APP and Alzheimer disease: APP can undergo two exclusive proteolytic pathways: cleavage by the α-secretase initiates the non-amyloidogenic pathway while cleavage by the β-secretase initiates the amyloidogenic pathway that leads, after a second cleavage by the γ-secretase, to amyloid-β (Aβ) peptides that can form toxic extracellular deposits, a hallmark of AD.