UGT1A1 and neutropenia: Since UGT1A1*28 homozygous individuals have only 35% of the activity in wild-type UGT1A1 individuals and metabolize irinotecan more slowly [10,12], cancer patients with the UGT1A1*28/*28 genotype are at an increased risk of high-grade neutropenia and/or diarrhea while being treated with irinotecan.