ABCG2 and breast carcinoma: In the case of DOX treatment in breast cancer, MDR reduces intracellular drug accumulation by activating the efflux of chemotherapeutic agents through membrane transporters, such as the ATP-binding cassette (ABC) proteins, which mainly include p-glycoprotein (Pgp, MDR1, ABCB1 (ATP-binding cassette sub-family B member 1)), the multidrug resistance protein 1 (MRP1, ABCC1 (ATP-binding cassette sub-family C member 1)), and the breast cancer resistance proteins (BCRP, ABCG2 (ATP-binding cassette sub-family G member 2)) [2].