ABCG2 and cancer: Of the 48 classified ABC transporters, the three most widely studied and characterized efflux glycoproteins, namely, P-glycoprotein (also known as Pgp or ABCB1), multidrug resistance protein 1 (also known as MRP1 or ABCC1), and breast cancer resistance protein (BCRP) (also known as BCRP or ABCG2), have significant roles in causing multidrug resistance in cancer.