Huntington's disease (HD) is a late onset, incurable, autosomal 
dominantly-inherited, progressive neuropsychiatric disease, characterised by 
chorea, changes in personality, mood and behaviour, and dementia. Huntington's 
disease is a clinical diagnosis. The advent of DNA diagnosis has made 
predictive, prenatal and preimplantation testing possible for at-risk persons or 
asymptomatic carriers. The prevalence is estimated to be 3-10/100,000 among 
individuals of European descent; HD is less common in other ethnic groups. 
Huntington's disease is caused by an expanded trinucleotide CAG repeat in the HD 
gene on chromosome 4. The gene encodes for the protein huntingtin, with an as 
yet unknown function. The mutated huntingtin has an elongated stretch of 
glutamines which leads to a gain of function such as overactivity, 
excitotoxicity, or to interactions with other proteins.