UGT1A1 and Decreased total neutrophil count: Furthermore, the UGT1A1 gene is accountable for the metabolism of SN-38, which is an active metabolite of irinotecan, and different variations of UGT1A1 (such as UGT1A1*6 and UGT1A1*28) increase myelosuppression, such as serious neutropenia [31].