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Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2021

Accounting Policies [Abstract]

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,
			2021						2020						2019
Options to purchase common stock			10,488,993						8,906,624						9,265,390
Service-based restricted stock units			2,448,868						2,558,052						2,069,936
Performance-based restricted stock units			663,353						663,353						—
Convertible preferred stock			3,309						3,309						8,456
Convertible notes			3,049,980						3,049,980						14,585,653
August 2019 Bonds			—						—						3,799,071
December 2019 Bonds			—						1,009,450						1,009,450
Total			16,654,503						16,190,768						30,737,956

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,
			2021						2020						2019
Risk-free interest rate			0.91%						0.63%						2.42%
Expected volatility			93%						78%						70%
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,
			2021						2020						2019
Risk-free interest rate			1.45%						0.82%						2.45%
Expected volatility			87%						76%						88%
Expected life in years			10						10						10
Dividend yield			—						—						—

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- Definition

Tabular disclosure of securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) in the future that were not included in the computation of diluted EPS because to do so would increase EPS amounts or decrease loss per share amounts for the period presented, by antidilutive securities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=124432515&loc=d3e3550-109257

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Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)(2)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

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