Many of the human ABC proteins are efflux transporters, and three of them, namely P-glycoprotein (P-gp/MDR1, gene symbol ABCB1), the multidrug resistance protein 1 (MRP1, gene symbol ABCC1), and the breast cancer resistance protein (BCRP, gene symbol ABCG2), have been implicated as major efflux transporters responsible for multidrug resistance in cancer cells [99]. This evidence concerns the gene ABCG2 and cancer.