- UGT1A1 polymorphism. The UGT1A1 gene polymorphisms refer to those genetic defects found to be associated with Gilbert's Syndrome, including a promoter defect (T-3263G) that disrupts a transcription regulatory site, the TA repeats promoter polymorphism, and four mutations within the coding region (G211A, C686A, C1091T, and T1456G).