NMDA receptors are multimeric glutamate-gated cation channels, which are major constituents of the postsynaptic density (PSD). PSD-95/SAP90 protein is a member of the membrane-associated guanylate kinase (MAGUK) family and a prominant component of the PSD and associates with NMDA receptors. The interaction is mediated by binding of the C-terminus of the NMDA receptor subunits to the first two PDZ (also known as GLGF or DHR) domains of PSD-95 (Kornau et al. 1995, Niethammer et al. 1996). PSD-95 acts as a scaffolding protein in NMDA receptor signalling by bringing together NMDA receptor and various proteins like enzyme neuronal nitric oxide synthase (nNOS) (Brenman et al. 1996), SynGAP (Kim et al. 1998), GKAP (Kim et al. 1997), SHANK (Naisbitt et al. 1999) and multiple non-receptor tyrosine kinases (Sala & Sheng 1999). In this way, the multidomain PSD-95 molecule connects NMDA receptors to a variety of intracellular signaling proteins and anchors the whole complex to the postsynaptic density. PSD-95 subfamily includes other three more members: PSD-93/chapsyn-110, SAP97/hDlg, and SAP102. All except SAP97 appear to be components of the PSD and associated with NMDA receptors.