Stock-Based Compensation (Tables)

6 Months Ended

Jun. 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	(55,712	)	55,712		70.53
Exercised	—		(1,677,990	)	3.28
Canceled	886		(288,655	)	6.08
Restricted stock units granted	(173,733	)	—		—
Restricted stock units canceled	30,410		—		—
Balance as of June 30, 2019	3,358,358		5,677,472		$	5.54	8.1	$	458,782
Vested and Exercisable as of June 30, 2019			2,370,282		$	4.12	7.7	$	194,852

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	173,733		55.25
Vested	(1,106	)	65.50
Canceled	(30,410	)	42.72
Unvested balance as of June 30, 2019	142,217		$	57.85

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 June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
	(unaudited)
	(in thousands)
Cost of precision oncology testing	$	126	$	79	$	296	$	142
Research and development expense	1,428		214		2,638		418
Sales and marketing expense	646		259		1,472		633
General and administrative expense	1,015		628		1,991		1,264
Total stock-based compensation expense	$	3,215	$	1,180	$	6,397	$	2,457

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 June 30,		Six Months Ended June 30,
	2019	2018	2019	2018
	(unaudited)
Expected term (in years)	5.50 – 6.16	5.85 – 6.20	5.50 – 6.22	5.01 – 10.00
Expected volatility	66.9% – 68.3%	80.7% – 81.0%	66.7% – 68.3%	80.7% – 86.5%
Risk-free interest rate	1.9%	3.0%	1.9% – 2.7%	2.5% – 2.9%
Expected dividend yield	—%	—%	—%	—%