Epigenetics is the study of gene expression that has been modified by various factors, such as DNA methylation, histone modifications, microRNAs, and long noncoding RNAs, typically in response to environmental exposures. Modified gene expression may last across the lifespan or even become heritable (Weaver et al. 2004). Histone and DNA methylation are epigenetic phenomena that change in the human brain from childhood to adulthood, suggesting that the epigenetic pattern may modify brain development. However, only one study examined the global cytosine methylation of DNA with age in humans (Siegmund et al. 2007). From 17 weeks of gestation to 104 years of age, DNA methylation levels increased in genes that show decreased mRNA expression. Emerging data from studies on gene-by-environment-by-age interactions of DNA methylation show that epigenetic effects may influence processes such as fetal programming, in which events that occurred during gestation may last across the lifespan. Stable and long-lasting epigenetic alterations in the brain also has been suggested to cause some psychiatric disorders (Feinberg et al., 2007; Gardner et al. 2007; Simmons 2009), leading to the hypothesis that many adult
