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Methyl-CpG-binding protein 2 (MECP2) deleterious variants, which are responsible for Rett syndrome in girls, are involved in a wide spectrum of developmental disabilities in males. A neuropsychiatric phenotype without intellectual disability is uncommon in patients with MECP2 deleterious variants. We report on two dizygotic twins with an MECP2-related psychiatric disorder without intellectual disability. Neuropsychological and psychiatric phenotype assessments were performed, and a genetic analysis was carried out. Both patients fulfilled the Pervasive Developmental Disorder criteria on Autism Diagnostic Observation Schedule and Asperger syndrome criteria on Diagnostic and Statistical Manual of Mental Disorders, 4th ed. (DSM-IV). One patient developed early-onset schizophrenia (DSM-IV criteria) with two acute psychotic episodes, the latest one following corticosteroids and sodium valproate intake, with major hyperammonemia. A novel MECP2 gene transversion c.491 G>T [p.(Ser164Ile)] was found in both twins. Pathogenicity of this variant was considered on the basis of strong clinical and molecular data. The underlying molecular basis of neuropsychiatric disorders may have important consequences on genetic counseling and therapeutic strategies.