The PSD-95/SAP 90 family of proteins, which are known to bind to and cluster various membrane proteins, are involved in the organization of synaptic structure. SynGAP (for synaptic GTPase activating protein), a novel synaptic RasGAP, is a brain-specific protein abundant in the cortex, hippocampus and olfactory bulb. SynGAP interacts with all three PDZ domains within PSD-95/ SAP 90 proteins. It represents one of three classes of mammalian RasGAPs and is specifically localized to excitatory synapses possessing NMDA receptors. SynGAP may be involved in the regulation of BDNF as well as Ras signaling. Its activity is inhibited by phosphorylation by CaMKII, which could result in the activation of the MAP kinase pathway.