Three genetic loci for Alzheimer's disease have been identified. These are the amyloid precursor gene on chromosome 21, a gene for early-onset autosomal dominant Alzheimer's disease on chromosome 14, and the risk-modifying gene APOE on chromosome 19. Additional Alzheimer's disease genes remain to be found. The genes identified by studying inherited forms of Alzheimer's disease are now being used to understand the initiating steps in the pathogenesis of the disease.