Alzheimer's disease is a senile dementia caused by progressive neurodegeneration of the central nervous system. One of the most prominent pathological characteristics is beta A4 amyloid deposition in senile plaques in the brain parenchyma and in cerebral blood vessels. beta A4 amyloid is processed from a larger integral membrane protein, the beta A4 amyloid precursor protein. Different pathogenic mutations in this protein have been detected in a small number of Alzheimer's disease families. Here functional implications of these mutations on the processing of the precursor protein and the beta A4 amyloid deposition will be discussed with respect to the pathogenesis of Alzheimer's disease and related disorders.