Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2019

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

Share-based Compensation, Stock Options, Activity

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, 2018	3,556,507		7,588,405		$	4.58	8.3	$	250,495
Granted	(306,958	)	306,958		89.37
Exercised	—		(2,627,486	)	3.72
Canceled	5,922		(352,459	)	6.17
Restricted stock units granted	(484,952	)	—		—
Restricted stock units canceled	36,140		—		—
Balance as of September 30, 2019	2,806,659		4,915,418		$	10.23	8.0	$	271,740
Vested and Exercisable as of September 30, 2019			1,851,781		$	4.52	7.4	$	109,974

Schedule of Restricted Stock 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
Unvested balance as of December 31, 2018	—		$	—
Granted	484,952		79.68
Vested	(5,545	)	53.20
Canceled	(36,140	)	48.83
Unvested balance as of September 30, 2019	443,267		$	82.52

Schedule of Compensation Cost for Share-based Payment Arrangements, Allocation of Share-based Compensation Costs by Plan

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

	Three Months Ended September 30,				Nine Months Ended September 30,
	2019		2018		2019		2018
	(unaudited)
	(in thousands)
Cost of precision oncology testing	$	266	$	112	$	562	$	254
Research and development expense	2,066		617		4,704		1,035
Sales and marketing expense	1,458		428		2,930		1,061
General and administrative expense	1,694		674		3,686		1,938
Total stock-based compensation expense	$	5,484	$	1,831	$	11,882	$	4,288

Schedule of Share-based Payment Award, Employee Stock Purchase Plan, Valuation Assumptions

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

	Three Months Ended September 30,		Nine Months Ended September 30,
	2019	2018	2019	2018
	(unaudited)
Expected term (in years)	6.03 – 6.08	5.87 – 6.09	5.50 – 6.22	5.01 – 10.00
Expected volatility	63.2% – 64.6%	69.7% – 70.2%	63.2% – 68.3%	69.7% – 86.5%
Risk-free interest rate	1.6% – 1.9%	2.7% – 2.9%	1.6% – 2.7%	2.5% – 2.9%
Expected dividend yield	—%	—%	—%	—%

The grant date fair value of the stock purchase right granted under the ESPP was estimated using a Black-Scholes option-pricing model with the following weighted-average assumptions:

	Three Months Ended September 30, 2019	Nine Months Ended September 30, 2019
	(unaudited)
Expected term (in years)	0.29	0.29 – 0.50
Expected volatility	60.3%	60.2% – 60.3%
Risk-free interest rate	2.1%	2.1% – 2.5%
Expected dividend yield	—%	—%