Crigler-Najjar syndrome (Monarch Disease Ontology id MONDO_0009044) (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. 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. Also known as: Crigler Najjar Syndrome, Crigler Najjar syndrome, UGT deficiency, bilirubin UDP glucuronyl transferase deficiency, bilirubin uridinediphosphate glucuronosyltransferase deficiency, bilirubin-UGT deficiency, hereditary unconjugated hyperbilirubinemia.