The last year has seen major advances in the study of Alzheimer's disease (AD). Four mutations involving amino acid substitutions in exons 16 and 17 of the amyloid precursor protein (APP) gene, have been identified which co-segregate with the disease in some families multiply affected by early onset Alzheimer's disease. These mutations are strongly suggestive of a causative role for the amyloid precursor protein in Alzheimer's disease. Despite their rarity, these mutations are important because they represent the first known cause of Alzheimer's disease. Processing of APP must be central to the pathogenesis of the disease although the precise effects of these amino acid substitutions are not understood. Work is now being undertaken to characterise the processing pathways of APP and to identify other causes of AD. The development of models of AD using the APP mutations offers the possibility of identifying drug targets and developing more effective treatments than are presently available.