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Alzheimer's disease (AD) is the major and common disease usually for aged people to show progressive neurodegenerative disorder with the dementia. Amyloid-beta （also β-protein and referred here to as Aβ） is a wellestablished seminal peptide in AD that is produced from the amyloid precursor protein （APP） by consecutive digestions with β-secretase of BACE and gamma-secretase of the presenilin complex. Abnormal cerebral accumulation of Aβ such as insoluble fi brils in senile plaques and cerebral amyloid angiopathy （CAA） are observed as a neuropathological hallmark of AD. In contrast to such insoluble fi brillary Aβ, a soluble oligomeric complex is discussed as ADDLs, Aβ oligomer, low-n oligomer Aβ, Aβ*56 or so. Despite their diff erent names, it is proposed as the current hypothesis that oligomeric Aβ is the direct molecule to cause synaptic toxicity and cognitive dysfunction in the early stages of AD. We identifi ed a novel APP mutation （E693delta; referred to as the Osaka mutation） in a pedigree with probable AD resulting in a variant Aβ lacking glutamate at position 22. Based on theoretical prediction and in vitro studies on synthetic mutant Aβ peptides, the mutated Aβ peptide showed a unique aggregation property of enhanced oligomerization but no fi brillization. This was further confi rmed by PiB-PET analysis on the proband patient. Collectively together, we conclude that the Osaka mutation is the fi rst human evidence for the hypothesis that oligomeric Aβ is involved in AD., 弘前医学. 61(Suppl.), 2010, p.S105-S110