APP and Alzheimer disease: One of the main hallmarks of Alzheimer’s disease (AD) is the presence of senile plaques in the brain, composed of insoluble deposits of abnormally folded β-amyloid (Aβ), an aggregation-prone fragment of 39–43 amino acids, most commonly Aβ1–40 (Aβ40) and Aβ42, formed through the proteolytic cleavage of the Amyloid Precursor Protein (APP).