Definition of Huntington's disease:

Huntington's disease is a disorder passed down through families in which nerve cells in certain parts of the brain waste away, or degenerate.

Causes, incidence, and risk factors:

Huntington's disease is caused by a genetic defect on chromosome 4. The defect causes a part of DNA, called a CAG repeat, to occur many more times than it is supposed to. Normally, this section of DNA is repeated 10 to 28 times. But in persons with Huntington's disease, it is repeated 36 to 120 times.

As the gene is passed down through families, the number of repeats tend to get larger. The larger the number of repeats, the greater your chance of developing symptoms at an earlier age. Therefore, as the disease is passed along in families, symptoms develop at younger and younger ages.

There are two forms of Huntington's disease.

• The most common is adult-onset Huntington's disease. Persons with this form usually develop symptoms in their mid 30s and 40s.
• An early-onset form of Huntington's disease accounts for a small number of cases and begins in childhood or adolescence.

If one of your parents has Huntington's disease, you have a 50% chance of getting the gene for the disease. If you get the gene from your parents, you will develop the disease at some point in your life, and can pass it onto your children. If you do not get the gene from your parents, you cannot pass the gene onto your children.