ABCG2 and neoplasm: Recently, several studies have discovered the relationship of the ATP-binding cassette (ABC) transporter family with the chemosensitivity of tumor cells, especially P-glycoprotein (P-gp, ABCB1), multidrug-resistance-associated protein 1 (MRP1, ABCC1), and the breast cancer resistance proteins (BCRPs, ABCG2).3