APP and Alzheimer disease: In particular, the formation of Aβ plaques in the brain has been proposed to play an important role in the pathogenesis of AD for several reasons: 1) Aβ deposits years before the onset of AD and can precede NFT formation; 2) mutation in the APP gene leads to some forms of early onset familial AD; and 3) Aβ is toxic to cultured neurons [3]–[5].