Resistance of tumors or infectious agents to a wide-range of therapeutic compounds. In cancer multidrug resistance ABC-MDR proteins actively expel the cytotoxic drugs from cells, maintaining the drug level below a cell-killing threshold. Drug extrusion mediated by these primary active transporters is driven by the energy of ATP hydrolysis. The most intriguing characteristic distinguishing the ABC-MDR proteins from other mammalian transporters is their wide substrate specificity. Unlike other selective (classical) transport proteins, multidrug transporters recognize and handle a wide range of substrates. This wide substrate specificity explains the cross-resistance to several chemically unrelated compounds, the characteristic feature found in the multidrug resistance phenotype.

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