Statement of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Cash flow from operating activities
Net loss	$ (25,520)	$ (22,389)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	4,528	2,061
Depreciation	448	436
Amortization of premiums and discounts on marketable securities	571	261
Change in fair value of preferred stock tranche liability		9,750
In-kind research and development expenses	989	2,190
Other non-cash items	(409)	(205)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(311)	243
Other non-current assets	57	14
Deferred revenue	(12,219)	57,604
Accounts payable	175	(1,013)
Accrued expenses	2,301	1,952
Other non-current liabilities		(186)
Lease incentive benefit		138
Net cash provided by (used in) operating activities	(29,390)	50,856
Cash flow from investing activities
Purchases of property and equipment	(1,160)	(909)
Change in restricted cash	(471)
Purchases of marketable securities	(66,627)	(100,283)
Proceeds from maturities of marketable securities	129,700	6,000
Net cash used in investing activities	61,442	(95,192)
Cash flow from financing activities
Proceeds from the issuance of redeemable convertible preferred stock net of discount and issuance costs		104,779
Deferred initial public offering costs		(954)
Proceeds from the exercise of stock options and issuance of restricted stock	146	9
Net cash provided by financing activities	146	103,834
Net increase in cash and cash equivalents	32,198	59,498
Cash and cash equivalents, beginning of period	31,309	7,035
Cash and cash equivalents, end of period	$ 63,507	66,533
Supplemental Disclosure of Cash and non-cash Activities
Accretion of redeemable convertible preferred stock to redemption value		6,045
Deferred initial public offering costs included in accrued expenses		$ 348