ABCB11 is involved in the ATP-dependent secretion of bile salts into the canaliculus of hepatocytes. It is expressed predominatly, if not exclusively, in the liver, where it is further localized to the canilicular microvilli and to subcanilicular vesicles fo the hepatocytes. Structurally, ABCB11 is a multifunctional polypeptide with two homologus halves, each containing a hydrophobic membrane-anchoring domain and an ATP binding cassette (ABC) domain. Defects in ABCB11 are the cause of progressive familial intrahepatic cholestasis 2 (PFIC2). PFIC2 is an inherited liver disease of childhood which is characterized by cholestasis and normal serum gamma-glutamyltransferase activity. Defects in ABCB11 are also found in cases of chronic intrahepatic cholestasis without obvious familial history of chronic liver disease.