A number sign (#) is used with this entry because Gilbert syndrome is caused by homozygous, compound heterozygous, or heterozygous mutation in the UDP-glucuronosyltransferase gene (UGT1A1; 191740) on chromosome 2q37. Description The hereditary hyperbilirubinemias (Wolkoff et al., 1983) include (1) those resulting in predominantly unconjugated hyperbilirubinemia: Gilbert or Arias syndrome, Crigler-Najjar syndrome type I (218800), and Crigler-Najjar syndrome type II (606785); and (2) those resulting in predominantly conjugated hyperbilirubinemia: Dubin-Johnson syndrome (237500), Rotor syndrome (237450), and several forms of intrahepatic cholestasis (147480, 211600, 214950, 243300). Detailed studies show that patients with Gilbert syndrome have reduced activity of bilirubin glucuronosyltransferase (Bosma et al., 1995, Koiwai et al., 1995). Genetic Heterogeneity of Hyperbilirubinemia See also Crigler-Najjar syndrome type I (HBLRCN1; 218800), Crigler-Najjar syndrome type II (HBLRCN2; 606785), and transient familial neonatal hyperbilirubinemia (HBLRTFN; 237900), all caused by mutation in the UGT1A1 gene (191740) on chromosome 2q37; Dubin-Johnson syndrome (DJS, HBLRDJ; 237500), caused by mutation in the ABCC2 gene (601107) on chromosome 10q24; and Rotor syndrome (HBLRR; 237450), caused by digenic mutation in the SLCO1B1 (604843) and SLCOB3 (605495) genes, both on chromosome 12p. Clinical Features The characteristics of Gilbert syndrome are normal liver function tests of the usual type, normal liver histology, delayed clearance of bilirubin from the blood, and mild jaundice that tends to fluctuate in severity, particularly after fasting (Nixon and Monahan, 1967).