Note 9 - Stock-based Compensation (Tables)

12 Months Ended

Dec. 31, 2015

Notes Tables

Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table Text Block]

	Year Ended December 31,
	2015		2014		2013
Research and development	$	650	$	114	$	106
General and administrative		1,514		345		49
Total	$	2,164	$	459	$	155

Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]

	Year Ended December 31,
	2015			2014			2013
Weighted average expected volatility		79.8	%		79.5	%		91.1	%
Expected term (in years)		6.11			6.00			5.95
Risk free interest rate		1.76	%		1.88	%		1.03	%
Expected dividend yield		0	%		0	%		0	%

Schedule of Share-based Compensation, Stock Options, Activity [Table Text Block]

	Options			Weighted-Average Exercise Price		Weighted-Average Remaining Contractual Term (years)		Aggregate Intrinsic Value
Outstanding at December 31, 2014		1,235,526		$	4.19		7.1	$	7,709
Granted		1,010,256		$	13.52
Exercised		(45,413	)	$	1.96
Outstanding at December 31, 2015		2,200,369		$	8.52		7.8	$	15,705
Exercisable at December 31, 2015		812,891		$	3.53		5.3	$	9,755
Vested or expected to vest at December 31, 2015 (1)		1,991,131		$	8.16		7.6	$	14,926