UGT1A1 and neutropenia: UGT1A1*6 and *28 alleles and their impact on the incidence of irinotecan toxicity (severe neutropenia and diarrhea) caused by elevated exposure to SN-38 have been the most extensively studied, with the majority of evidence focused on the UGT1A1*28 allele [41,45,46,47,48,49].