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Background: Family history is one of the most consistent risk factors for dementia. Therefore, analysis of families with a distinct inheritance pattern of disease can be a powerful approach for the identification of previously unknown disease genes.

Objective: To map susceptibility regions for Alzheimer's disease.

Methods: A complete genome scan with 369 microsatellite markers was carried out in 12 extended families collected in Sweden. Age at disease onset ranged from 53 to 78 years, but in 10 of the families there was at least one member with age at onset of <= 65 years. Mutations in known early-onset Alzheimer's disease susceptibility genes have been excluded. All people were genotyped for APOE, but no clear linkage with the epsilon 4 allele was observed.

Results: Although no common disease locus could be found in all families, in two families an extended haplotype was identified on chromosome 8q shared by all affected members. In one of the families, a non-parametric multi-marker logarithm of the odds (LOD) score of 4.2 (p = 0.004) was obtained and analysis based on a dominant model showed a parametric LOD score of 2.4 for this region. All six affected members of this family shared a haplotype of 10 markers spanning about 40 cM. Three affected members in another family also shared a haplotype in the same region.

Conclusion: On the basis of our data, we propose the existence of a dominantly acting Alzheimer's disease susceptibility locus on chromosome 8.