Statement of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Cash flow from operating activities
Net loss	$ (16,523)	$ (18,472)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	2,739	1,204
Depreciation	288	276
Amortization of premiums and discounts on marketable securities	427	110
Change in fair value of preferred stock tranche liability		9,750
In kind Research And Development Expenses	752	2,065
Increase (Decrease) in Prepaid Rent	75	(134)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	73	612
Other non-current assets	57	(969)
Deferred revenue	(8,550)	62,541
Accounts payable	(158)	(157)
Accrued expenses	1,619	811
Other non-current liabilities		(169)
Lease incentive benefit		138
Net cash provided by (used in) operating activities	(19,201)	57,606
Cash flow from investing activities
Purchases of property and equipment	(657)	(650)
Change in restricted cash	(471)
Purchases of marketable securities	(12,041)	(88,029)
Proceeds from marketable securities	68,000	4,000
Net cash used in investing activities	54,831	(84,679)
Cash flow from financing activities
Proceeds from issuance of Series A redeemable convertible preferred stock and tranche rights, net of issuance costs		104,779
Proceeds from issuance of common stock and restricted stock	93	7
Net cash provided by financing activities	93	104,786
Net increase in cash and cash equivalents	35,723	77,713
Cash and cash equivalents, beginning of period	31,309	7,035
Cash and cash equivalents, end of period	$ 67,032	84,748
Supplemental Disclosure of Cash and non-cash Activities
Accretion of redeemable convertible preferred stock to redemption value		$ 3,451