Epigenetics refers to information transmitted during cell division other than the DNA sequence per se, and it is the language that distinguishes stem cells from somatic cells, one organ from another, and even identical twins from each other. Examples include (1) DNA methylation, a covalent modification of the nucleotide cytosine, that is copied during cell division at CpG dinucleotides by the maintenance enzyme DNA methyltransferase I; (2) posttranslational modifications of nucleosome proteins about which the DNA double helix is wrapped; and (3) the density of nucleosomes and higher-order packaging of chromatin within the nucleus, including its relationship to the nuclear lamina.
Feinberg AP, Fallin MD. Epigenetics at the Crossroads of Genes and the Environment. JAMA. 2015;314(11):1129-1130. doi:10.1001/jama.2015.10414