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Stockholders' Equity (Tables)
 12 Months Ended
Dec. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]  
Restricted Stock Activity

11.

Stockholders’ Equity (continued)

The activity for restricted stock is summarized as follows:

 

 

 

Shares

 

 

Weighted-

Average

Grant-Date

Fair Value

 

Balance at December 31, 2018

 

467,632

 

 

$

8.47

 

Granted

 

 

229,821

 

 

 

11.54

 

Vested

 

 

(290,963

)

 

 

7.99

 

Canceled

 

 

(6,667

)

 

 

—

 

Unvested at December 31, 2019

 

399,823

 

 

$

10.54

 
Summary of Stock Option Activity under its Stock Plans

The following table summarizes stock option activity and related information under the Company’s Stock Plans:

 

 

 

Available

for Grant

 

 

Shares

 

 

Weighted

Average

Exercise

Price

 

 

Weighted

Average

Remaining

Contractual

Term (in years)

 

 

Aggregate

Intrinsic Value

 

Balance at December 31, 2018

 

 

1,708,516

 

 

 

10,720,200

 

 

$

8.39

 

 

 

 

 

 

 

 

 

Shares authorized

 

 

2,000,000

 

 

 

—

 

 

 

 

 

 

 

 

 

 

 

 

 

Granted (1)

 

 

(2,531,063

)

 

 

2,301,242

 

 

 

11.69

 

 

 

 

 

 

 

 

 

Exercised (2)

 

 

125,526

 

 

 

(2,850,642

)

 

 

7.74

 

 

 

 

 

 

 

 

 

Forfeited and expired (3)

 

 

889,566

 

 

 

(882,899

)

 

 

7.97

 

 

 

 

 

 

 

 

 

Outstanding at December 31, 2019

 

 

2,192,545

 

 

 

9,287,901

 

 

$

9.44

 

 

 

6.54

 

 

$

279,636,183

 

Vested and expected to vest, net of estimated

   forfeiture at December 31, 2019

 

 

 

 

 

 

8,966,722

 

 

$

9.38

 

 

 

6.45

 

 

$

270,570,850

 

Exercisable at December 31, 2019

 

 

 

 

 

 

5,750,443

 

 

$

8.62

 

 

 

5.14

 

 

$

177,869,577

 

 

(1)

The difference between shares granted in the number of shares available for grant and outstanding options represents the RSUs and RSAs granted for the period.  

(2)

Shares presented as available for grant represents shares repurchased for tax withholding upon vesting of RSUs.  

(3)

The difference between shares forfeited and expired in the number of shares available for grant and outstanding options represents the RSUs canceled during the period.
Stock Options Outstanding

As of December 31, 2019, stock options outstanding were as follows:

 

 

 

Options Outstanding

 

Exercise Price Range

 

Shares

 

 

Weighted-

Average

Contractual Life

 

$2.10 - $3.57

 

 

940,493

 

 

 

6.14

 

$4.50 - $6.60

 

 

574,174

 

 

 

5.36

 

$6.62

 

 

991,898

 

 

 

7.07

 

$6.66 - $7.98

 

 

890,168

 

 

 

5.42

 

$8.19

 

 

961,252

 

 

 

5.15

 

$8.22 - $10.82

 

 

590,192

 

 

 

6.44

 

$10.86

 

 

980,448

 

 

 

8.13

 

$10.91 - $10.94

 

 

78,800

 

 

 

6.80

 

$11.02

 

 

1,109,583

 

 

 

9.17

 

$11.56 - $29.76

 

 

2,170,893

 

 

 

5.80

 

 

 

 

9,287,901

 

 

 

6.54

 
Employee Stock-based Compensation Expense Recognized

11.

Stockholders’ Equity (continued)

Total employee stock-based compensation expense recognized associated with restricted stock, stock options, and the ESPP, was as follows (in thousands):

 

 

 

Year Ended December 31,

 

 

 

2019

 

 

2018

 

 

2017

 

Research and development

 

$

4,530

 

 

$

3,632

 

 

$

3,154

 

General and administrative

 

 

6,819

 

 

 

6,339

 

 

 

4,965

 

Total

 

$

11,349

 

 

$

9,971

 

 

$

8,119

 
Assumptions for Fair Values of Employee Stock Options Granted under Company's Stock Plans

The fair values of the employee stock options granted under the Company’s Stock Plans and the option component of the shares purchased under the ESPP during 2019, 2018 and 2017 were estimated at the date of grant using the Black-Scholes option-pricing model with the following assumptions:  

 

 

 

Employee Stock Options

 

 

Employee Stock Purchase Plan

 

 

 

2019

 

 

2018

 

 

2017

 

 

2019

 

 

2018

 

 

2017

 

Dividend yield

 

 

0

%

 

 

0

%

 

 

0

%

 

 

0

%

 

 

0

%

 

 

0

%

Volatility

 

 

71.3

%

 

 

67.8

%

 

 

68.3

%

 

 

56.4

%

 

 

73.8

%

 

 

52.9

%

Weighted-average expected life (in

   years)

 

 

6.0

 

 

 

6.0

 

 

 

6.0

 

 

 

0.5

 

 

 

0.5

 

 

 

0.5

 

Risk-free interest rate

 

 

2.28

%

 

 

2.66

%

 

 

2.04

%

 

 

1.87

%

 

 

2.33

%

 

 

1.22

%

Weighted-average grant date fair

   value

 

$

7.54

 

 

$

6.22

 

 

$

4.30

 

 

$

4.10

 

 

$

3.73

 

 

$

2.16

 
Stock-based Compensation Expense in Connection with Grants of Stock Options to Nonemployees

In connection with grants of stock options to nonemployees, the Company recorded stock-based compensation expense as follows (in thousands):

 

 

 

Year Ended December 31,

 

 

 

2019

 

 

2018

 

 

2017

 

Research and development

 

$

186

 

 

$

862

 

 

$

629

 

General and administrative

 

 

103

 

 

 

—

 

 

 

—

 

Total

 

$

289

 

 

$

862

 

 

$

629

 
Assumptions for Fair Values of Stock Options Granted are Calculated Related to Stock Options Granted to Nonemployees

The estimated fair values of the stock options granted are calculated at each reporting date using the Black-Scholes option-pricing model, with the following assumptions:

 

 

 

Year Ended December 31,

 

 

 

2019

 

 

2018

 

 

2017

 

Dividend yield

 

0%

 

 

0%

 

 

0%

 

Volatility

 

68-87%

 

 

67-68%

 

 

69-70%

 

Weighted-average expected life (in years)

 

5.5-6.0

 

 

5.7-9.9

 

 

5.5-10.0

 

Risk-free interest rate

 

1.6-2.2%

 

 

2.7-3.0%

 

 

1.9-2.5%