The mechanisms through which environmental factors exert lasting effects on genome function are under active study. Classical genetics has long recognized the role of molecular modifications to the primary DNA backbone as a determinant of gene expression; in the era of genomics a wide array of epigenetic changes are known to affect gene expression.40 The classical pediatric example of methylation resulting in differential gene expression is Prader-Willi syndrome, in which the pattern of methylation of cytosine residues at a single genomic locus determines which parental allele is expressed, and thus whether the condition is manifest.41 There is increasing recognition that methylation plays a substantial role in cancer biology, and considerable research is directed at developing drugs that inhibit epigenetic changes.42 There is also great interest in learning how environmental factors interact with the genome to alter the course of common conditions such as obesity, asthma, and diabetes in pediatric populations.43,44
