APP and Alzheimer disease: The pathological hallmarks of AD are cortical and hippocampal atrophy caused by the loss of neurons and synapses with the presence of two types of abnormal structures: neurofibrillary tangles consisting of an aggregation of misfolded tau proteins, and amyloid plaques consisting of a fibrillary aggregation of amyloid-β (Aβ) peptides derived from amyloid precursor protein (APP).