BEGAIN is a 593 amino acid protein that localizes to cytoplasm and membrane. BEGAIN interacts with PSD-95 and SAPAP1 and forms a ternary complex and may sustain the structure of the postsynaptic density (PSD). BEGAIN is a novel PSD component associated with the core complex of PSD-95 and SAPAP. Because BEGAIN and SAPAP interact with the same region of PSD-95, BEGAIN and SAPAP may compete for the binding to PSD-95 and cannot interact with PSD-95 simultaneously. The C-terminal region of BEGAIN is involved in the interaction with PSD-95 whereas the N-terminal region has a coiled-coil structure that may interact with other molecules. BEGAIN is specifically expressed in brain and enriched in the PSD fraction. BEGAIN is also expressed in neurons and enriched at synaptic junctions, and is likely involved in the organization of synaptic junction components.