Epigenetic mechanisms act to change the accessibility of chromatin to transcriptional regulation locally and globally via modifications of the DNA and by modification or rearrangement of nucleosomes. Epigenetic changes can be defined as stable molecular alterations such as the gene expression they are heritable during cell divisions but do not involve changes in the DNA sequence. Epigenetics consist in several molecular mechanisms: histone modifications, small non-coding or antisense RNAs
and DNA methylation; that are closely interconnected.