Neuroligins are Type I membrane proteins enriched in synaptic plasma membranes and clustered in synaptic clefts and postsynaptic densities. They have been characterized as neuronal cell surface proteins and are thought to be involved in cell-cell-interactions by forming intercellular junctions through binding to beta-neurexins. They play a major role in the formation or maintenance of synaptic junctions. They are also thought to be involved in the specification of excitatory synapses. Neuroligins interact with beta-neurexins and this interaction is involved in the formation of functional synapses.