Required, with CNTNAP2, for radial and longitudinal organization of myelinated axons (PubMed:25378149). Plays a role in the formation of functional distinct domains critical for saltatory conduction of nerve impulses in myelinated nerve fibers. Demarcates the paranodal region of the axo-glial junction. In association with contactin involved in the signaling between axons and myelinating glial cells (PubMed:25378149, PubMed:11395000). Interacts with CNTN1/contactin in cis form. Expressed in brain. In myelinated nerve fibers predominantly found in paranodal axoglial junctions. In the internodal region of myelinated axons in the CNS and the PNS also found as a thin line apposing the inner mesaxon of the myelin sheath. In PNS neurons this line forms a circumferential ring that apposes the innermost aspect of Schmidt-Lanterman incisures. Mutant mice exhibit tremor, ataxia, and significant motor paresis. Normal paranodal junctions fail to form, and the organization of the paranodal loops is disrupted. Contactin is undetectable in the paranodes, and potassium channels are displaced from the juxtaparanodal into the paranodal domains. Also results in a severe decrease in peripheral nerve conduction velocity (PubMed:11395000, PubMed:25378149). Double mutants CNTNAP1 and CNTNAP2 have wider Ranvier nodes compared to wild-type littermates (PubMed:25378149). Belongs to the neurexin family.