9. INCOME TAXES

No provision or benefit for federal or state income taxes has been recorded, as the Company has incurred a net loss for all of the periods presented, and the Company has provided a full valuation allowance against its deferred tax assets.

At December 31, 2015, the Company had U.S. federal and Massachusetts net operating loss carryforwards of $77,456 and $69,682, respectively, of which federal carryforwards will expire in varying amounts beginning in 2026. Massachusetts net operating losses began to expire in 2011. Utilization of net operating losses may be subject to substantial annual limitations due to the "change in ownership" provisions of the Internal Revenue Code, and similar state provisions. The annual limitations may result in the expiration of net operating losses before utilization. The Company also had research and development tax credit carryforwards at December 31, 2015 of $1,010 which will begin to expire in 2021 unless previously utilized.

Significant components of the Company's net deferred tax asset are as follows:

	December 31,
	2015			2014
Net operating loss carryforward	$	30,014		$	22,490
Research and development credit carryforward		942			801
Stock-based compensation		3,307			2,265
Depreciation and amortization		(48	)		(124	)
Accrued expenses		186			226
Charitable contributions		96			112

Subtotal		34,497			25,770
Valuation allowance		(34,497	)		(25,770	)

Net deferred taxes	$	—		$	—

The Company has maintained a full valuation allowance against its deferred tax assets in all periods presented. A valuation allowance is required to be recorded when it is more likely than not that some portion or all of the net deferred tax assets will not be realized. Since the Company cannot be assured of generating taxable income and thereby realizing the net deferred tax assets, a full valuation allowance has been provided. In the years ended December 31, 2015 and 2014, the valuation allowance increased by $8,727 and $6,290, respectively.

The Company has no uncertain tax positions at December 31, 2015 and 2014 that would affect its effective tax rate. The Company does not anticipate a significant change in the amount of uncertain tax positions over the next twelve months. Since the Company is in a loss carryforward position, the Company is generally subject to U.S. federal and state income tax examinations by tax authorities for all years for which a loss carryforward is available.

Income tax benefits computed using the federal statutory income tax rate differs from the Company's effective tax rate primarily due to the following:

	December 31,
	2015			2014			2013
Statutory rate		(34.0	)%		(34.0	)%		(34.0	)%
State taxes, net of benefit		(3.5	)%		(4.8	)%		(2.6	)%
Permanent differences:
Derivative losses		11.0	%		0.7	%		17.1	%
Other		0.3	%		2.6	%		0.0	%
R&D tax credit		(0.4	)%		(1.0	)%		(0.4	)%
Other		0.4	%		2.2	%		0.0	%
Increase in valuation reserve		26.2	%		34.3	%		19.9	%

Effective tax rate		0.0	%		0.0	%		0.0	%