Accumulating studies indicate that there are several major mechanisms of drug resistance in cancer cells, such as increased detoxification of anticancer drugs by glutathione system, defective apoptosis pathway, increased levels of DNA repair or DNA tolerance, decreased uptake of water-soluble drugs and enhanced drug efflux from cancer cells mediated by ATP-binding cassette (ABC) transporters [3–5]. Here, ABCG2 is linked to cancer.