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Stock-based compensation (Tables)
9 Months Ended
Sep. 30, 2021
Stock-based compensation  
Summary of stock option activity and related information

    

Shares

    

Weighted-average exercise price per share

    

Weighted-average remaining contractual term (years)

    

Aggregate intrinsic value (in thousands)

 

Outstanding at December 31, 2020

 

12,690,745

$

3.90

 

6.5

$

3,390

Granted

 

1,652,863

2.78

Exercised

 

(565,891)

1.59

Forfeited/cancelled

 

(830,708)

3.71

Outstanding at September 30, 2021

 

12,947,009

$

3.87

 

6.1

$

10,140

Vested at September 30, 2021

 

8,454,091

$

4.54

 

4.8

$

6,720

Schedule of assumptions used to estimate fair value of each stock option on grant date

September 30,

2021

2020

Risk-free interest rate

 

0.76

%  

0.75

%  

Volatility

 

95

%  

94

%  

Dividend yield

 

Expected term (years)

 

5.9

5.8

Schedule of restricted stock units

    

Shares

    

Weighted-average grant date fair value per share

 

Outstanding at December 31, 2020

 

2,649,317

$

1.73

Granted

 

795,962

$

2.76

Vested

 

(1,426,160)

$

1.40

Forfeited/cancelled

(269,140)

$

2.11

Outstanding at September 30, 2021

 

1,749,979

$

2.41

Schedule of assumptions used to estimate fair value of each employee stock purchase plan on grant date

Nine months ended September 30,

2021

2020

Risk-free interest rate

 

 

0.07

%  

1.04

%  

Volatility

 

 

68

%  

109

%  

Dividend yield

 

 

Expected term (years)

 

 

0.5

0.5