Chapsyn-110, is a part of the membrane-associated putative guanylate kinase (MAGUK) family. It binds directly to the NMDA receptor, and Shaker K+ channel subunits, and is 70-80% identical to PSD-95/SAP90 and SAP97 (1). It associated tightly with the postsynaptic density I brain, and mediates the clustering of both NMDA recpetors and K+ channels in heterologous cells. The encoded protein forms a heterodimer with PSD-95 that may interact at postsynaptic sites to form a multimeric scaffold for the clustering of receptors, ion channels, and associated signaling proteins (1, 2).