ABCB4 (MDR3) is a member of the ABC transporter superfamily. ABC transporters comprise a large number of membrane proteins, which mediate the translocation of a variety of molecules across cellular membranes. MDR3 is a transporter expressed at the canalicular membrane of hepatocytes, where it translocates phosphatidylcholine (PC) in bile in order to protect the hepatobiliary epithelial against the damage from free bile acids^1-3.

Genetic variants in the ABCB4 gene sequence are associated with rare biliary diseases, in particular progressive familial intrahepatic cholestasis type 3. Different phenotypes are detected in ABCB4 mutations such as, low phospholipid‐associated cholelithiasis and intrahepatic cholestasis of pregnancy^1-3.

ABCB4 is a polytopic transmembrane glycoprotein with two transmembrane modules each spanning the membrane six times and two ATP-binding domains. The protein possesses a conserved C-terminal region. The presence of a hydrophobic amino acid at the C-terminus suggests properties of a PDZ-binding-like motif^2,3.