Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2018

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 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	8.8	$	7,595
Granted	(1,966,069	)	1,966,069		6.45
Exercised	—		(864,418	)	2.98
Canceled	775,633		(775,633	)	4.10
Repurchase of early exercised shares	1,230		—
Balance as of September 30, 2018	509,584		7,717,070		$	4.38	8.5	$	91,809
Vested and Exercisable as of September 30, 2018			2,797,623		$	3.38	7.7	$	36,085

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 option-related stock‑based compensation expense:

	Three Months Ended September 30,					Nine Months Ended September 30,
	2018		2017			2018		2017
	(in thousands)
Cost of precision oncology testing	$	112	$	(25	)	$	254	$	104
Research and development expense	617		17			1,035		307
Sales and marketing expense	428		(526		)	1,061		14
General and administrative expense	674		1,342			1,938		1,673
Total stock-based compensation expense	$	1,831	$	808		$	4,288	$	2,098

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

The grant date fair value of employee 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,
	2018	2017	2018	2017
Expected term (in years)	5.87 – 6.09	6.02 – 6.08	5.01 – 6.51	6.02 – 6.08
Expected volatility	69.7% – 70.2%	74.3% – 75.1%	68.7% – 72.5%	74.3% – 75.1%
Risk-free interest rate	2.7% – 2.9%	2.0% – 2.0%	2.5% – 2.9%	1.9% – 2.0%
Expected dividend yield	—%	—%	—%	—%

Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions

The fair value of the stock options granted to non-employees is calculated at each reporting date using the Black-Scholes options-pricing model with the following assumptions:

	Three Months Ended September 30,		Nine Months Ended September 30,
	2018	2017	2018	2017
Expected term (in years)	5.4 – 9.5	6.0 – 10.0	5.4 – 10.0	6.0 – 10.0
Expected volatility	64.9% – 71.5%	72.3% – 72.8%	64.9% – 71.5%	66.3% – 73.4%
Risk-free interest rate	2.8% – 3.1%	1.8% – 2.0%	2.3% – 3.1%	1.8% – 2.1%
Expected dividend yield	—%	—%	—%	—%