MRP4 (multidrug resistance protein 4) is a member of the ABC (ATP-binding cassette) superfamily of transporters and the MRP (multidrug resistance protein) subfamily involved in multi-drug resistance. MRPs can confer resistance to many drug and chemical agents. They have become clinically important due to their possible involvement in the resistance to chemotherapeutic agents in cancer. MRP4 was first identified as a transporter that could efflux antiretroviral compounds. The physiological functions of MRP4 have been proposed to be related to bile-acid homeostasis and cAMP-dependent signaling.