UGT1A1 and Hyperbilirubinemia: Crigler-Najjar syndrome is classified into 2 types, depending on serum total bilirubin (TBIL) concentration: the more severe (CNS-I) is characterized by high levels of TBIL (342–684 μmol/L), whereas the milder form (CNS-II) is characterized by TBIL values ranging from 103 to 342 μmol/L.[8] In CNS-I, the most severe UGT1A1-associated hereditary disorder, described in 1952 by Crigler and Najjar, there is complete, or almost complete, absence of UGT1A1 enzyme activity, with severe jaundice, resulting in life-threatening hyperbilirubinemia and a lack of response to phenobarbital.