(From Medical News Today). You may recall that it wasn't all that long ago that researchers announced results from a study of changes in gene expression with age in the human kidney. Now we have more of the same, but for the aging brain. Researchers "found a pattern of age-related changes in the expression of hundreds of genes. ... One common theory of aging holds that damage is done to DNA and proteins by "free-radicals" (highly reactive molecules produced by the metabolic activity of mitochondria). It predicts that more metabolically active tissues will show greater age-related differential gene expression. The results of this study support this theory since the more metabolically active cortex shows a greater reduction in gene activity." Use it more and it tends to wear out faster.