APP and Alzheimer disease: The characteristic neuritic plaques representing oneof the classical neuropathological hallmarks in the brains of AD patientsare composed of aggregated and highly insoluble Aβ peptides.1 Soluble Aβ peptides are generated duringnormal cellular metabolism by consecutive proteolytic cleavages ofthe amyloid-beta precursor protein (APP) by β- and γ-secretasesand can be found under physiological conditions in biological fluids,such as cerebrospinal fluid (CSF) and blood plasma (reviewed in ref (2)).