Rare autosomal recessive disorder of bilirubin metabolism caused by mutation in the UDP-glucuronosyltransferase (UDPGT) gene. Type I is characterized by complete absence of bilirubin UDPGT enzyme, resulting in severe neonatal jaundice, pure unconjugated hyperbilirubinemia, and kernicterus (bilirubin encephalopathy). Left untreated, it leads to neurological impairment, brain damage, and death. Therapies include exchange transfusions, phototherapy, and liver transplantation. Type II, or Arias Syndrome, is characterized by reduced level of bilirubin UDPGT enzyme; it has later onset and a better prognosis. Phenobarbital treatment for jaundice continues into adulthood. Liver function tests appear normal in both types.