Mutations in the Aristaless-related homeobox gene (ARX) may promote an encephalopathic phenotype with either normal imaging (X-linked West syndrome, X-linked myoclonic epilepsy with spasticity and intellectual disability, Partington syndrome with mental retardation, ataxia and dystonia, and nonsyndromic forms of mental retardation) or associated to brain malformations (X-linked lissencephaly with abnormal genitalia). The gene discussed is ARX; the disease is Intellectual disability.