ABCG2 and cancer: ABCG2 also plays an important role in the development of MDR in cancer cells by providing the effective efflux of a broad spectrum of chemotherapeutic agents, including anthracyclines, methotrexate, mitoxantrone, topotecan, irinotecan, flavopiridol, and camptothecin-derived topoisomerase I inhibitors [11,12].