AD is the main form of dementia (~70%) in Western countries, and is characterized by the presence in postmortem brain of extracellular amyloid plaques composed of ‘Aβ’ generated by the aggregation of toxic peptide fragments of the Alzheimer precursor protein, APP, and intraneuronal deposition of highly phosphorylated filamentous aggregates (neurofibrillary tangles, NFT) of the microtubule-associated protein Tau. This evidence concerns the gene APP and Alzheimer disease.