A characteristic of cells that makes them resistant to the effects of several different classes of drugs. There are several forms of drug resistance. They are each determined by genes that govern how the cell will respond to the chemical agents. One type of multidrug resistance involves the ability to force several drugs out of cells. The outer wall, or membrane, of the cell contains a pump that ejects chemicals, preventing them from reaching a toxic concentration. The resistance to drugs can be traced to the expression of genes that direct the formation of high amounts of the protein that prevents the drugs from affecting the malignant cells. If the gene or genes involved are not expressed or are weakly expressed, the cells are more sensitive to the drug's effect. If the genes are highly expressed, the cells are less sensitive to the drug's effect.