4. Intangible assets, net

Intangible assets other than goodwill consist of the following:

September 30, 2018

December 31, 2017

(In thousands)

Amortization

Period

Gross Amount

Accumulated Amortization

Net

Gross Amount

Accumulated Amortization

Net

Software developed for internal use

5-10 years

21,455

(2,524

)

18,931

16,642

(1,289

)

15,353

The gross amount associated with software developed for internal use represents capitalized costs of internally-developed software, including eligible salaries and staff benefits, share-based compensation, traveling expenses incurred by relevant employees, and other relevant costs.

Amortization expenses of $445 and $217 for the three months ended September 30, 2018 and 2017, respectively, and $1,235 and $528 for the nine months ended September 30, 2018 and 2017, respectively, were included in depreciation and amortization expense. As of September 30, 2018, intangible assets of $3,719, included in the gross amounts of software developed for internal use, have not started amortization, as they are not ready for their intended use.

As of September 30, 2018, estimated amortization expense related to the Company’s intangible assets for the remainder of 2018 through 2023 and thereafter are as follows:

(In thousands)
Year	September 30, 2018
Remainder of 2018	$	458
2019		2,331
2020		2,330
2021		2,328
2022		2,327
2023 and thereafter		9,157
Total	$	18,931