ABCG2 and cancer: Recently, many studies have shown a close relationship between the occurrence of breast cancer MDR and the expression of the ATP-binding cassette (ABC) transporter family, especially P-glycoprotein (P-gp/ABCB1), multidrug-resistance-associated protein 1 (MRP1/ABCC1), and the breast cancer resistance proteins (BCRP/ABCG2) (3, 4).