Molecular model of a human brain-derived beta-amyloid fibril isolated from a patient with Alzheimer's disease. The fibril is made up of beta-amyloid peptides (chains of protein, red, orange and green) stacked in a three-fold symmetry. These insoluble fibres resist degradation and so build-up in brain tissue, forming the amyloid plaques found in the brain's of Alzheimer's disease patients.