MECP2 duplication syndrome is characterized by moderate to severe intellectual disability. Most boys with this condition also have weak muscle tone, feeding difficulties, poor or absent speech, seizures that may not improve with treatment, or muscle stiffness (spasticity). Individuals with MECP2 duplication syndrome have delayed development of motor skills such as sitting and walking. Many individuals with MECP2 duplication syndrome have recurrent respiratory tract infections. The respiratory infections are a major cause of death, with almost half succumbing by age 25.