The Cdc42 guanine nucleotide exchange factor (GEF) 9 gene (ARHGEF9) encodes collybistin, a Rho GTPase that has been implicated in regulating localization of gephyrin, thereby GABAA and glycine receptors, in neuronal synapses. TheARHGEF9 gene is located at Xq11.1 and contains 10 exons. A missense mutation and complex rearrangements that disrupt the ARHGEF9 gene, have been reported in patients with mental retardation, sometimes in conjunction with epileptic seizures (Harvey K et al. J Neurosci. 2004;24(25): 5816-5826, Marco EJ et al. J Med Genet. 2008;45:100-105, and Kalscheuer VM et al. Hum Mutat. 2009;30:61-68).