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LETTERS TO THE EDITOR•trained personnel or sophisticated laboratory facilities. Such a test exists in the form of an immunoassay to measure the diphtheria antitoxin in vitro, rather than the established and more complex in vivo (guinea pig) or Vero cell culture assays. A rapid dot-ELISA anti-diphtheria toxin IgG assay would provide an adequately sensitivity and reproducible test allowing the primary care worker to establish the presence of antitoxin levels exceeding 0.01 IU per ml, the level agreed as providing basic or better protection against the disease'.

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We found a new familial Alzheimer's disease kindred in which the disease cosegregates with the APP717Val-->Ile mutation and in which all of the three most common apolipoprotein E (ApoE) alleles are represented. We studied the relationship between ApoE genotype and the clinical expression of the disease and found that in this amyloid precursor protein-mutated family, ApoE genotype influences the age at onset of the disease. Three mutated subjects heterozygous for the epsilon 4 allele had the earliest age at onset in this family, subjects heterozygous for the epsilon 2 allele had the latest age at onset, and subjects homozygous for the epsilon 3 allele had an intermediate age at onset. In this large kindred we also found an amyloid precursor protein-mutated subject 2.4 standard deviations older than the mean age at onset without clinical signs and symptoms of the disease and carrying the epsilon 2/epsilon 3 genotype.

Our data support the evidence that CTG expansions may be linked to SCA8, since the pathogenic expansions have been found only among patients with genetically unidentified forms of hereditary and sporadic ataxia. Patients carrying expanded alleles present peculiar phenotypic features, thus suggesting that unknown additional factors could probably predispose to the disease.