When someone has Huntington's disease, part of the DNA sequence:        

(CAG: Cytosine — Adenine — Guanine) repeats more times than normal.

It is important to understand that everyone on the planet is born with the Huntingtin gene - CAG repeat. The healthy gene repeat averages around 17, however, a person's CAG repeat can range from less than 10 to more than 120. There are two forms of this disease: Adult on-set Huntington's disease, and Juvenile Huntington's disease. Please review the resources on this page for more information about CAG.