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Spinal muscular atrophy (SMA) causes progressive muscular weakness and degeneration by loss of anterior horn cells in the brain stem and spinal cord. Age of onset can vary from prenatal onset to adult onset: SMA I, with onset before six months of age; SMA II, with onset between six and 12 months; SMA III, with onset in childhood after 12 months; and SMA IV, with adult onset. Approximately 95% of individuals with SMA are homozygous for a deletion of exons 7 & 8 of the SMN1 gene. This test will only look for deletions in exons 7 & 8.
About 5% of individuals with SMA have the deletion on one allele and a point mutation on the other allele. These individuals will have normal results by this method. For individuals with a high clinical suspicion of SMA but normal test results, SMA dosage testing by MLPA is recommended.
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