APP and Alzheimer disease: Alzheimer's disease (AD) is a neurodegenerative disorder characterized by a progressive decline in cognitive abilities and associated with neuropathological features, including extensive loss of neurons (particularly cholinergic neurons), intracellular neurofibrillary tangles composed of the tau protein, and extracellular deposition of plaques composed of β-amyloid (Aβ), which is a cleavage product of amyloid precursor protein (APP).