UGT1A1 and Hyperbilirubinemia: Moreover, bilirubin-induced hyperbilirubinemia was more severe at ZT2 than ZT14 in wild-type mice consistent with lower expressions of Ugt1a1 and Mrp2 (two bilirubin detoxification proteins) at ZT2 than ZT14 (Figure 5B & Figure 3B).