NLGN4X and attention deficit-hyperactivity disorder: Recent studies have shown an association of XLI with behavioral disorders, including autism, attention deficit-hyperactivity disorder (ADHD), and social communication deficits; however, in the affected subjects, large gene deletions in the proximity of the STS locus have been found that included the NLGN4 gene encoding neuroligin 4, a synaptic peptide that has been previously implicated in X-linked autism and mental retardation (228).