MDR stands for multidrug resistance. Proteins which confer multidrug resistance are often endogenous culprits that inhibit the successful decrease/destruction of cancer cells. The major multidrug resistance protein is P-gp, or P-glycoprotein. It is encoded by the gene mdr1. High amounts of P-gp confer drug resistance by acting as an ATP-driven efflux pump. By reducing the amount of P-gp expressed, chemotherapy effectiveness increases. It is hypothesized that P-gp's normal function is to toss non-toxic molecules across the cell membrane as well as acting as an inducer for a Chloride pump.