UGT1A1 and Crigler-Najjar syndrome type 1: Genetic defects in the human UGT1A1 gene are associated with unconjugated hyperbilirubinemia, which can be either asymptomatic as in individuals with Gilbert syndrome [6] or severe as in the case of Crigler-Najjar syndrome types I and II [7], depending on the remaining UGT1A1 activity.