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Rett syndrome is a progressive neurologic disorder in girls characterized by normal birth and apparently normal psychomotor development during the first six to 18 months of life. The girls then enter a short period of developmental stagnation followed by rapid regression in language and motor skills. The hallmark of the disease is the loss of purposeful hand use and its replacement with repetitive stereotyped hand movements. Screaming fits and inconsolable crying are common by age 18-24 months. Additional characteristics include autistic features, panic-like attacks, bruxism, episodic apnea and/or hyperpnea, gait ataxia and apraxia, tremors, and acquired microcephaly. After this period of rapid deterioration, the disease becomes relatively stable, though girls will likely develop dystonia and foot and hand deformities as they grow older. Seizures occur in up to 90% of affected females; generalized tonic-clonic seizures and partial complex seizures are the most common. Females with classic Rett syndrome typically survive into adulthood, but the incidence of sudden, unexplained death (which may be caused by cardiac arrhythmias) is significantly higher than in controls of similar age. The diagnosis of Rett syndrome rests on clinical diagnostic criteria established for the classic syndrome and/or molecular testing of the MECP2 gene. Molecular genetic testing identifies MECP2 mutations in approximately 80% of females with classic Rett syndrome. The lab identifies mutations in the MECP2 gene by Sanger sequencing.