Multidrug Resistance 3 (MDR3), also known as ATP Binding Cassette Subfamily B Member 4 (ABCB4), is a membrane-associated protein belonging to the superfamily of ATP-binding cassette transporters. ABC proteins transport various molecules across extra- and intra-cellular membranes. ABC genes are divided into seven distinct subfamilies (ABC1, MDR/TAP, MRP, ALD, OABP, GCN20, White). This protein is a member of the MDR/TAP subfamily. Members of the MDR/TAP subfamily are involved in multidrug resistance.  MDR3 is an energy-dependent phospholipid efflux translocator that mediates the translocation of phosphatidylcholine across the canalicular membrane of the hepatocyte, and also acts as a positive regulator of biliary lipid secretion. Defects in MDR3 are associated with progressive familial intrahepatic cholestasis type 3 and gallbladder disease type 1. Co-overexpression of MDR3 and MRP1 has been documented as correlating with blastemal subtype and high-risk prognosis of Wilms' tumor patients.