Members of the postsynaptic density-95 (PSD-95)/SAP90 family of membrane-associated guanylate kinase (MAGUK) proteins function as multimodular scaffolds that organize protein-signaling complexes at neuronal synapses. PSD-95/SAP90 binds guanylate kinase-associated protein (GKAP), also designated GK domain-binding protein, DAP-1-a, DAP-1-b, PSD-95 binding protein, PSD-95/SAP90 associated protein, or SAPAP, through the guanylate kinase domain. GKAP is expressed widely in neurons of the cortex and hippocampus and in the Purkinje and granule cells of the cerebellum. GKAP is localized specifically in the PSD of glutamatergic synapses, consistent with its direct interaction with PSD-95 family proteins.