Multiple drug resistance(MDR) is a major clinical obstacle in cancer chemotherapy. Acquirement of MDR phenotype in cancer cells is often associated with enhanced expression of human MDR-1 gene: MDR-1 gene codes membranous P-glycoprotein which catalyses energy-dependent outward transport of anticancer agents. By contrast, MDR cancer cell lines without overexpression of P-glycoprotein are called as atypical MDR (at MDR) cells. The acquirement of at MDR has been shown to be partly associated with altered DNA topoisomerase II. Furthermore, a new ATP binding cassette (ABC) family, MRP gene has just recently shown to involve in acquirement of at-MDR in cancer cell lines, which do not express both altered topoisomerase II and P-glycoprotein.