ABCG2 and breast cancer: Mechanistically, resistance to chemotoxic agents in breast cancer patients is most often associated with the action of ATP-binding cassette (ABC) drug transporters, which include ABC transporter-subfamily B member 1 (ABCB1, or P-glycoprotein/P-gp), subfamily C member 1 (ABCC1, or multidrug resistance-associated protein 1/MRP1) and subfamily G member 2 (ABCG2, or breast cancer resistance protein/BCRP) [2,4].