Amyloid precursor protein cleavage. Illustration of three stages (before, during and after) in the cleavage of the amyloid precursor protein (APP). At left, APP (blue) is inbetween two enzyme molecules: gamma and beta secretase. At centre, the enzymes have bound to APP. At right, the cleavage has occurred and the APP molecule has separated into four fragments, one of which is still embedded in the cell membrane. It is this fragment that is the beta amyloid peptide that is implicated in conditions such as Alzheimer's disease. APP is a complex protein with many functions. It is found on the surface of cells throughout the body.