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The age at which symptoms usually appear depends on the nature of the mutations in the Huntington's Disease gene. It codes for a protein called huntingtin, which has poorly understood but essential functions in the brain.
The gene contains what are called CAG repeats - all people have CAG repeats in the gene but it is the number of them that is important:
Somebody with 25 or fewer repeats will not get the disease.
Somebody with up to 35 repeats will not get the disease but their children are at slightly greater risk of it.
Somebody with 36-39 repeats may get the disease but, if so, not until old age.
Somebody with 40 or more repeats will get the disease but when symptoms appear depends on the number of repeats - symptoms usually appear in middle age for people with between 40 and 45 repeats though there is considerable variation in this.
People with 60 or more repeats will show symptoms in their teens - this is called Juvenile Huntington's Disease.
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