APOE and Alzheimer disease: In that study, young-adult participants in both groups did not have amyloid or tangle deposition in the posterior cingulate cortex (PCC); yet, APOE4 carriers, i.e., those at greater risk for developing AD, showed reduced mitochondrial activity in PCC neurons, as measured by cytochrome oxidase levels, suggesting that a change in mitochondrial function may be the earliest manifestation of AD risk32.