Huntington’s disease (HD) is a genetic neurodegenerative disorder that affects muscle coordination and leads to cognitive decline and psychiatric problems. It typically becomes noticeable in mid-adult life. HD is the most common genetic cause of abnormal, involuntary twitching movements called chorea. Physical symptoms can begin at any age from infancy to old age, but typically begin between 35 and 44 years of age. HD is a rare disease affecting approximately 4 people in a million. However, the chances that a person in a family with a history of the disease will get HD can be as high as 50%. The disease is caused by an autosomal dominant mutation in one of the two copies of a gene called huntingtin. The gene includes the genetic code for a protein of the same name (huntingtin), which contains a long stretch of the amino acid glutamine. In normal people, the glutamine-repeat stretch contains up to 36 copies of this amino acid, whereas in affected individuals, the number of glutamines typically is over 40, and can be over 100. The longer the glutamine repeat is, the earlier the disease onset. Huntingtin containing a long glutamine repeat aggregates abnormally and deposits inside affected neurons, leading to dysfunction and death of these neurons.