Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2020

Summary of common shares that were excluded from the diluted net loss per share calculation because of their anti-dilutive effect

The following table summarizes potential shares of common stock that were excluded from diluted net loss per share calculation because of their anti-dilutive effect:


	Year Ended December 31,
	2020	2019	2018
Options to purchase common stock	8,906,624	9,265,390	8,752,677

Service-based restricted stock units	2,558,052	2,069,936	1,688,017
Performance-based restricted stock units	663,353	—	—
Convertible preferred stock	3,309	8,456	8,456
Convertible notes	3,049,980	14,585,653	—
August 2019 Bonds	—	3,799,071	—
December 2019 Bonds	1,009,450	1,009,450	—
Total	16,190,768	30,737,956	10,449,150

Summary of assumptions used to estimate the fair value of stock options

The weighted average assumptions used in the Black-Scholes model for option grants to employees and directors are presented below:


	Year Ended December 31,
	2020	2019	2018
Risk-free interest rate	0.63%	2.42%	2.73%
Expected volatility	78%	70%	72%
Expected life in years	6	6	6
Dividend yield	—	—	—

The weighted average assumptions used in the Black-Scholes model for option grants to non-employees are presented below:


	Year Ended December 31,
	2020	2019	2018
Risk-free interest rate	0.82%	2.45%	2.71%
Expected volatility	76%	88%	78%
Expected life in years	10	10	9
Dividend yield	—	—	—