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. This evidence concerns the gene ABCC1 and cancer.