Goodwill is attributable to the acquisition of our pharma services in July 2019. The carrying value of the intangible assets acquired was $15.6 million, with goodwill of approximately $8.3 million and identifiable intangible assets of approximately $7.3 million. The goodwill balance at March 31, 2021 was $8.4 million. The net carrying value of the identifiable intangible assets from all acquisitions as of March 31, 2021 and December 31, 2020 are as follows:

Amortization expense was approximately $1.1 million for both the three-month periods ended March 31, 2021 and 2020, respectively. Estimated amortization expense for the next five years is as follows:

The following table displays a roll forward of the carrying amount of goodwill from December 31, 2020 to March 31, 2021: