A hereditary disease characterised by involuntary movements and DEMENTIA. Each child of a parent with the disease has a 50:50 chance of also developing it. Onset is most common between the ages of 35 and 45, but 10 per cent of cases occur under the age of 20. Some patients show more severe mental disturbance; others more severe disturbances of movement; but in all it pursues an inexorable downward course over a period of 10–20 years to a terminal state of physical and mental helplessness. It is estimated that there are around 6,000 cases in Britain. The defective gene is located on chromosome no. 4 so GENETIC SCREENING is possible for those at risk.