"We describe a novel mechanism of signal transduction enriched in medium spiny neurons of striatum that likely mediates effects of the neurotransmitter dopamine acting on these cells. We find that the protein ARPP-16 which is highly expressed in striatal medium spiny neurons acts as a selective inhibitor of certain forms of the serine/threonine protein phosphatase PP2A when phosphorylated by the kinase MAST3."xsd:string