Globally, age-related decreases in expression levels (i.e., the expression of genes goes down) and increases in the variation of the profile of gene expression (i.e., the patterns of expressed genes diversify) have been observed. Regarding the latter, with minor effects on specific genes, this age-correlated heterogeneity of expression is widespread throughout the transcriptome. This effect was explained as an outcome of accumulating stochastic changes in the somatic cells, such as mutations, cellular and DNA damage, especially in gene promoters (Lu, Pan, Kao, Li, Kohane, & Chan, 2004; Somel, et al., 2010; Somel, et al., 2006). In general, stochastic age-related changes in gene expression have a systematic directed component: a set of age-related genes has been detected, which have shown a special trajectory of age-related changes of their expression levels.
