The pathological assembly of Ab, tau, and alpha-synuclein is at the heart of Alzheimer's and Parkinson's diseases. Extracellular deposits of Ab and intraneuronal tau inclusions define Alzheimer's disease, whereas intracellular inclusions of alpha-synuclein make up the Lewy pathology of Parkinson's disease. Most cases of disease are sporadic, but some are inherited in a dominant manner. Mutations frequently occur in the genes encoding Ab, tau, and alpha-synuclein. Overexpression of these mutant proteins can give rise to disease-associated phenotypes. Protein assembly begins in specific regions of the brain during the process of Alzheimer's and Parkinson's diseases, from where it spreads to other areas.