APP and Alzheimer disease: The extracellular deposition of the amyloid β-peptide (Aβ) (which is  derived from the processing of the amyloid precursor protein [APP]) in  senile plaques and intracellular accumulation of neurofibrillary tangles  (principally composed of phosphorylated tau protein) are the main  pathological features of Alzheimer's disease (AD) [1].