UGT1A1 and Hyperbilirubinemia: Unconjugated hyperbilirubinemia occurs primarily in newborns since they display an increased bilirubin production and a decreased disposal, caused by a combination of increased erythrocyte breakdown and very low or absent expression of the bilirubin conjugation enzyme, UDP-glucuronosyltransferase 1A1 (UGT1A1)2.