Alzheimer's disease (AD), by far the most common form of dementia in the elderly, is clinically characterized by gradual, progressive loss in cognitive functioning and changes in personality, ultimately leading to death. It is now well established that genetic factors play an important role in AD. So far, three genes have been identified in which mutations cause autosomal-dominant AD: the amyloid precursor protein (APP) gene on chromosome 21, the presenilin 1 (PSEN1) gene on chromosome 14, and the homologous presenilin 2 (PSEN2) gene on chromosome 1. A major susceptibility gene, the apolipoprotein E (APOE) gene, was identified on chromosome 19.