4.	Balance Sheet Details

Short-Term Investments

The following is a summary of our short-term, available-for-sale securities (in thousands):

	December 31, 201 5
			Gross		Gross
	Amortized		Unrealized		Unrealized			Estimated
	Cost		Gains		Losses			Fair Value
United States corporate debt	$	28,750	$	—	$	(35	)	$	28,715
Foreign corporate debt		4,927		—		(5	)		4,922
United States commercial paper		7,770		—		—			7,770
Foreign commercial paper		15,579		—		—			15,579
Total	$	57,026	$	—	$	(40	)	$	56,986

The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts to maturity. We regularly monitor and evaluate the realizable value of our marketable securities. We did not recognize any impairment losses for the year ended December 31, 2015.

Property and Equipment

Property and equipment is comprised of the following (in thousands):

	December 31,
	2015			2014
Scientific equipment	$	5,593		$	4,793
Computer equipment and software		1,272			1,133
Furniture, fixtures and office equipment		352			351
Leasehold improvements		1,384			1,376
		8,601			7,653
Less: accumulated depreciation and amortization		(5,552	)		(4,833	)
	$	3,049		$	2,820

Depreciation and amortization expense for the years ended December 31, 2015, 2014 and 2013 was approximately $734,000, $578,000 and $333,000, respectively.

During the year ended December 31, 2014, we recognized $905,000 as an impairment loss due to the write-down of a piece of manufacturing-related equipment. As of December 31, 2014, the estimated fair value of the equipment was approximately $115,000. In 2015, we sold the equipment for cash proceeds of $241,000.

Accrued Payroll and Employee Liabilities and Other Accrued

Expense

Accrued payroll and employee liabilities and other accrued expense consisted of the following (in thousands):

	December 31,
	20 15		2014
Accrued employee salaries and benefits	$	1,863	$	719
Accrued bonuses		2,965		2,012
Total accrued payroll and employee liabilities	$	4,828	$	2,731

	December 31,
	2015		2014
Accrued consulting and professional fees	$	3,914	$	1,775
Accrued accounts payable		36		886
Deferred rent		61		105
Other accrued liabilities		143		165
Total other accrued expense	$	4,154	$	2,931