A MUTATION at codon 717 of the β-amyloid precursor protein gene has been found to cosegregate with familial Alzheimer's disease in a single family1. This mutation has been reported in a further five out of ∼ 100 families multiply affected by Alzheimer's disease1-4. We have identified another family, F19, in which we have detected linkage between the β-amyloid precursor protein gene and Alzheimer's disease. Direct sequencing of exon 17 (ref. 5) in affected individuals from this family has revealed a base change producing a Val → Gly substitution, also at codon 717. The occurrence of a second allelic variant at codon 717 linked to the Alzheimer's phenotype supports the hypothesis that they are pathogenic mutations.
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