Aβ is composed of a family of peptides produced by proteolytic cleavage of the type I transmembrane spanning glycoprotein amyloid precursor protein (APP). Senile plaque Aβ protein species ends in residue 40 or 42, but it is suspected that Aβ42 form is crucial in the pathogenesis of AD. Although Aβ42 makes up less than 10% of total Aβ, it aggregates at much faster rates than Aβ40. Aβ42 is the initial and major component of senile plaque deposits. While the most prevalent hypothesis for mechanisms of Aβ-mediated "neurotoxicity" is structural damage to the synapse, various mechanisms such as oxidative stress, altered calcium homeostasis, induction of apoptosis, structural damage, chronic inflammation and neuronal formation of amyloid has been proposed. Observation of AB42/AB40 ratio has been a promising biomarker for AD. However, as AB42 fails to be a reliable biomarker in plasma, attention was drawn for alternative biomarkers.