QUARTERLY RESULTS OF OPERATIONS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
QUARTERLY RESULTS OF OPERATIONS (UNAUDITED)
Laboratory service revenue	$ 14,420	$ 12,632	$ 8,119	$ 4,266	$ 1,504				$ 39,437	$ 1,504
License fee revenue								$ 294		294	$ 4,144
Cost of revenue	7,667	7,528	5,094	4,212	3,401	$ 924			24,501	4,325
Gross margin	6,753	5,104	3,025	54	(1,897)	(924)		294	14,936	(2,527)	4,144
Research and development	9,365	9,863	8,115	6,571	4,992	9,073	$ 7,174	7,430	33,914	28,669	27,678
General and administrative	15,864	15,432	13,683	12,971	10,625	8,994	6,230	4,586	57,950	30,435	13,649
Sales and marketing	21,944	23,079	20,593	16,524	15,069	13,217	6,166	4,456	82,140	38,908	9,578
Loss from operations	(40,420)	(43,270)	(39,366)	(36,012)	(32,583)	(32,208)	(19,570)	(16,178)	(159,068)	(100,539)	(46,761)
Investment income	491	365	193	222	150	160	146	86	1,271	542	316
Interest income (expense)	(62)	(40)	107	(11)
Interest expense					(11)	(12)	(13)	(15)	(6)	(51)	(69)
Net loss	$ (39,991)	$ (42,945)	$ (39,066)	$ (35,801)	$ (32,444)	$ (32,060)	$ (19,437)	$ (16,107)	$ (157,803)	$ (100,048)	$ (46,514)
Net loss per share-basic and diluted (in dollars per share)	$ (0.41)	$ (0.45)	$ (0.44)	$ (0.40)	$ (0.38)	$ (0.39)	$ (0.24)	$ (0.23)	$ (1.71)	$ (1.25)	$ (0.69)
Weighted average common shares outstanding-basic and diluted (in shares)	96,404	94,444	88,919	88,662	84,734	82,941	82,048	70,987	92,135	80,232	67,493