UGT1A1 Genotyping: Clinical Insight into Bilirubin Metabolism and Irinotecan Toxicity The UGT1A1 Promoter Polymorphism The most clinically relevant genetic variation in UGT1A1 involves the number of TA repeats in the promoter region (TATA box) of the gene: Allele TA Repeats Clinical Relevance UGT1A1*1 6TA Wild-type (normal enzyme activity) UGT1A1*28 7TA Reduced transcription and decreased enzyme