5. CAPITALIZED FEES PAID TO A RELATED PARTY

We capitalize fees paid to licensors related to agreements for approved products or commercialized products. We capitalize these fees as capitalized fees paid to a related party ("Capitalized Fees") and amortize these Capitalized Fees on a straight-line basis over their estimated useful lives upon the commercial launch of the product, which is expected to be shortly after regulatory approval of such product. The estimated useful lives of these Capitalized Fees are based on a country-by-country and product-by-product basis, as the later of the expiration or termination of the last patent right covering the compound in such product in such country and 15 years from first commercial sale of such product in such country, unless the agreement is terminated earlier. Capitalized fees paid to a related party, which consist of registrational and launch-related milestone fees paid to GSK, were as follows:

(In thousands)	Amortization period		December 31, 2016			December 31, 2015
United States		2013 - 2030	$	120,000		$	120,000
Europe		2013 - 2029		60,000			60,000
Japan		2013 - 2029		40,000			40,000

Gross carrying value				220,000			220,000
Accumulated amortization				(39,455	)		(25,632	)

Net carrying value			$	180,545		$	194,368

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. As of December 31, 2016, the weighted average remaining amortization period is 13.1 years.

Additional information regarding these milestone fees is included in Note 3, "Collaborative Arrangements." Amortization expense for the years ended December 31, 2016, 2015 and 2014 were $13.8 million, $13.8 million and $11.1 million. The remaining estimated amortization expense is $13.8 million for each of the years from 2017 to 2021 and $111.4 million thereafter.