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What does a positive allele for HD mean?

February 24, 2016

Huntington's Disease Youth Organization

HDYO has more information about HD available for young people, parents and professionals on our site:


What does a positive allele for HD mean?

Q. My mom has paper work that she showed a + allele for huntingtons. What does this mean?

Leanne, Young Adult, USA

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A. Dear Leanne,

Many thanks for your question. I am sorry that it is a bit difficult to give you a definite answer on this, as over the years different laboratories have sometimes used slightly different ways of reporting the results of huntington’s disease tests, and I would not want to give you incorrect information. The safest way to make sure that you get an accurate explanation of what the report means would be to contact the clinic or clinician who arranged the test (usually either a Genetic Counsellor or Doctor – their details may be on the report). However, if you were to contact them (rather than your mom) they would likely need to check that your mom had given her consent for this information to be released.

There should also be more information on the report itself that helps explain what the result means. Testing the HD gene involves counting the number of ‘CAG repeats’, and if both copies (alleles) of the gene have 26 or fewer CAG repeats (sometimes called the normal or unaffected range), that person would not be at risk of HD. However, if one allele has 40 or more repeats (called the ‘expanded’ range), this would be expected to cause the symptoms of Huntington’s disease to develop at some stage. This HDYO article has more detail about the HD gene and numbers of CAG repeats, and this may also help you understand the report.

I hope this is helpful, and please do not hesitate to get back in touch if you have any further questions.

Best wishes