Evaluating epigenetic changes in humans is more difficult because epigenetic marks can be tissue specific. Access to human brain tissue is limited to postmortem studies of donated brains, which are generally unique and unrepresentative samples and must be interpreted in the context of those limitations. Nonetheless, a recent study of human brain samples from the Quebec Suicide Brain Bank found evidence of increased DNA methylation of the exon 1F promoter in hippocampal samples from suicide victims compared with controls—but only if suicide was accompanied with a history of childhood maltreatment (McGowan et al. 2009). Importantly, this paralleled epigenetic changes originally observed in rat brain in the ortholog of this locus. Another line of evidence suggesting epigenetic changes that may be relevant in humans is the observation of increasing discordance in epigenetic marks in MZ twins across time. This is significant because MZ twins have identical genotypes, and therefore, differences between them are attributed to environmental influences. In a study by Fraga and colleagues (2005), MZ twins were found to be epigenetically indistinguishable during the early years of life, but older
