ABCG2 and cancer: Today, the most extensively studied and characterized ABC transporters, found to be involved in cancer MDR, are (a) multidrug resistance protein 1/P-glycoprotein (MDR1/P-gp), encoded by the ABCB1 gene; (b) multidrug resistance-associated protein 1 (MRP1), encoded by the ABCC1 gene; and (c) breast cancer resistance protein (BCRP), encoded by the ABCG2 gene [8].