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)	September 30, 2016			December 31, 2015
United States	$	120,000		$	120,000
Europe	60,000			60,000
Japan	40,000			40,000

Gross carrying value	220,000			220,000
Accumulated amortization	(36,000		)	(25,632		)

Net carrying value	$	184,000		$	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 September 30, 2016, the weighted average remaining amortization period is 13.4 years.

Additional information regarding these milestone fees is included in Note 3, “Collaborative Arrangements.” Amortization expense for the three and nine months ended September 30, 2016 and 2015 was $3.5 million and $10.4 million, respectively. As of September 30, 2016, the remaining estimated amortization expense is $3.5 million for 2016, $13.8 million for each of the years from 2017 to 2021, and an aggregate of $111.4 million thereafter.