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Stock-Based Compensation and Employee Stock Purchase Program (Tables)
12 Months Ended
Dec. 31, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]  
Summary of Activity Under Stock Incentive Plans

The following is a summary of stock option information and weighted average exercise prices under the Company’s stock incentive plans (in thousands, except share data and price per share):

 

 

Shares

Subject to

Outstanding

Options

 

 

Weighted-

Average

Exercise

Price Per

Share

 

 

Aggregate

Intrinsic

Value

 

Outstanding — Balance at December 31, 2018

 

 

530,044

 

 

$

35.53

 

 

$

 

Options granted

 

 

208,571

 

 

 

5.13

 

 

 

 

 

Options exercised

 

 

(546

)

 

 

4.63

 

 

 

 

 

Options forfeited

 

 

(30,132

)

 

 

33.95

 

 

 

 

 

Options expired

 

 

(46,236

)

 

 

98.69

 

 

 

 

 

Outstanding — Balance at December 31, 2019

 

 

661,701

 

 

 

21.60

 

 

$

305

 

Options granted

 

 

502,494

 

 

 

4.28

 

 

 

 

 

Options exercised

 

 

 

 

 

 

 

 

 

 

Options forfeited

 

 

(174,508

)

 

 

5.91

 

 

 

 

 

Options expired

 

 

(99,928

)

 

 

25.56

 

 

 

 

 

Outstanding — Balance at December 31, 2020

 

 

889,759

 

 

 

14.46

 

 

$

240

 

Vested and expected to vest — December 31, 2020

 

 

841,991

 

 

 

15.03

 

 

$

240

 

Exercisable —December 31, 2020

 

 

502,973

 

 

$

22.20

 

 

$

 

Weighted-Average Fair Value Assumption of Stock Option Awards

The fair value of stock option awards was estimated at the date of grant using a Black-Scholes option-pricing model with the following weighted-average assumption:

 

 

Year Ended December 31,

 

Assumptions

 

2020

 

 

2019

 

Expected term (years)

 

 

6.48

 

 

 

7.04

 

Expected volatility

 

134%

 

 

99%

 

Risk-free interest rate

 

1.01%

 

 

2.01%

 

Expected dividend yield