APP and Alzheimer disease: Analyses of post-mortem human brains reveal two key features of AD, namely the presence of amyloid plaques—extra-cellular accumulations of amyloid-β (Aβ) peptides, which are derived from the proteolytic processing of the amyloid precursor protein (APP); and neurofibrillary tangles—intra-cellular accumulations of the microtubule-associated protein tau.