UGT1A1 and Decreased total neutrophil count: Homozygosity or double heterozygosity for UDP-glucuronosyl transferase 1A1 (UGT1A1) polymorphisms (UGT1A1*28 and UGT1A1*6) may relate to increased serious adverse events, such as neutropenia and diarrhea, in patients treated with an irinotecan-based regimen.