Huntington's disease (HD) is an inherited, degenerative brain disorder named after physician George Huntington, who first described the illness in 1872. It is also sometimes referred to as Huntington's chorea, from the Greek word choreia meaning dance, due to the characteristic movement disorder that is a part of the illness. In former times it was called St. Vitus' Dance.

HD is a genetic (hereditary) disease, meaning that it can be passed down from one generation to the next. In 1993, scientists identified the gene that causes HD.

Genes are comprised of long strings of chemical building blocks. In individuals with the  HD gene, there are too many of these building blocks (designated as CAG) in a particular place on chromosome 4. The protein encoded by this gene does not work properly and eventually leads to the symptoms of HD. Although every cell of the body has two copies of every gene, only one abnormal or expanded gene is needed to cause HD.

Every child of a parent who carries the HD gene has a 50:50 probability of inheriting the abnormal gene. A child who inherits the HD gene will eventually develop the illness, although usually not until adult life. If a child does not inherit the HD gene, he or she will not develop the disease, nor will his or her childen: HD does not skip generations.

About 30,000 people in North America have HD, and another 150,000 are considered at risk for inheriting the illness because they have (or had) a parent with HD. Most of those affected by the illness develop it in middle adult life, but about 10% will have onset prior to age 20 (juvenile HD) and another 10% after age 55. Men and women are equally likely to inherit the gene and develop the illness. HD can affect people of all ethnic groups, but is more common among those of European descent.