Rett syndrome is a progressive, neuro-developmental disorder that affects approximately 1 in 10,000 females. Classic Rett syndrome is diagnosed based on a defined set of clinical criteria and characterized by apparently normal development in the first 6-18 months, followed by an arrest in development and subsequent regression in language and motor skills. Frequent symptoms include loss of speech and purposeful hand use, stereotypic hand movements, ataxia, microcephaly, and seizures. “Atypical” Rett syndrome can be milder or more severe than typical Rett syndrome and is diagnosed when some but not all clinical criteria for Rett syndrome are present. The milder form may include mental retardation, mild learning disablilities and/or autism. Mutations in the MECP2 gene have been found to cause Rett syndrome and “atypical” Rett syndrome in females. In males, MECP2 mutations are not as common and responsible for a broad spectrum of neurodevelopmental phenotypes, ranging from severe neonatal encephalopathy to a variety of neuropsychiatric features or mild mental retardation. Rarely, males with a progressive neurodevelopmental syndrome, including mental retardation, spasticity, speech and social problems, have been found to have a duplication or triplication of the MECP2 gene.