DARPP-32 (dopamine and cyclic AMP-regulated phosphoprotein) is a cytosolic protein highly enriched spiny neurons of the neostriatum. DARPP-32 is a bifunctional signal transduction molecule that controls serine/threonine kinase and serine/threonine phosphatase activity depending on the phosphrylation state of the protein. Dopamine stimulates phosphorylation of DARPP-32 through D1 receptors and activation of PKA. PKA phosphorylation of DARPP-32 at threonine 34 converts it into an inhibitor of protein phosphatase 1. DARPP-32 is converted into an inhibitor of PKA when phosphorylated at threonine 75 by cyclin-dependent kinase 5 (cdk5). Mice containing a targeted deletion of the DARPP-32 gene exhibit an altered biochemical, electrophysiological and behavioral phenotype.