Epigenetics is the study of heritable changes in gene expression without changes to the DNA sequence. Epigenetic mechanisms, such as DNA methylation, chromatin remodeling, and noncoding  RNAs, regulate cell growth, development, and differentiation.

There are two types of epigenetic modifications – DNA methylation and histone modifications.

● DNA methylation

DNA methylation involves the addition of a methyl or hydroxymethyl group to bases in the DNA sequence. The most commonly studied modification is the methylation of the C5 position on cytosine bases, or m5c. The methylation of CpG sites is performed by DNA methyltransferases (DNMTs). The canonical DNMT enzymes are DNMT1, DNMT3A, and DNMT3B. DNA methylation regulates transcription, chromatin structure, DNA repair, and X chromosome inactivation. It is also important for the developmental control of gene expression.

● Histone Modifications

Histones are a family of proteins that are found in the nucleus and function to package DNA into nucleosomes. A histone modification is a covalent post-translational modification (PTM) to histone proteins which includes methylation, phosphorylation，acetylation, ubiquitylation and sumoylation. The PTMs made to histones can impact gene expression by altering chromatin structure or recruiting histone modifiers. Histone proteins act to package DNA, which wraps around the eight histones, into chromosomes. Histone modifications act in diverse biological processes such as transcriptional activation/inactivation，chromosome packaging and DNA damage/repair.