Neuroligin 4, X-Linked (NLGN4X) is a single-pass type I membrane protein that belongs to the type-B carboxylesterase/lipase family. NLGN4X is detected at higher levels in heart and at lower levels in the liver, skeletal muscle, and pancreas. NLGN4X is a putative neuronal cell surface protein involved in cell-cell-interactions. NLGN4X may act as splice site-specific ligands for β-neurexins. It has been shown that NLGN4X is involved in the formation and remodeling of central nervous system synapses. NLGN4X also interacts with discs, large (Drosophila) homolog 4 (DLG4). Defects in NLGN4X have been associated with autism and Asperger syndrome.