Epigenetics is an important mechanism for the regulation of tumor. DNA methylation is one of the extensively studied epigenetic alterations. Methylation regulates gene expression and can influence embryogenesis and tumorigenesis. The hypomethylation of promoter region of oncogene can lead to the transcriptional activation while the hypermethylation of the tumor suppressor gene promoter region can result in transcriptional repression, both of which are important ways for tumor formation.