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Friedreich ataxia (FA) is a slowly progressive ataxia with typical onset between age 10-25 years. It is characterized by depressed tendon reflexes, speech disturbance (dysarthria), muscle weakness, visual disturbance due to optic nerve atrophy and spasticity. Individuals with FA often have diabetes mellitus and/or cardiomyopathy. For about 25% of affected individuals, the age of onset is later and progression of the disease is much slower than usual. FA is caused by a GAA expansion in the Frataxin gene. Normal GAA repeat length is 5-33 GAA repeats. 34-65 uninterrupted GAA repeats are mutable normal alleles. They do not cause disease, but may expand during transmission to the next generation and become disease-causing alleles. Disease-causing alleles have 66-1700 repeats. 96% of affected individuals are homozygous for a triplet repeat expansion. About 4% of affected individuals will have a GAA expansion on one allele and a point mutation on the other allele.
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