APP and Alzheimer disease: Aβ is an aggregation-prone and toxic polypeptide with 39–43 residues, derived from the AD proteolysis process of amyloid precursor protein.19 20 Among all Aβ isoforms, Aβ40 and Aβ42 are believed to be the most important ones.21 Aβ40 and Aβ42 are quite similar in their sequences; the only difference between them is an extra isoleucine and an alanine at the C-terminus of Aβ42.22 In this meta-analysis, we figured out plasma Aβ40 values might not differ between PET (+) and PET (−) subjects.