XML 52 R26.htm IDEA: XBRL DOCUMENT v3.24.0.1
Stockholders' Equity (Deficit) (Tables)
 12 Months Ended
Dec. 31, 2023
Stockholders' Equity Note [Abstract]  
Schedule of Shares Reserved for Future Issuance
The Company has reserved shares of common stock for future issuances as follows:
December 31,
 20232022
For exercise of outstanding common stock options18,336,656 15,076,365 
For delivery upon vesting of outstanding restricted stock units895,361 920,533 
For future equity awards under the 2013 Equity Incentive Plan2,257,711 1,466,603 
For future purchases under the 2013 Employee Stock Purchase Plan2,179,399 2,186,097 
     Total shares of common stock reserved for future issuances23,669,127 19,649,598 
Schedule of Fair Value Assumptions, Stock Options
The following table illustrates the assumptions for the Black-Scholes model used in determining the fair value of the stock options granted:
 Years Ended December 31,
 202320222021
Expected volatility83.20 %74.27 %95.84 %
Expected term (in years)5.76.06.0
Weighted-average risk-free interest rate3.87 %1.91 %0.71 %
Expected dividend yield— %— %— %
Weighted-average fair value per option$1.13 $3.33 $6.67 
Schedule of Activity Related to Stock Options
A summary of activity related to the Company’s stock options is as follows:
 Number of Options
Outstanding		Weighted-Average
Exercise Price		Weighted-Average
Remaining Contractual
Life (in Years)		Total Intrinsic Value
Balance, December 31, 202111,649,592 $6.27 7.65
     Granted4,217,275 5.12 — 
     Exercised(271,079)2.24 — 
     Forfeited and expired(519,423)6.93 — 
Balance, December 31, 202215,076,365 $6.00 7.32
     Granted3,861,060 1.59 — 
     Exercised— — — 
     Forfeited and expired (600,769)5.56 — 
Balance, December 31, 202318,336,656 $5.09 6.57$— 
Exercisable at December 31, 202312,395,052 $5.68 5.61$— 
Vested or expected to vest at December 31, 202317,530,966 $5.18 6.46$— 
Schedule of Other Information Regarding Stock Options
Other information regarding the Company’s stock options is as follows (in thousands, except per share data):
Years Ended December 31,
202320222021
Weighted-average grant-date fair value per share of options granted$1.13 $3.33 $6.67 
Total intrinsic value of options exercised$— $114 $3,496 
Total fair value of shares vested$12,834 $12,721 $8,642 
Schedule of Share-based Compensation, Shares Authorized
The following table summarizes, at December 31, 2023, by price range: (1) for stock option awards outstanding under the 2013 Plan, the number of stock option awards outstanding, their weighted-average remaining life and their weighted-average exercise price; and (2) for stock option awards exercisable under the 2013 Plan, the number of stock option awards exercisable and their weighted-average exercise price:
OutstandingExercisable
Exercise Price Range ($)NumberWeighted-Average Remaining Contractual Life (in years)Weighted-Average Exercise PriceNumberWeighted-Average Exercise Price
0.00 to 1.36
1,293,800 9.84$1.03 1,857 $1.11 
1.37 to 2.08
4,901,027 7.41$1.94 2,987,651 $1.95 
2.09 to 3.13
3,198,545 5.392.37 3,039,410 2.35 
3.14 to 5.60
1,473,488 4.364.63 1,399,404 4.63 
5.61 to 7.86
3,664,378 7.595.81 1,996,046 5.95 
7.87 to 53.74
3,805,418 5.2412.29 2,970,684 13.16 
0.00 to 53.74
18,336,656 6.57$5.09 12,395,052 $5.68 
Schedule of Fair Value Assumptions, Employee Stock Purchase Plan
The following table illustrates the assumptions for the Black-Scholes model used in determining the fair value of the ESPP purchase rights:
 Years Ended December 31,
 202320222021
Expected volatility94.64 %104.88 %97.54 %
Expected term (in years)1.501.280.71
Weighted-average risk-free interest rate4.63 %2.63 %0.25 %
Expected dividend yield— %— %— %
Weighted-average option value per share$0.75 $1.97 $6.55 
Schedule of Activity Related to Restricted Stock Units
A summary of activity related to the Company’s RSUs is as follows:
 Number of Restricted
Stock Units Outstanding		Weighted-Average Grant-Date Fair Value
Balance, December 31, 2022920,533 $4.82 
Granted561,404 1.94 
Share issuance(445,940)3.83 
Forfeited(140,636)4.84 
Balance, December 31, 2023895,361 $3.49 
Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs
For awards with only service conditions and graded-vesting features, the Company recognizes compensation expense on a straight-line basis over the requisite service period. Total stock-based compensation expense was as follows (in thousands): 
 Years Ended December 31,
 202320222021
Income Statement Classification:
Research and development expense$7,092 $8,267 $6,611 
General and administrative expense10,365 7,018 5,649 
          Total stock-based compensation expense$17,457 $15,285 $12,260