The adaptor-related protein complex 1, sigma 2 subunit gene (AP1S2) encodes a component of the AP1 adaptor complex, which is involved in trafficking of membrane proteins in eukaryotes. Since AP1 has been shown to associate with synaptophysin, AP1S2 has been implicated in neurotransmitter processing and release. The AP1S2 gene is located at Xp22 and contains 4 coding exons. Nonsense and splicing mutations, as well as a small deletion, in the AP1S2gene have been detected in patients with variable degree of mental retardation. Other associated clinical features observed in these patients include hypotonia, delays in walking and speech, aggressive behavior, brain calcification, and elevated levels of protein in the cerebrospinal fluid (Borck G et al. Hum Mutat. 2008;29(7):966-974).