The following table summarizes the components of the Company’s intangible assets for each of the periods indicated:

	As of December 31, 2018		As of December 31, 2017
	(in thousands)
Patents	$	5,773	$	7,660
In-licensed rights		5,626		6,491
Software licenses		175		204
Total	$	11,574	$	14,355

The in-licensed rights relate to agreements with BioMarin and UWA. Following the execution of the settlement and license agreements with BioMarin in July 2017, the Company recorded a $6.6 million intangible asset related to EXONDYS 51 in the U.S. Additionally, as a result of the FDA approval and the subsequent commercial sale of EXONDYS 51, the Company made a $1.0 million sales milestone to UWA and, accordingly, recorded an in-licensed right. The in-licensed rights are being amortized on a straight-line basis over the remaining life of the related patent because the life of the related patent reflects the expected time period that the Company will benefit from the in-licensed right. For the years ended December 31, 2018, 2017 and 2016, the Company recorded $0.9 million, $1.1 million and less than $0.1 million, respectively, of amortization related to the in-license rights.

Patent costs consist primarily of external legal costs, filing fees incurred to file patent applications and renewal fees on proprietary technology developed or licensed by the Company. Patent costs associated with applying for a patent, being issued a patent and annual renewal fees are capitalized. Costs to defend a patent and costs to invalidate a competitor’s patent or patent application are expensed as incurred. Patent costs are amortized on a straight-line basis over the shorter of the estimated economic lives or the initial term of the patents, which is generally 20 years. Patent amortization expense was $0.7 million, $0.6 million and $0.6 million for the years ended December 31, 2018, 2017 and 2016, respectively. The Company also expensed the remaining net book value of previously capitalized patents that were later abandoned of $0.1 million, $0.6 million and $0.3 million for the years ended December 31, 2018, 2017 and 2016, respectively, which were included in research and development expenses on the consolidated statements of operations and comprehensive loss.

Additionally, the Company reviewed its patent portfolio and identified technology that the Company will no longer pursue. As a result, the Company impaired these patent assets and recorded $3.8 million in impairment loss, which was included in research and development expense on the consolidated statement of operations and comprehensive loss.

The following table summarizes the estimated future amortization for intangible assets for the next five years:

	As of December 31, 2018 (in thousands)
2019		1,490
2020		1,300
2021		1,299
2022		1,298
2023		1,297
Total	$	6,684