Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2017	Jun. 30, 2016
Operating activities
Net loss	$ (14,358)	$ (12,209)
Adjustment to reconcile net loss to net cash used in operating activities:
Restructuring charges	2,723	0
Depreciation and amortization	18	45
Stock-based compensation	693	689
Net amortization of premium/ discounts on marketable securities	127	98
Changes in operating assets and liabilities:
Prepaid expenses and other assets	496	(118)
Accounts payable	(314)	(2,584)
Accrued expenses	(4,801)	(21)
Net cash used in operating activities	(15,416)	(14,100)
Investing activities
Purchases of marketable securities	(27,528)	(50,848)
Maturities of marketable securities	38,952	9,000
Change in restricted cash	2,432	(2,430)
Proceeds from the sale of equipment	325	0
Purchases of property and equipment	(3)	(548)
Net cash provided by/ (used in) investing activities	14,178	(44,826)
Financing activities
Proceeds from the exercise of stock options	25	9
Cash provided by financing activities	25	9
Net decrease in cash and cash equivalents	(1,213)	(58,917)
Cash and cash equivalents at beginning of period	16,432	89,713
Cash and cash equivalents at end of period	$ 15,219	$ 30,796

The sum of the periodic adjustments of the differences between securities' face values and purchase prices that are charged against earnings. This is called accretion if the security was purchased at a discount and amortization if it was purchased at premium. As a noncash item, this element is an adjustment to net income when calculating cash provided by or used in operations using the indirect method.

Name:	us-gaap_AccretionAmortizationOfDiscountsAndPremiumsInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- References

No definition available.