Acts as a synaptic organizer in specific subsets of neurons in the brain (PubMed:29691328, PubMed:21410790). Essential for long-term maintenance but not establishment of excitatory synapses (PubMed:29691328 PubMed:30287486). Homohexamer; disulfide-linked homotrimers. The trimers are assembled via the globular C1q domains. The trimers associate via N-terminal cysteine residues to form disulfide-linked hexamers (By similarity). May form homooligomers or heterooligomers with CBLN1 and CBLN3 prior to secretion (PubMed:29782851). Once secreted, does not interact with other CBLN family members. Interacts with GRID2, and more weakly with GRID1. Interacts with NRXN1 and NRXN2 long and short isoforms produced by alternative promoter usage (PubMed:21410790, PubMed:22220752, PubMed:29782851). Weakly interacts with NRXN3 short isoform and not at all with NRXN3 long isoform (PubMed:22220752, PubMed:29782851). Expressed in various brain regions with higher levels in the olfactory bulb, cerebral cortex, certain thalamic and hypothalamic nuclei, superior and inferior colliculi and some brainstem nuclei. Highly expressed in the dorsal medial habenula. In the developing brain, expressed as early as 10-13 dpc. Expression level peaks at 18 dpc and gradually decreases afterwards. Mice show impaired synaptic transmission within 3 weeks in interpeduncular target neurons whereas significant decrease in both the synapse density and size seen only after 3 months (PubMed:30287486). Display impaired passive avoidance learning, spatial learning and short-term memory (PubMed:30287486). Double CBLN1 and CBLN2 knockout mice exhibit gait abnormalities, impairments in balance and coordination and develop seizures (PubMed:29691328). Synapse density in the hippocampus is normal in 1-2 months old mice, but severely decreased in 6 month old mice (PubMed:29691328). Triple CBLN1, CBLN2 and CBLN4 knockout mice exhibit impairments in sensory processing and sensorimotor gating, in addition to severe motor deficits, seizures and reduced synapse density in the hippocampus of aging mice (PubMed:29691328).