Mdr-3 Background Information Cells selected for resistance to a single cytotoxic drug may become cross-resistant to a broad range of drugs having different structures and cellular targets. This phenomenon is called multiple drug resistance (MDR). The MDR proteins (Mdrs) are members of a highly conserved superfamily of ATP-binding cassette transport proteins. Mdr functions as an energy-dependent efflux pump for structurally diverse agents ranging from ions to peptides and is implicated in the development of the multiple drug resistance observed in human cancer cells following prolonged chemotherapy. The classic form of MDR is associated with an increase in the Mdr protein, but not all cases of MDR can be attributed to a rise in Mdr levels. Mdr-3 (also known as ABCB4) is a member of the Mdr family that may be associated with a more malignant phenotype in B cell lymphocytic leukemias.
