Epigenetics is the study of heritable changes in genome function that occur without a change in DNA sequence. This includes the study of how chromatin allows patterns of gene expression to be passed from one cell to its descendants, and how gene expression changes during the differentiation of one cell type into another.

Epigenetic inheritance can potentially persist over hundreds if not thousands of years, providing an alternative to genetic mutations as a substrate for natural or artificial selection (i.e breeding). Epigenetic changes can also be reversible and can require resetting (e.g. during reproduction), allowing a flexible but potentially stable, approach for gene regulation. Epigenetic inheritance can be propagated by differences in DNA methylation, post-translational histone modifications, deposition of histone variants and combinations of these modifications. A large portfolio of enzymes catalyzing and removing epigenetic marks orchestrates epigenetic inheritance. Small interfering RNA (siRNA)-mediated mechanisms can play central roles in setting up and maintaining states of gene activity. Epigenetic mechanisms likely arose to combat viruses and ‘selfish' transposable elements, but have also been co-opted for regulation of endogenous genes There are a wide range of epigenetic phenomena including maternal effects, dosage effects, genomic imprinting, paramutation, gene silencing, quelling, X-chromosome inactivation and transvection. The study of epigenetics is now beginning to impact on our understanding of human disease, cancer, ageing and stem cells, as well as on agriculture. The 2006 Nobel Prize for Physiology/Medicine was awarded for the discovery of the epigenetic phenomenon of gene silencing by RNAi.