In humans, methylation, hydroxymethylation, and possibly other modifications of the fifth carbon of cytosine occur within CpG dinucleotides. Methylation of CpG dinucleotides in genomic DNA is the most studied epigenomic mark. An epigenome of a specific cell population is a comprehensive genome-wide map of these chromatin tags. Other layers may reflect altered epigenetic states that may be propagated over long periods of time and during cell division by other mechanisms. Some layers of this information, such as cytosine methylation tags within more than twenty eight million cytosine-phosphate-guanine (CpG) dinucleotides in the human genome, are replicated concurrently with the DNA. The genomic DNA sequence in a living cell is tagged by methylation of specific cytosines, and by modifications of DNA-attached protein and RNA fractions of the chromatin. In humans and other species epigenetic memory integrates genetic, physiological and environmental influences through the lifetime of an organism and is particularly sensitive to those influences during early development.Д®pigenetic memory may reside in various cellular compartments and, most conspicuously, within the nucleus. These mechanisms are essential for differentiated cells, which acquire distinct вЂњprogramsвЂќ, not encoded in DNA, to guide their cell-type specific homeostatic behavior within a multicellular organism. Within mainstream molecular biology, epigenetics currently has several specific definitions, all focusing on mechanisms other than changes in DNA sequence that perpetuate altered cellular activity states. Spotting epigenetic memory within layers of epigenomic informationД®pigenomics is a new arrival into an old lineage that includes the classical concept of epigenesis and the evolving concept of epigenetics. The development of new bioinformatic frameworks for comparative epigenome analysis is putting epigenome maps within reach of researchers across a wide spectrum of biological disciplines. Finally, comparisons of epigenomes from monozygotic twins help dissect genetic and environmental influences on human phenotypes and longitudinal comparisons reveal aging-associated epigenomic drift.

At the organismal level, comparisons of epigenomes from diverse cell types provide insights into cellular differentiation. At the human population level, epigenomic changes provide footprints of the effects of genomic variants within the vast non-protein coding fraction of the genome while comparisons of the epigenomes of parents and their offspring point to quantitative epigenomic parent-of-origin effects confounding classical Mendelian genetics. Comparisons of methylation profiles among species reveal evolutionary conservation of gene body methylation patterns, pointing to the fundamental role of epigenomes in gene regulation. Fuelled by new sequencing technologies, epigenome mapping projects are revealing epigenomic variation at all levels of biological complexity, from species to cells.

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