MRPs belong to a subfamily of the ATP binding cassette (ABC) transporter family [9, 10]. In higher animals, MRP4 is a unidirectional and distinctive bilaterally localized transporter in polarized cells, such as baso-lateral membrane of hepatocytes  and choroid plexus epithelial cells [12, 13]. Such tissue-specific distribution suggests that MRP4 has multiple functions. MRP4 is responsible for pumping out a broad range of substrates, including bile acids, as well as for physiological regulation via transport of cyclic nucleotides out of cells .