Furthermore, within the ABCB, C and G subfamilies, many genes code for proteins that contribute to resistance of cancer cells against chemotherapeutic agents: the multidrug resistance proteins or P-glycoproteins (MDR or P-gps, members of the ABCB subfamily), the multidrug resistance-associated proteins (MRP, members of the ABCC subfamily) and the breast cancer protein (BCRP or human ABCG2) [19,20]. The gene discussed is ABCG2; the disease is cancer.