Drug resistance is a major problem in cancer chemotherapy. P-glycoprotein plays a major role in multidrug resistance in cancer cells. P-glycoprotein is expressed in some normal tissues and has physiological functions. These include protecting the brain against toxic substances at the blood-brain barrier site, excreting toxic substances from the liver, kidney, and gastrointestinal tracts, and transporting steroidal hormones in the adrenal grand. Once expressed in cancer cells, P-glycoprotein effluxes a variety of anticancer drugs, such as doxorubicin, vinca alkaloids, etoposide and taxol, and thereby allows cancer cells to show resistance to these drugs.