The X-linked neuroligin 4 gene (NLGN4X) encodes an adhesion protein that participates in the formation, organization, and remodeling of neuronal synapses. The NLGN4X gene is located at Xp22.3 and contains 6 exons. Various types of mutations in NLGN4X have been implicated in autism spectrum disorders and/or mental retardation (Daoud, H. et al. Biol Psychiatry 2009;66:906-910).