Editor—Huntington’s disease (HD) is an autosomal dominant disorder characterised by the association of choreic movements and cognitive/psychiatric changes. In 1993, the HD Collaborative Research Group reported the identification of the IT15 gene, which encodes a protein named huntingtin that carries an unstable and expanded CAG repeat in patients.1 Normal alleles are polymorphic with 11 to 35 CAG repeats, whereas expanded alleles in patients contain 36 or more repeats.

HD is thought to be a true dominant disorder, since homozygous carriers of the disease are no more severely affected than heterozygous carriers.2 However, precise clinical evaluations have not yet been reported in homozygous patients with known expansion sizes. We report here the clinical, neuropsychological, and molecular characterisation of such a patient in comparison to his heterozygous brother.

The patients underwent neurological examination, including the motor scale of HD,3 neuropsychological testing known to be sensitive to subcortical dementia.4 CAG repeats of the IT15 gene were sized by PCR using Hex labelled primer HD1 and HD2.5 PCR products were run on a 4% polyacrylamide gel and the CAG repeat was estimated using Genescan and the Genotyper software package.

Three HD patients (IV.4, IV.5, and IV.6) were born to parents who were first cousins (fig 1). The mother (III.2), who died at the age of 62, was known to be affected by history but age at onset could not be determined. The father (III.1, 20/41 CAG repeats), examined at the age of 68, had severe choreic movements (motor score 16/20), dysarthria, and cognitive changes with a Mini Mental State examination of 13/30, but was not aware of his symptoms …