MONDO_0021020 (Crigler-Najjar syndrome type 1) (CNS1) is the most severe form of CNS, a hereditary disorder of hepatic bilirubin conjugation, characterized by severe neonatal unconjugated hyperbilirubinemia due to a complete absence of hepatic bilirubin glucuronosyltransferase (BGT). Also known as: Crigler-Najjar syndrome, type 1, Crigler-Najjar syndrome, type I, UGT deficiency type 1, bilirubin uridinediphosphate glucuronosyltransferase deficiency type 1, bilirubin-UGT deficiency type 1, hereditary unconjugated hyperbilirubinemia type 1, hyperbilirubinemia, Crigler-Najjar type 1.