DNA methylation is the most basic epigenetic modification. In eukaryotes, the most common methylation modification is 5-methylcytosine (5mC), which is generated by attaching the methyl group to the fifth carbon of the pyrimidine ring of CpG island cytosine with the help of DNA methyltransferase (DNMT). Besides, DNA methylation occurs at the N-6 position of adenine (N6-methyladenine, 6mA), the N-4 position of cytosine (N4-methylcytosine, 4mC), as well as the N-7 position of guanine (N7-methylguanine, 7mG), which are respectively catalyzed by different DNA methyltransferases. The gene sequence is not altered during this process, but it regulates gene expression. DNA methylation is a dynamic process, and the DNA demethylation is mediated by DNA demethylases.