Cell surface protein involved in cell-cell-interactions via its interactions with neurexin family members. Plays a role in synapse function and synaptic signal transmission, and probably mediates its effects by recruiting and clustering other synaptic proteins. May promote the initial formation of synapses, but is not essential for this. May also play a role in glia-glia or glia-neuron interactions in the developing peripheral nervous system. Interacts with NRXN1, NRXN2 and NRXN3. Interacts (via its C-terminus) with DLG4/PSD-95 (via PDZ domain 3) (By similarity). Homodimer, and heterodimer with NLGN1 and NLGN2 (By similarity). Detected at both glutamatergic and GABAergic synapses. Detected in brain and on hippocampus neurons, especially at excitatory synapses. Detected in retina (at protein level). Expressed in brain, spinal cord and dorsal root ganglion. The N-terminus is blocked. Belongs to the type-B carboxylesterase/lipase family.