Alzheimer's disease (AD) is characterised neuropathologically by the accumulation of neuritic plaques and neurofibrillary tangles as well as by cerebrovascular amyloid deposition and neuronal cell loss. The major component of neuritic plaques and cerebrovascular amyloid is a 40-42 amino acid peptide termed beta-amyloid, derived as a proteolytic fragment from the large amyloid precursor protein (APP), a membrane-bound protein expressed in most tissues. The last few years have seen considerable advances in understanding the pathogenesis of Alzheimer's disease through genetic studies. The importance of the beta-amyloid peptide in the pathogenesis of AD has been strengthened by the identification of pathogenic mutations in the APP gene on chromosome 21.