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Stock Option Plans (Tables)
12 Months Ended
Dec. 31, 2015
Summary of Stock Option Activity for Employee and Nonemployee Awards under Plan

A summary of stock option activity for employee and nonemployee awards under the 2007 Plan and the 2014 Plan during the year ended December 31, 2015 is presented below (Aggregate Intrinsic Value in thousands):

 

 

 

Number of

Shares

 

 

Weighted-

Average

Exercise

Price

 

 

Weighted-

Average

Remaining

Contractual

Life (Years)

 

 

Aggregate

Intrinsic

Value

 

Outstanding — January 1, 2015

 

 

2,126,176

 

 

$

4.97

 

 

 

6.7

 

 

$

2,701

 

Granted

 

 

2,360,682

 

 

 

5.66

 

 

 

 

 

 

 

 

 

Exercised

 

 

(370,230

)

 

 

4.40

 

 

 

 

 

 

 

 

 

Forfeited

 

 

(661,702

)

 

 

5.86

 

 

 

 

 

 

 

 

 

Outstanding — December 31, 2015

 

 

3,454,926

 

 

$

5.39

 

 

 

8.9

 

 

$

 

Options expected to vest — December 31, 2015

 

 

2,336,878

 

 

$

5.32

 

 

 

9.4

 

 

$

 

Options exercisable — December 31, 2015

 

 

1,001,415

 

 

$

5.56

 

 

 

7.7

 

 

$

 

 

Schedule of Stock-based Compensation Expense

Stock-based compensation expense recorded as research and development and general and administrative expenses is as follows (in thousands):

 

 

 

As of December 31,

 

 

 

2015

 

 

2014

 

 

2013

 

Research and development

 

$

795

 

 

$

317

 

 

$

284

 

General and administrative

 

 

1,580

 

 

 

568

 

 

 

338

 

Total

 

$

2,375

 

 

$

885

 

 

$

622

 

 

Summary of Assumptions Used in Black-Scholes Option-Pricing Model for Stock Options Granted to Employees and Non Employees

The assumptions used in the Black-Scholes option-pricing model for stock options granted to employees during the years ended December 31, 2015, 2014, and 2013 are as follows:

 

 

 

December 31,

 

 

2015

 

2014

 

2013

Expected life

 

6 years

 

6 years

 

6 years

Risk-free interest rate

 

1.45%-2.02%

 

1.71%-2.00%

 

1.09%-1.92%

Expected volatility

 

51%-63%

 

54%-60%

 

59%-79%

Expected dividend rate

 

—%

 

—%

 

—%

 

Non Employee Awards [Member]  
Summary of Assumptions Used in Black-Scholes Option-Pricing Model for Stock Options Granted to Employees and Non Employees

The Black-Scholes assumptions used to estimate the fair value of these awards for the years ended December 31, 2015, 2014, and 2013 were as follows:

 

 

 

December 31,

 

 

2015

 

2014

 

2013

Expected life

 

10 years

 

8 years

 

7 years

Risk-free interest rate

 

2.10%-2.25%

 

1.86%-2.53%

 

1.11%-2.86%

Expected volatility

 

60%-61%

 

56%-62%

 

56%-79%

Expected dividend rate

 

—%

 

—%

 

—%