Statement of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Cash flow from operating activities
Net loss	$ (16,648)	$ (7,188)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	2,165	1,422
Depreciation	317	136
Amortization of premiums and discounts on marketable securities	92	231
In-kind research and development expenses	44
Other non-cash items	855	37
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	958	(149)
Other non-current assets	(1,490)	57
Deferred revenue	(1,464)	(4,349)
Accounts payable	280	1,085
Accrued expenses	(504)	(769)
Other non-current liabilities	1,000
Lease incentive benefit	(101)
Net cash provided by (used in) operating activities	(14,294)	(9,487)
Cash flow from investing activities
Purchases of property and equipment	(2,945)	(460)
Change in restricted cash		(471)
Proceeds from maturities of marketable securities	23,600	13,000
Net cash used in investing activities	20,655	12,069
Cash flow from financing activities
Deferred initial public offering costs	602	4
Net cash provided by financing activities	602	4
Net increase in cash and cash equivalents	6,963	2,586
Cash and cash equivalents, beginning of period	36,641	31,309
Cash and cash equivalents, end of period	43,604	$ 33,895
Supplemental Disclosure of Cash and non-cash Activities
Capital Expenditures Incurred but Not yet Paid	$ 387