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Genes responsible for multidrug resistance resulting from their overexpression in mammalian cells. Mammalian P-glycoproteins are encoded by small MDR gene familes. The human multidrug resistance 1; (MDR1); gene responds to environmental stress including various anticancer agents. It is a major determinant in the development of resistance to a large number of cancer chemotherapeutic agents. (Biochem Biophys Res Commun 1994;199 (3):1428-35; Cancer Res 1994:54 (6):1536-41)