A characteristic of cells that make them resistant to the effects of several different classes of drugs. These are several forms of multidrug resistance. They each are determined by genes that govern how the cell will respond to the chemical agents. The first identified mechanism of multidrug resistance (or MDR) involves the cell’s ability to pump several drugs out of cells. A pump in the cell wall rapidly rejects drugs out of the cell preventing them from reaching a toxic concentration. In cells, 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 form having their effects in the malignant cells.