Condensed Statements of Cash Flows - USD ($)	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Operating activities
Net loss	$ (7,271,884)	$ (5,979,664)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	26,725	17,096
Stock-based compensation expense	901,924	821,672
Amortization of premium on marketable securities	22,122	102,775
Changes in operating assets and liabilities:
Prepaid and other current assets	224,354	272,957
Accounts payable and accrued expenses	(252,818)	(939,801)
Accrued compensation	(544,584)	(278,069)
Deferred rent	(3,850)	35,295
Net cash used in operating activities	(6,898,011)	(5,947,739)
Investing activities
Maturities of marketable securities	13,875,000	14,578,000
Purchase of marketable securities	(4,240,142)	(10,211,361)
Capital expenditures	(105,114)
Net cash provided by investing activities	9,529,744	4,366,639
Financing activities
Proceeds from issuance of common stock, net of offering costs	3,076,709
Deferred public offering costs		(30,965)
Proceeds from stock issuances under employee stock purchase plan and exercise of stock options	1,503	2,106
Net cash provided by (used in) financing activities	3,078,212	(28,859)
Net increase (decrease) in cash and cash equivalents	5,709,945	(1,609,959)
Cash and cash equivalents at beginning of period	13,876,090	9,912,674
Cash and cash equivalents at end of period	19,586,035	8,302,715
Supplemental disclosure of cash flow information:
Cash paid for interest	$ 17,500	17,500
Supplemental schedule of noncash investing and financing activities:
Deferred public offering costs included in accounts payable		$ 185,688