ABCB1 and cancer: ATP-binding cassette (ABC) transporters such as p-glycoprotein (PGP, ABCB1), multidrug resistance protein 1 (MRP1, ABCC1) and breast cancer resistance protein (BCRP, ABCG2) have been reported to be closely related to drug resistance, and they are usually overexpressed in cancer patients who are not responsive to chemotherapy drugs.