If this in fact happens, one possible mechanism that ties aging to AD could occur as follows: age-related changes in mitochondrial function occur, these changes reach a point that they turn on neuron APOE expression, the apolipoprotein E or a cleavage product localizes to the mitochondria to further interfere with mitochondrial function, and the degree of mitochondrial compromise within the cell worsens to surpass the ability of the cell to compensate (Fig. 2). This evidence concerns the gene APOE and Alzheimer disease.