ABCG2 and cancer: The major mechanism of pump resistance is involved in the increased ability of cancer cells to actively efflux drugs, which is induced by the ATP-binding cassette (ABC) superfamily of membrane transporters including ABCB1 (multidrug resistance protein 1, P-gp/MDR1), ABCC (multidrug resistance-associated protein, MRP) subfamily, and ABCG2 (breast cancer resistance protein, BCRP) [10].