4. Intangible assets, net

Intangible assets other than goodwill consist of the following:

(In thousands)	Amortization period	June 30, 2018			December 31, 2017
Gross amount:
Software developed for internal use	3 years	$	3,737		$	2,972
Acquired proprietary technology	5 years		11,416			11,382
Customer relationships	7-10 years		34,986			34,986
Trade names	20 years		16,357			16,357
Domain names	20 years		191			191
Databases	5-10 years		31,292			31,292
Non-competition agreements	2-5 years		1,768			1,768
Total gross amount			99,747			98,948
Accumulated amortization:
Software developed for internal use			(826	)		(490	)
Acquired proprietary technology			(5,837	)		(4,693	)
Customer relationships			(12,023	)		(9,628	)
Trade names			(2,095	)		(1,686	)
Domain names			(24	)		(20	)
Databases			(8,768	)		(6,964	)
Non-competition agreements			(1,446	)		(1,113	)
Total accumulated amortization			(31,019	)		(24,594	)
Net intangible assets:
Software developed for internal use			2,911			2,482
Acquired proprietary technology			5,579			6,689
Customer relationships			22,963			25,358
Trade names			14,262			14,671
Domain names			167			171
Databases			22,524			24,328
Non-competition agreements			322			655
Total net intangible assets		$	68,728		$	74,354

The gross amount associated with software developed for internal use primarily represents capitalized costs of internally developed software. The amounts relating to acquired proprietary technology, customer relationships, trade names, domain names, databases and non-competition agreements primarily represent the fair values of intangible assets acquired as a result of the acquisition of Fluent, LLC (“Fluent LLC”) effective on December 8, 2015 (the “Fluent LLC Acquisition”) and the acquisition of Q Interactive, LLC (“Q Interactive”) effective on June 8, 2016 (the “Q Interactive Acquisition”).

On January 18, 2017, the Company’s management and Board of Directors approved a plan to merge and fully integrate Q Interactive’s business into Fluent LLC (the “Q Interactive Integration”). As a result, the remaining balance of long-lived assets of $3,626, relating primarily to the acquired proprietary technology and trade names acquired in the Q Interactive Acquisition, was written off to costs and expenses as a write-off of long-lived assets during the first quarter of 2017.

Amortization expense of $3,213 and $3,147 for the three months ended and $6,421 and $6,258 for the six months ended June 30, 2018 and 2017, respectively, were included in depreciation and amortization expenses. As of June 30, 2018, intangible assets with the carrying amount of $1,655, included in the gross amount of software developed for internal use, have not started amortization, as they are not ready for their intended use.

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

(In thousands)
Year	June 30, 2018
Remainder of 2018	$	6,450
2019		12,901
2020		12,246
2021		9,199
2022		7,877
2023 and thereafter		20,055
Total	$	68,728