Genetic testing allows an individual who is at-risk of having Huntington's Disease know with certainty if they have the disease. Every individual has two copies of each gene, one inherited from the mother and the other from the father. Huntington's disease patients have an unusually high number of CAG repeats in either of the two HTT gene copies.
Those with a repeat of 26 or fewer are "normal." A repeat of between 27 to 35 are in "the intermediate range." In this area, although the patient will not develop the disease, they are still at-risk of passing it on to they children. With CAG repeats between 36-39, it cannot be predicted with certainty whether the individual will develop Huntington’s disease. CAG repeats that are 40 and above are considered "gene-positive." This means at some point in your life you will develop the symptoms of the disease, but there currently is no way to know for certain when and how severely the onset of those symptoms will come.
Children who become symptomatic at a young age and have a very unusually high CAG repeat number have what is known as Juvenile Huntington's Disease. This is a children's version of the disease that has a much earlier onset age and a more rapid onset of symptoms. Most JHD patients usually have between 80 and 100 CAG repeats.