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The term epigenetic literally means "above the genome". The main components of the epigenetic code are DNA methylation, histone modifications, and non-coding RNAs. The epigenetic programs in cells are normally faithfully reproduced during mitosis and maintained in the post-mitotic cell. They can also be maintained during meiosis, resulting in epigenetic transgenerational disease inheritance, and potentially introducing phenotypic variation that is selected for in the evolution of species.

In the broadest sense of the term, epigenetic changes are all changes in gene function that do not involve a change in the underlying sequence of the gene. Epigenetic changes can switch genes on or turn them off, or modify the way in which genes are transcribed. Whereas some epigenetic modifications are heritable (passed through the germline), others can persist throughout the life of the organism.

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