Multidrug resistance (MDR) is a major impediment to the success of cancer chemotherapy.  Multidrug resistance-associated proteins (MRPs) play an important role in the process of MDR.  As an ATP- binding cassette (ABC) transporter superfamily, MRPs are selective and specific drug efflux pumps.  In this paper, physiological characteristics, structural characteristics and resistance profile of MRPs and the associated reversal studies are reviewed.