One of the major pathological hallmarks of Alzheimer's disease is the deposition in the brain parenchyma and cerebral blood vessel walls of amyloid beta-protein, a 4kDa proteolytic product of the longer beta-amyloid precursor protein (APP). Six different single base mutations in the APP gene have been reported causing early-onset Alzheimer's disease (age at onset < or = 65 years) or related amyloidosis in a small number of families. Cell transfection experiments using wild-type and mutant APP cDNA indicated that APP mutations result in the production of more or longer, aggregation prone, A beta peptides.