["Protein cross-linking and aggregation are important molecular processes in Alzheimer's disease (AD), and tissue transglutaminase (tTG) catalyzes protein cross-linking.", 'To measure tTG, tTG enzyme activity and isopeptide, which is the product of tTG, in brain and to relate them to cognitive scores.', 'tTG and isopeptide levels were measured in frontal gray matter of 10 normal (NCI), 10 mild cognitive impairment (MCI) and 9 AD brains from the Religious Orders Study. tTG enzymatic activity was measured with a fluorescence assay.', 'tTG protein and enzyme activity were highest in AD, but not significantly greater than MCI or NCI. In contrast, isopeptide immunoreactivity in formic acid extracts was significantly greater in AD than NCI and MCI. The level of insoluble formic acid extractable isopeptide correlated with several measures of cognitive function, including word generation and perceptual speed. Multiple linear regression analyses indicated that insoluble isopeptide immunoreactivity could be accounted for by a combination of factors in the formic acid extract, including Abeta, ubiquitin and tau.'] Does cognitive performance correlate with cortical isopeptide immunoreactivity as well as Alzheimer type pathology?