Alzheimer’s Disease (AD) is a progressive neurodegenerative disease and future treatments are likely to be most effective in early stages, prior to the onset of dementia. The first detectable pathology in AD is the accumulation of a 42 amino acid Aβ peptide (Aβ42) in extracellular plaques in the brain. This amyloid deposition, which is the characteristic feature of AD, occurs decades before clinical onset of dementia. The availability of biomarkers is crucial for early detection of, and monitoring of, the progression of AD. Cerebrospinal fluid (CSF) concentrations of Aβ42 in AD patients are lower than controls and correlate with the amount of plaque determined either by positron emission tomography or at autopsy. Blood assays for Aβ42 would clearly be preferable, but are more problematic. Currently a ratio of Aβ42 to Aβ40 is used. A significant problem with plasma Aβ42/40 measurement is that Aβ is also produced by extracerebral tissues that are not affected by AD. Biomarkers, such as phosphorylated tau peptides, have been employed, together with Aβ42/40 ratio for defining AD. What is urgently needed are better blood biomarkers of AD, in particular one that mirrors the state of amyloid in the brain. Our hypothesis is that measurement of the levels of a newly identified, i.e previously unreported, covalently crosslinked Aβ peptide dimer will provide this biomarker. Elucidation of the structure of this novel crosslinked Aβ dimer has recently been achieved in our laboratories, and its structure has been proven by independent synthesis. We were able to determine the structure of this crosslink as a result of an ongoing, well- established collaboration between our groups on age-related protein crosslinking. Importantly, this novel crosslink forms spontaneously, is abundant in brain, and covalent crosslinking of two adjacent Aβ peptides can occur only once fibrils are intimately aligned within amyloid in the brain. It is therefore amyloid-specific and cannot form in extracerebral tissues. Crucially, the Aβ dimer fraction from brain, has been shown to be neurotoxic in several bioassays. Under the same conditions, the Aβ monomer was quite inert. Therefore, unlike other currently used biomarkers, levels of blood Aβ dimer may reflect damage that is occurring in the brain of AD patients. Our ultimate aim is to make monoclonal antibodies to this crosslink to enable its specific detection. Manufacture of these monoclonal antibodies and development of an ELISA assay for use in potential bioassays, forms the major part of this NOSI proposal. A bioassay specific for a brain-derived neurotoxic form of Aβ would be a major advance in AD diagnosis.