The gross carrying amount and net book value of goodwill was $0.1 million at December 31, 2021, all of which resulted from the Merger. During the year ended December 31, 2021, we identified and recorded measurement period adjustments primarily for taxes related to the Merger, which reduced goodwill by $22.3 million from the preliminary purchase price allocation. The measurement period adjustments were the result of additional analysis performed and information identified during 2021 based on facts and circumstances that existed as of the Merger date. Refer to Note 3 “Reverse Merger and Contingent Value Rights” for more information.

We test goodwill for impairment on an annual basis or more frequently if an impairment indicator exists. To determine if an impairment has occurred, we perform a quantitative test in which the fair value of a single reporting unit is compared to its carrying value. If the carrying value of the reporting unit exceeds the fair value of the reporting unit, we record an impairment loss equal to that difference.

The gross carrying amounts and net book value of intangible assets were as follows (in thousands):

	December 31, 2021
	Gross Carrying Amount		Accumulated Amortization		Net Book Value
Intangible assets with finite lives:
Acquired license agreement	$	26,685	$	1,968	$	24,717
In-place lease		1,433		141		1,292
Total intangible assets with finite lives		28,118		2,109		26,009
Acquired IPR&D assets		36,550		—		36,550
Total intangible and acquired IPR&D assets	$	64,668	$	2,109	$	62,559

	December 31, 2020
	Gross Carrying Amount		Accumulated Amortization		Net Book Value
Intangible assets with finite lives:
Acquired license agreement	$	26,685	$	398	$	26,287
In-place lease		1,433		24		1,409
Total intangible assets with finite lives		28,118		422		27,696
Acquired IPR&D assets		39,295	—			39,295
Total intangible and acquired IPR&D assets	$	67,413	$	422	$	66,991

Intangible assets are carried at cost less accumulated amortization and impairment. Amortization is over periods of 9 to 17 years, with an original weighted average period of 16.7 years, and the amortization expense is recorded in operating expenses. We test our Acquired IPR&D assets for impairment on an annual basis, or more frequently if an impairment indicator exists.

Acquired IPR&D decreased by $2.7 million from the sale of certain of our non-renal assets in exchange for stock during the year ended December 31, 2021. Refer to Note 10 “Equity Method Investment” for more information.

Amortization expense was $1.7 million and $0.4 for the years ended December 31, 2021, and 2020, respectively. Based on finite-lived intangible assets recorded as of December 31, 2021, the estimated future amortization expense for the next five years is as follows (in thousands):

Year Ending December 31,	Estimated Amortization Expense
2022	$	1,722
2023		1,733
2024		1,733
2025		1,733
2026		1,733
Thereafter		17,355