MDR1 is an ATP-dependent efflux pump with broad substrate specificity meaning it is an important protein of the cell membrane that pumps many foreign substances out of cells. It is extensively distributed and expressed in the intestinal epithelium, in liver cells, in the cells of the proximal tubule of the kidney, and in the capillary endothelial cells composing the bloodÐbrain barrier and blood-testis barrier. Some cancer cells express large amounts of MDR1, further amplifying the effect of reducing the efficacy of some pharmaceutical drugs, and rendering these cancers multidrug resistant.