Involved in the development and/or maintenance of neuronal circuitry. Accessory subunit of the neuronal N-methyl-D-aspartate receptor (NMDAR) critical for maintaining the abundance of GRIN2A-containing NMDARs in the postsynaptic density. Regulates long-term NMDA receptor-dependent synaptic plasticity and cognition, at least in the context of spatial learning and memory (By similarity). Interacts with PLZ domains of DLG2, DLG3 and DLG4 via its C-terminal TRV domain. Interacts with GRIN2A and GRIN2B via its CUB domains (By similarity). Component of the postsynaptic density (PSD) of excitatory synapses. Isoform 1 and isoform 2 are retina-specific. Isoform 3 is found in retina as well as at lower levels in adult and fetal brain.