Alzheimer’s disease is the most common form of dementia, affecting about 10% of the elderly population. Four genetic loci have so far been implicated in Alzheimer’s disease, either in rare family pedigrees in which the defective gene (amyloid precursor protein, presenilin-1, presenilin-2) cosegregates with early onset Alzheimer’s disease, or in late onset cases in which polymorphism of the apolipoprotein E (ApoE) gene on the chromosome 19 is identified as an important individual risk trait. Apolipoprotein E is a normal constituent of plasma lipoproteins and has an important role in the maintenance of the integrity of the neuronal membrane and myelin sheath, but is also deposited in senile plaques in Alzheimer’s disease. Three allelic forms of the ApoE gene have been characterised—Apo ε2, Apo ε3, and Apo ε4. In a previous study, we found that the Apo ε4 allele constitutes a major risk factor for Alzheimer’s disease in the Portuguese population,1similar to that described for …