Epigenetics is one of the most rapidly expanding fields in biology. On a molecular level, covalent modifications of cytosine bases and histones, and changes in the positioning of nucleosomes are commonly regarded as the driving epigenetic mechanisms. They are fundamental to the regulation of many cellular processes, including gene and microRNA expression, DNA protein interactions, suppression of transposable element mobility, cellular differentiation, embryogenesis, X-chromosome inactivation and genomic/ imprinting. Genomic imprinting is an epigenetic genemarking phenomenon that occurs in the germ line, leads to parental-origin-specific expression of a small subset of genes in mammals and oviparus. The epigenetic imprints regarding the parental origin are established during male and female gametogenesis, passed to the zygote through fertilization, Maintained throughout development and adult life, and erased in primordial germ cells before the new imprints are set.