FIXED ASSETS

Fixed assets, summarized by major category, consist of the following (in thousands):



	March 31,			December 31,
	2016			2015
Machinery and laboratory equipment	$	31,086		$	29,864
Leasehold improvements	32,359			30,834
Computer equipment and software	4,724			4,007
Office furniture and equipment	1,440			1,439
Construction in progress	54,234			49,097
Total	123,843			115,241
Less: accumulated depreciation	(27,997		)	(24,917		)
Fixed assets, net	$	95,846		$	90,324

For the three months ended March 31, 2016 and 2015, depreciation expense was $3.1 million and $2.7 million, respectively. For the three months ended March 31, 2016 and 2015, capitalized interest on the construction of manufacturing sites was $0.3 million and $0.2 million, respectively. As of March 31, 2016 and December 31, 2015, total fixed assets, net, includes leasehold improvements and manufacturing process equipment located in England in the amount of $28.4 million and $25.9 million, respectively.