These findings were generated through the development of humanized UGT1 (hUGT1) mice,14, 15 which express the entire human UGT1 locus in a murine Ugt1-null background.16 Taking advantage of the power of reverse genetics, it will be shown that PXR plays a crucial role in pregnancy-induced glucuronidation in addition to the early development of hyperbilirubinemia in neonatal hUGT1 mice. Here, UGT1A4 is linked to Hyperbilirubinemia.