Although the constitutional genome sequence tends to be quite stable, gene expression may change with age, gender, diet, season, time of day, drug and medication use, and exposure to a wide variety of environmental stimuli. Epigenetic events include methylation of genes, modifications of the histone proteins that interact with genes, and other chemical changes influencing gene expression; these effects may persist throughout life and in some cases may even be passed on to the next generation.45 It is possible to measure gene expression in many peripheral tissues including blood and saliva. Blood gene expression appears to parallel brain gene expression for some genes,46 but there are many differences as well.47 We can also observe gene expression changes in cultured cells obtained from the tissue of patients with psychiatric disorders (skin or blood cells). Such cells may offer the opportunity for in vitro models of psychiatric disorders.48
