Crigler-Najjar syndrome (CNS) is a hereditary disorder of bilirubin metabolism characterized by unconjugated hyperbilirubinemia due to a hepatic deficit of bilirubin glucuronosyltransferase (GT) activity. Two types have been described, CNS types 1 and 2 (see these terms). CNS1 is characterized by a complete deficit of the enzyme and is unaffected by phenobarbital induction therapy, whereas the enzymatic deficit is partial and responds to phenobarbital in CNS2.