Gilbert syndrome (GS) is an inherited form of chronic mild unconjugated hyperbilirubinemia (1)(2)(3), although many patients do not have a clear family history (4). Hepatic glucuronidation of bilirubin is catalyzed by isoenzyme 1A1 of UDP-glucuronosyl transferase (UGT1A1). The majority of GS subjects were found to be homozygous for an extra TA in the TATA-box in the promoter region of UGT1A1 (5)(6)(7). Transcription of the (TA)7 allele is reduced by at least 70% compared with the wild-type (TA)6 allele. Because bilirubin UGT1A1 is the only enzyme with substantial bilirubin glucuronidating activity in humans (8), the presence of this extra TA in both alleles can explain the impaired conjugation of bilirubin found in Caucasoid GS patients (6).
