Laboratory investigations indicate that cancer cells can become simultaneously resistant to many different chemotherapeutic drugs that are natural products via the expression of an energy-dependent drug efflux pump. This multidrug transporter is a plasma membrane glycoprotein encoded in the human by the MDR1 gene. Recent clinical studies indicate that expression of the multidrug transporter plays a major role in the intrinsic and acquired resistance to chemotherapy of many human cancers. Strategies aimed at inactivating this drug efflux pump may have significant impact on the treatment of human cancer.