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Stockholders' Equity (Tables)
3 Months Ended
Mar. 31, 2015
Share Based Compensation [Abstract]  
Option Activity under Stock-Based Compensation Plans

Option activity under our stock-based compensation plans during the three months ended March 31, 2015 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, 2014

 

 

1,820

 

 

$

27.48

 

 

 

 

 

 

 

 

 

Options granted

 

 

619

 

 

 

16.30

 

 

 

 

 

 

 

 

 

Options exercised

 

 

(2

)

 

 

10.22

 

 

 

 

 

 

 

 

 

        Options cancelled:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Options forfeited (unvested)

 

 

(54

)

 

 

17.88

 

 

 

 

 

 

 

 

 

Options cancelled (vested)

 

 

(70

)

 

 

48.75

 

 

 

 

 

 

 

 

 

Balance at March 31, 2015

 

 

2,313

 

 

 

24.06

 

 

 

6.94

 

 

$

8,276

 

Vested and expected to vest at March 31, 2015

 

 

2,313

 

 

 

24.06

 

 

 

6.94

 

 

$

8,276

 

Exercisable at March 31, 2015

 

 

1,126

 

 

 

30.20

 

 

 

4.55

 

 

$

2,298

 

 

Summary of Restricted Stock Units Activity

Restricted stock unit activity under our stock-based compensation plans during the three months ended March 31, 2015 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, 2014

 

179

 

 

$

17.13

 

Granted

 

24

 

 

$

16.16

 

Vested

 

(3

)

 

$

16.90

 

Forfeited or expired

 

(5

)

 

$

18.80

 

Non-vested as of March 31, 2015

 

195

 

 

$

16.97

 

 

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

 

 

Employee Stock Purchase Plan

 

 

 

Three Months Ended

 

 

Three Months Ended

 

 

 

March 31,

 

 

March 31,

 

 

 

2015

 

 

2014

 

 

2015

 

 

2014

 

Weighted-average fair value

 

$

10.85

 

 

$

15.70

 

 

$

17.32

 

 

$

9.20

 

Risk-free interest rate

 

 

1.6

%

 

 

1.8

%

 

 

0.4

%

 

 

0.2

%

Expected life (in years)

 

 

5.8

 

 

 

5.9

 

 

 

1.2

 

 

 

1.1

 

Volatility

 

 

0.8

 

 

 

1.4

 

 

 

2.4

 

 

 

0.9

 

 

Stock-Based Compensation Expense

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

 

 

 

Three Months Ended

 

 

 

March 31,

 

 

 

2015

 

 

2014

 

Research and development

 

$

885

 

 

$

692

 

General and administrative

 

 

1,069

 

 

 

578

 

Total

 

$

1,954

 

 

$

1,270