UGT1A1 and Gilbert syndrome: The most common genetic cause for reduced bilirubin conjugation is an insertion polymorphism in the TATA box of UGT1A1 – the (TA)7 variant allele UGT1A1*28; homozygosity for this allele confers a reduced transcriptional activity of 18 to 33% [22, 38], corresponding to the residual glucuronidation activity of ~ 30% determined in patients with Gilbert’s syndrome (GS) [39, 40].