UGT1A1 and neutropenia: In particular, when compared with the wild-type six-repeat allele (UGT1A1*1), the seven-repeat variant (UGT1A1*28) is responsible for a dramatically reduced expression of UGT1A1, resulting in poor metabolism of the SN38 active metabolite of irinotecan and increased neutropenia and, to a lesser extent, diarrhea [13, 23].