Several polymorphisms of UGT1A1 have been described with the best studied the one defined by a variable length “TA” tandem repeat in the regulatory TATA box of the UGT1A1 gene promoter (UGT1A1∗28) that leads to reduced expression of the isozyme, associated with Gilbert syndrome, the most common inherited cause of unconjugated hyperbilirubinemia. Here, UGT1A1 is linked to Gilbert syndrome.