Huntington's disease is a rare inherited disorder that causes the progressive breakdown (degeneration) of nerve cells in the brain. Huntington's disease has a major impact on human functional ability and typically results in movement, thinking (cognitive) and mental disorders.
HD is caused by a mutation in the gene for a protein called huntingtin. The defect causes the DNA building blocks cytosine, adenine, and guanine (CAG) to repeat themselves much more frequently than normal. Most people have fewer than 27 CAG repeats in the HD gene, so they are not at risk of developing the disease. That is the reason it makes huntington's disease an unusual genetic disorder.