Human DNA methylomes at base resolution show widespread epigenomic differences

Human genetic variation extends beyond differences in the underlying genetic sequence. A recent study reported in Nature provides a more robust understanding of DNA cytosine methylation, an alternate source of genetic variation. While DNA methylation was primarily associated with repeat CpG (Cytosine-phosphate-Guanine) rich portions of the genome, Lister et al. report that approximately 25% of methylation in embryonic stem cells occurs in non CpG portions of the genomes. CpG regions are associated with promoter regions of genes, and methylation of these regions are thought to regulate gene expression. The report of methylation in non CpG regions raises new questions about the role of this epigenetic modification. The publication also represents one of the first papers describing a relatively complete map of the human epigenome.