Alzheimer's disease, the most common cause of dementia in the elderly, exists in both familial and sporadic forms. Genetic studies have identified three genes; beta-amyloid precursor protein (APP), Presenilin-1 and Presenilin-2 which, when mutated, can cause familial forms of Alzheimer's disease. APP and APP-like proteins are transmembrane glycoproteins with a similar modular domain structure.