APP and Alzheimer disease: AD, on the other hand, is characterized by both extracellular accumulation of amyloid β (Aβ), a proteolytic fragment of the amyloid precursor protein (APP) by beta and gamma secretases, into senile plaques and the intracellular formation of neurofibrillary tangles (NFTs), composed of hyperphosphorylated tau (pTau) protein.