What is Huntington disease? Huntington disease (HD) is a
progressive, neurodegenerative genetic disorder characterized by
involuntary movements (chorea), lack of coordination, cognitive
decline, and behavioral/personality changes. The symptoms of HD
are as a result of loss of neurons (nerve cells) in certain
regions of the brain and usually develop in affected individuals
between the ages of 30 and 50 years, although there is a form
that affects children called juvenile-onset Huntington disease.
The disease affects men and women alike, with a higher frequency
among populations of European descent. It is estimated that
30,000 people in the US have this disease. Huntington disease is
an autosomal dominant disorder involving a defective gene on
chromosome 4. Only one copy of that abnormal gene is needed to
cause the disease in an affected person, inherited from either
of that person's parents. The mutation associated with HD is an
expansion of the trinucleotide repeat sequence "CAG" in the
IT-15 gene (HD gene),