ABCG2 and cancer: Three human MDR-type ABC transporters (P-glycoprotein (ABCB1), multidrug resistance associated protein-1 (ABCC1/MRP1) and breast cancer resistance protein (ABCG2/BCRP)) have been the subject of intensive investigation both to understand their contribution to cancer MDR and to understand the protein biochemical mechanisms of multidrug recognition and export [[5], [6], [7]].