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rare progressive neurological disorder that causes mental retardation, compulsive hand movements, reduced muscle tone, difficulties in walking, autism, decreased body weight, failure of the head to grow with age, and the increased presence of ammonia in the blood (hyperammonemia). Rett syndrome causes progressive disabilities in intellectual and motor development after about the first year of life. It is a sex-linked hereditary disorder that exclusively affects girls and occurs in about one of every 15,000 female births. The disease was identified by the Austrian physician Andreas Rett. There is no cure for Rett syndrome; however, the majority of individuals with the disorder reach middle adulthood.