Multidrug resistance proteins (MRPs) are a subset of the ATP-binding cassette genes that are phase III detoxification proteins involved in transporting exogenous and endogenous compounds across cellular membranes. Energy-dependent efflux pump responsible for decreased drug accumulation in multidrug-resistant cells. The MRPs characterized thus far are organic anion transporters. Many good substrates for these transporters are highly charged and do not penetrate the cell membrane.