UGT1A1 and Crigler-Najjar syndrome type 1: Left untreated, severe neonatal hyperbilirubinemia, which is also the hallmark of Crigler-Najjar syndrome type I (a genetic disorder caused by mutations in the UGT1A1 gene), can damage the nervous system, leading first to bilirubin encephalopathy and then to kernicterus (‘yellow’ staining of the brain tissue due to accumulation of bilirubin).