DNA methylation is an enzymatic modification of the DNA molecule that confers unique differential identities upon similar DNA sequences. DNA methylation plays a critical role in cellular differentiation by conferring cell-type identity upon differentiated tissues in multicellular organisms by an innate developmentally programmed process. Recent data points to the possibility that DNA methylation plays a role in responding to external cues and conferring environment-context identity to DNA. DNA methylation is implicated in the response to early life social environment and might be playing an important role in setting up stable behavioral phenotypes in response to early-life social environment. The critical question is whether these responses are limited to the brain or involve the immune system as well. Addressing this question has important implications on understanding the mechanisms involved in DNA methylation mediated responses to the environment and how they impact the phenotype as well as on the possibility of studying the associations between DNA methylation and behavior and behavioral pathologies in living humans. A model is presented suggesting that DNA methylation acts as a mechanism of genome adaptation to the environment that is genomewide and systemwide. New data suggesting associations between DNA methylation patterns in white blood cells and the social environment will be discussed.
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