Epigenetics is defined as heritable changes in gene expression patterns that occur without changing the DNA sequence itself [192], and includes DNA methylation and posttranslational modifications of histone proteins. DNA methylation, i.e. a covalent binding of a methyl group to the 5-position of the cytosine ring within the sequence 5’-CG-3’ (CpG), can be tissue- and cell-type specific and is found in all vertebrates, and many invertebrates and plants. CpG clusters with a minimum of 200 base pairs, a CG percentage greater than 50% and an observed/expected CpG ratio greater than 0.6 are called CpG islands. These islands are often found in gene promoter regions and can protect single CpGs within a CpG island from DNA methylation.
