Phosphodiesterase 9A is a cGMP-specific phosphodiesterase. PDE9A activity is inhibited by zaprinast, but it is insensitive to rolipram, vinpocetine, SKF-94120, dipyridamole, and IBMX. More than 20 alternatively spliced variants have been reported (Rentero et al. 2003). PDE9A has been implicated in penile erectile dysfunction, as well as in functional disturbances in which intraneuronal signal transmission via second messengers are important in the pathophysiology (example, bipolar affective disorder). PDE9A is the only cGMP-specific PDE with significant expression in the forebrain, and as such is likely to function in NO-cGMP signaling.