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Stock-Based Compensation (Tables)
 3 Months Ended
Mar. 31, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]  
Schedule of Allocated Stock-Based Compensation Expense

Stock-based compensation expense was allocated to research and development and general and administrative expense as follows (in thousands):

 

 

 

Three Months Ended March 31,

 

 

 

2016

 

 

2015

 

Research and development

 

$

4,496

 

 

$

4,222

 

General and administrative

 

 

2,844

 

 

 

2,224

 

Total stock-based compensation expense

 

$

7,340

 

 

$

6,446

 

 
Schedule of Assumptions used to Estimate Fair Value of Stock Options Granted and Employee Stock Purchase Plans

The assumptions used to estimate the fair value of stock options granted and ESPPs using the Black-Scholes option valuation model were as follows:

 

 

 

Three Months Ended March 31,

 

 

 

2016

 

 

2015

 

Stock Options

 

 

 

 

 

 

 

 

Expected term (in years)

 

 

5.3

 

 

 

5.2

 

Expected volatility

 

 

69

%

 

 

70

%

Risk-free interest rate

 

 

1.4

 

 

 

1.4

 

Expected dividend yield

 

 

—

 

 

 

—

 

Weighted average estimated fair value

 

$

11.46

 

 

$

17.49

 

 

 

 

 

 

 

 

 

 

ESPPs

 

 

 

 

 

 

 

 

Expected term (in years)

 

0.4 - 2.0

 

 

0.4 - 2.0

 

Expected volatility

 

58.5 - 70.3%

 

 

58.5 - 68.3%

 

Risk-free interest rate

 

0.1 - 0.9%

 

 

0.1 - 0.4%

 

Expected dividend yield

 

 

—

 

 

 

—

 

Weighted average estimated fair value

 

$

12.80

 

 

$

13.27