Therefore, there is considerable interest in investigating dominantly-inherited Alzheimer’s disease, which is caused by mutations in the amyloid precursor protein (APP), presenilin 1 (PSEN1), and presenilin 2 (PSEN2) genes, and which provides the opportunity to identify asymptomatic ‘at risk’ individuals prior to the onset of cognitive decline for observational studies and clinical trials. This evidence concerns the gene PSEN1 and Alzheimer disease.