ABCC4 (Multidrug resistance protein 4 /MOAT-B) is a member of the MRP/ABCC subfamily of ATP-binding cassette transporters, capable of pumping a wide variety of endogenous and xenobiotic anionic compounds out of the cell. ABCC4 also transports molecules involved in cellular signalling, including ADP, cyclic nucleotides, eicosanoids, urate and conjugated steroids. ABCC4 utilises the energy generated from the hydrolysis of ATP for these transport activities. ABCC4 has a dual localisation in polarised cells that suggests a key function in cellular protection and extracellular signalling pathways. It is also highly expressed on platelet dense granule membranes where it is believed to contribute to the accumulation of dense granule content.