Amyloid beta (Aβ or A beta) is a protein fragment of 39–43 amino acids that is the main constituent of amyloid plaques in the brains of Alzheimer's disease patients. Aβ is formed after sequential cleavage of the amyloid precursor protein (APP, transmembrane glycoprotein) by the β- and γ-secretases. . The major species generated are Aβ40 and Aβ42. The latter is more hydrophobic and more apt to aggregate and thus is considered to be primarily pathogenic, consistent with the phenotype of the major familial ADcausing mutations. Increases in either total Aβ levels or the relative concentration of the 42-amino acid form have been implicated in the pathogenesis of both familial and sporadic Alzheimer's disease. The 42-mers are the most amyloidogenic form of the peptide.