5. Capitalized Fees paid to a Related Party

Capitalized fees paid to a related party, which consist of registrational and launch-related milestone fees paid to GSK, were as follows:

	December 31, 2015									December 31, 2014
(In thousands)	Weighted Average Remaining Amortization Period (Years)		Gross Carrying Value		Accumulated Amortization			Net Carrying Value		Gross Carrying Value		Accumulated Amortization			Net Carrying Value
Approval and launch related milestone payments under the LABA Collaboration		14.1	$	220,000	$	(25,632	)	$	194,368	$	220,000	$	(11,809	)	$	208,191

These milestone fees are being amortized over their estimated useful lives commencing upon the commercial launch of the product in their respective regions with the amortization expense recorded as a reduction in revenue from collaborative arrangements. Additional information regarding these milestone fees is included in Note 3, "Collaborative Arrangements." Amortization expense for the years ended December 31, 2015, 2014 and 2013 were $13.8 million, $11.1 million and $0.7 million. The remaining estimated amortization expense is $13.8 million for each of the years from 2016 to 2020 and $125.3 million thereafter.