The genetic data implicating mutations framing the beta-amyloid segment of the amyloid precursor protein as causes of Alzheimer's disease are reviewed and integrated with information on the normal processing of the amyloid precursor protein. The data indicating that there is a second and quantitatively major locus for early-onset Alzheimer's disease on the long arm of chromosome 14 are reviewed. The prediction that this second genetic locus will produce a protein intimately involved in the metabolism of the amyloid precursor protein is reiterated, together with the prediction that all causes of Alzheimer's disease will directly involve this process.