The gene is present on chromosome 4. Gradually, this protein accumulates within brain cells. Brain cells are damaged when levels of huntingtin rise. Common medical opinion holds that nearly all cases are inherited and that appearance of the disease through new mutations is extremely rare. When a patient presents with HD, identification of a parent with an intermediate allele who lacks the full clinical manifestations of the disease is often possible.
Family history is the principal identified risk factor.
A small number of patients exist who have DNA-proven HD but a completely negative family history. The DNA test is highly reliable and can confirm or refute the diagnosis in most cases. Careful counseling and follow-up care is essential if asymptomatic family members are to be tested. CT scan or MRI may show loss of a normally convex bulge of the caudate nucleus into the lateral ventricles. Enlarged lateral ventricles usually are visible on head CT scan.