UGT1A1 and neutropenia: SN-38 is an active metabolite of IRI and the UGT1A1 gene encodes an intrahepatic metabolizing enzyme, which converts the active form SN-38 to the inactive form SN-38 G. In patients who are double heterozygotes for *6 and *28 or homozygotes for *6 or *28 of the UGT1A1 gene, the glucuronic acid conjugation capacity of UGT1A1 is known to be decreased and the metabolism of SN-38 to be delayed, and serious adverse drug reactions such as neutropenia may occur as a result.