APP and Alzheimer disease: Alzheimer's disease (AD) is a neurodegenerative disease characterized by progressive cognitive impairment and, at the cellular level, by synaptic damage, intracellular neurofibrillary tangles and beta-amyloid precursor protein (βAPP) processing dysfunction that leads to overabundance of the 42 amino acid amyloid-beta (Aβ42) peptide.