ABCG2 and cancer: ABCB1 and ABCG2 are capable of utilizing energy derived from ATP hydrolysis to actively transport a large majority of conventional cytotoxic chemotherapeutic drugs including but not limited to, etoposide, anthracyclines, Vinca alkaloids, methotrexate, paclitaxel, topotecan, SN-38 and mitoxantrone, out of cancer cells and consequently reduced the intracellular accumulation of these drugs [2,5,6].