Several Xp22.3 genes represent good candidates for conferring vulnerability to psychiatric illness: functional mutations within NLGN4X (encoding the synaptic neuroligin 4X protein) have been reported in cases of autism (Jamain et al., 2003; Laumonnier et al., 2004), whilst mutations within ASMT (encoding the acetylserotonin-O-methyltransferase enzyme involved in the biosynthesis of melatonin from serotonin) have been seen in individuals with autism (Melke et al., 2008; Jonsson et al., 2010) and ADHD (Chaste et al., 2011). The gene discussed is ASMT; the disease is attention deficit-hyperactivity disorder.