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,
(In thousands)	Amortization period	2018		2017
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			(67,101)		(53,278)
Net carrying value		$	152,899	$	166,722

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, 2018, the weighted average remaining amortization period is 11.1 years.

Additional information regarding these milestone fees is included in Note 3, “Collaborative Arrangements.” Amortization expense for each of the years ended December 31, 2018, 2017 and 2016 was $13.8 million. The remaining estimated amortization expense is $13.8 million for each of the years from 2019 to 2023 and $83.9 million thereafter.