Rather than developing synthetic inhibitors of ABCB1 and ABCG2, we and others have focused on the repositioning of molecularly targeted therapeutic agents to modulate the function and/or expression of ABCB1 and ABCG2, which is an alternative approach to resensitize multidrug-resistant cancer cells to conventional chemotherapeutic agents [11,17,20,34,56,57,58,59,60,61]. This evidence concerns the gene ABCG2 and cancer.