Beta-amyloid peptide (beta-APP) is a 40-residue peptide implicated in the pathogenesis of Alzheimer’s disease (AD) and aged Down's Syndrome, which is promoted by the acquisition of an additional copy of chromosome 21. The peptide is a proteolytic product of the much larger amyloid precursor protein (APP) encoded by a gene on chromosome 21. The peptide comprises a large extracellular N-terminal domain and a short hydrophobic membrane-spanning domain, followed by a short C-terminal region. Beta-APP both precedes and forms part of the transmembrane region.