The mutation responsible for Huntington's disease is a polymorphic (CAG)n repeat sequence which is expanded on affected chromosomes. The number of repeat units observed on 229 affected chromosomes varied from 27 to 102, while the control chromosomes showed a range of 7 to 34 repeats. There was a highly significant relationship between the size of the expanded region and age at onset, larger mutations being associated with earlier onset. This association was strongest in those with onset before 25 years of age but less clear cut with later onset, and is therefore unlikely to be useful for predicting age at onset in the context of presymptomatic testing.
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