Epigenetics is a study of the heritable gene expression changes that do not involve alternations in DNA sequence.

Prefix epi- signifies “on top of” or “in addition to”, hence epi-genetic is understood as something on top of genetics that changes the expression of the genes.

The “epi- mechanisms” that influence gene expression are most frequently classified as:

• Covalent modification of the DNA bases. DNA methylation is the most studied DNA modification leading to gene expression changes (see also: What is DNA methylation?).
• Histone modifications. DNA in the nucleus does not exist by itself but in a complex with proteins called histones. Those proteins can regulate access of the transcription machinery to the DNA and hence influence gene expression. This type of gene expression regulation is accomplished by covalent modifications of the histones.
• Regulation of gene expression by non-coding RNA mechanisms such as micro-RNA or siRNA.