The number of CAG repeats in the HTT gene has a number of implications for a patient from a family with Huntington Disease (HD).
Each patient will have 2 HTT alleles. Inheritance of 1 mutated allele is sufficient to cause Huntington Disease.
Number of CAG Repeats
Risk of Clinical HD in Patient
Risk of HD-Causing Allele in a Single Child
27 to 35
low to moderate
36 to 39
HD-causing, reduced penetrance
variable (onset may be late)
high (usually 50%)
HD-causing, full penetrance
• A HD-causing allele is any with >= 36 CAG repeats.
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Specialty: Genetics, Neurology