Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2019

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

A summary of the Company’s stock option activity under the 2012 Plan and the 2018 Plan and related information is as follows:



			Options Outstanding
	Shares Available for Grant		Shares Subject to Options Outstanding		Weighted-Average Exercise Price		Weighted-Average Remaining Contractual Life (Years)	Aggregate Intrinsic Value

								(in thousands)
Balance as of December 31, 2017	1,698,790		7,391,052		$	3.63	8.6	$	3,325
Shares authorized	3,658,602		—
Shares forfeited	(508,847	)	—
Granted	(2,088,639	)	2,088,639		7.19
Exercised	—		(1,007,387	)	3.09
Canceled	795,371		(883,899	)	4.57
Repurchase of early exercised shares	1,230		—
Balance as of December 31, 2018	3,556,507		7,588,405		$	4.58	8.3	250,495
Shares authorized	—		—
Shares forfeited	—		—
Granted	(324,579	)	324,579		88.18
Exercised	—		(2,999,419	)	3.87
Canceled	12,636		(418,676	)	6.64
Repurchase of early exercised shares	—		—
Balance as of December 31, 2019	3,244,564		4,494,889		$	10.90	7.7	$	306,392
Vested and Exercisable as of December 31, 2019			1,908,216		$	5.27	7.2	$	139,337

Schedule of Restricted Stock Unit Activity

A summary of the Company’s restricted stock unit activity under the 2012 Plan and the 2018 Plan and related information is as follows:




	Restricted Stock Units Outstanding		Weighted-Average Grant Date Fair Value
Balance as of December 31, 2018	—		$	—
Granted	567,425		78.61
Vested	(22,208	)	47.78
Canceled	(49,086	)	57.51
Balance as of December 31, 2019	496,131		$	82.08

Schedule of Stock Based Compensation Expense

The following table presents the effect of employee and non‑employee related stock‑based compensation expense:



	Year Ended December 31,
	2019		2018		2017

	(in thousands)
Cost of precision oncology testing	$	863	$	512	$	162
Research and development expense	5,907		1,684		507
Sales and marketing expense	4,716		1,727		80
General and administrative expense	5,468		2,928		2,921
Total stock-based compensation expense	$	16,954	$	6,851	$	3,670

Schedule of Stock Options Valuation Assumptions

The grant date fair value of employee and nonemployee stock options was estimated using a Black-Scholes option-pricing model with the following weighted-average assumptions:



	Year Ended December 31,
	2019	2018	2017

Expected term (in years)	5.50 – 6.22	5.01 – 6.51	6.02 – 6.08
Expected volatility	63.2% – 68.7%	68.7% – 78.8%	74.1% – 75.1%
Risk-free interest rate	1.6% – 2.7%	2.5% – 3.0%	1.9% – 2.2%
Expected dividend yield	—%	—%	—%

Schedule of Employee Stock Purchase Plan Valuation Assumptions

The following assumptions were used to calculate the stock-based compensation for each stock purchase right granted under the ESPP:



	Year Ended December 31,
	2019	2018

Expected term (in years)	0.29 – 0.5	0.33
Expected volatility	58.8% – 60.3%	43.6%
Risk-free interest rate	1.6% – 2.5%	2.4%
Expected dividend yield	—%	—%