual Term

(years)

 

 

Aggregate Intrinsic Value

(in thousands)

 

Balance at December 31, 2015

 

 

2,891

 

 

$

23.34

 

 

 

 

 

 

 

 

 

Options granted

 

 

1,351

 

 

 

19.95

 

 

 

 

 

 

 

 

 

Options exercised

 

 

(16

)

 

 

13.81

 

 

 

 

 

 

 

 

 

Options cancelled:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Options forfeited (unvested)

 

 

(36

)

 

 

19.22

 

 

 

 

 

 

 

 

 

Options cancelled (vested)

 

 

(54

)

 

 

42.90

 

 

 

 

 

 

 

 

 

Balance at September 30, 2016

 

 

4,136

 

 

 

22.06

 

 

 

6.76

 

 

$

182

 

Vested and expected to vest at September 30, 2016

 

 

4,098

 

 

 

22.10

 

 

 

6.74

 

 

$

182

 

Exercisable at September 30, 2016

 

 

1,755

 

 

 

25.70

 

 

 

5.30

 

 

$

182

 

 

Summary of Restricted Stock Units Activity

Restricted stock unit activity under our stock-based compensation plans during the nine months ended September 30, 2016 was as follows (in thousands except per share amounts):

 

 

Number of Shares (In thousands)

 

 

Weighted-Average Grant-Date Fair Value

 

Non-vested as of December 31, 2015

 

195

 

 

$

17.52

 

Granted

 

387

 

 

$

21.35

 

Vested

 

(26

)

 

$

15.59

 

Forfeited or expired

 

(13

)

 

$

20.33

 

Non-vested as of September 30, 2016

 

543

 

 

$

20.28

 

 

Fair Value-Based Measurements and Weighted-Average Assumptions

The fair value-based measurements and weighted-average assumptions used in the calculations of these measurements are as follows:

 

Stock Options

 

 

Stock Options

 

 

Employee Stock Purchase Plan

 

 

Three Months Ended

 

 

Nine Months Ended

 

 

Nine Months Ended

 

 

September 30,

 

 

September 30,

 

 

September 30,

 

 

2016

 

 

2015

 

 

2016

 

 

2015

 

 

2016

 

 

2015

 

Weighted-average fair value

$

8.90

 

 

$

17.63

 

 

$

9.67

 

 

$

13.23

 

 

$

7.86

 

 

$

9.18

 

Risk-free interest rate

 

1.1

%

 

 

1.7

%

 

 

1.4

%

 

 

1.7

%

 

 

0.6

%

 

 

0.4

%

Expected life (in years)

 

4.5

 

 

 

5.8

 

 

 

4.9

 

 

 

5.9

 

 

 

1.2

 

 

 

1.2

 

Volatility

 

0.7

 

 

 

0.7

 

 

 

0.7

 

 

 

0.7

 

 

 

0.6

 

 

 

0.6

 

 

Stock-Based Compensation Expense

The components of stock-based compensation expense were (in thousands):

 

 

Three Months Ended

 

 

Nine Months Ended

 

 

September 30,

 

 

September 30,

 

 

2016

 

 

2015

 

 

2016

 

 

2015

 

Research and development

$

1,639

 

 

$

1,232

 

 

$

4,490

 

 

$

2,985

 

General and administrative

 

2,022

 

 

 

1,398

 

 

 

5,540

 

 

 

3,517

 

Total

$

3,661

 

 

$

2,630

 

 

$

10,030

 

 

$

6,502