APP and Alzheimer disease: Increasingly, recent studies have shown more direct molecular links between AD and prion diseases, and the proteins at the centre of these diseases; namely the amyloid precursor protein (APP) and its proteolytic cleavage product the amyloid-β (Aβ) peptide which deposits as plaques in the AD brain, and the normal cellular prion protein (PrPC) and the disease-associated isoform PrPSc, which accumulates in prion diseases.