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Infantile spasms, mental retardation, autism, and dystonia represent disabling diseases for which little etiologic information is available. Mutations in the Aristaless related homeobox gene (ARX) have been found in patients with these conditions. This discovery provides important genetic information and may ultimately offer treatment options for these patients.Recent work has demonstrated that mutations in ARX cause X-linked West syndrome, X-linked myoclonic epilepsy with spasticity and intellectual disability, Partington syndrome (mental retardation, ataxia, and dystonia), as well as nonsyndromic forms of mental retardation. Patients with these aforementioned diseases and ARX mutations were not reported to have brain imaging abnormalities. In contrast, mutations in ARX mutations have also been found in X-linked lissencephaly with abnormal genitalia, which typically includes severe brain malformations (lissencephaly, agenesis of the corpus callosum, and midbrain malformations), intractable seizures, and a severely shortened lifespan. ARX knockout mice manifest defects in overall neuroblast proliferation as well as selective abnormalities in γ-aminobutyric acid-ergic interneuron migration. Consistent with these findings in mice, phenotype/genotype studies in humans suggest that truncating mutations cause X-linked lissencephaly with abnormal genitalia, and insertion/missense mutations result in epilepsy and mental retardation without cortical dysplasia.Mutations in the homeobox gene, ARX, cause a diverse spectrum of disease that includes cognitive impairment, epilepsy, and in another group of patients severe cortical malformations. Although the precise prevalence of ARX mutations is unclear, ARX may rival Fragile X as a cause of mental retardation and epilepsy in males.