Implications of the Number of CAG Repeats in an HTT Allele

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	Allele	Risk of Clinical HD in Patient	Risk of HD-Causing Allele in a Single Child
<= 26	normal	none	none (0%)
27 to 35	intermediate	none	low to moderate
36 to 39	HD-causing, reduced penetrance	variable (onset may be late)	high (usually 50%)
>= 40	HD-causing, full penetrance	very high	high (usually 50%)

where:

• A HD-causing allele is any with >= 36 CAG repeats.

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