The gross carrying amount and net book value of goodwill was $18.5 million at September 30, 2021, all of which resulted from the Merger. During the second quarter of 2021, we identified and recorded measurement period adjustments for taxes related to the merger, which reduced goodwill by $3.9 million from the preliminary purchase price allocation and reduced our deferred tax liabilities by $0.8 million and reduced the CVR liability by $3.1 million. The measurement period adjustments were the result of additional analysis performed and information identified during the second quarter of 2021 based on facts and circumstances that existed as of the merger date. As of September 30, 2021, the preliminary purchase price allocation for the merger with Aduro is subject to change as we use the measurement period, not to exceed one-year, to adequately analyze all the factors used in establishing the asset and liability fair values as of the merger date.

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):

	September 30, 2021
	Gross Carrying Amount		Accumulated Amortization		Net Book Value
Intangible assets with finite lives:
Acquired license agreement	$	26,685	$	1,575	$	25,110
In-place lease		1,433		111		1,322
Total intangible assets with finite lives		28,118		1,686		26,432
Acquired in-process research and development assets ("IPR&D")		36,550		—		36,550
Total intangible and acquired IPR&D assets	$	64,668	$	1,686	$	62,982

	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 a period 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 nine months ended September 30, 2021. Refer to Note 15, “Equity Method Investment,” for more information.

Amortization expense was $0.4 million and $0 for the three months ended September 30, 2021 and 2020, respectively, and was $1.3 million and $0 for the nine months ended September 30, 2021 and 2020, respectively. Based on finite-lived intangible assets recorded as of September 30, 2021, the estimated future amortization expense for the next five years is as follows (in thousands):

Year Ending December 31,	Estimated Amortization Expense
2021 (remaining three months)	$	423
2022		1,722
2023		1,733
2024		1,733
2025		1,733
Thereafter		19,088