Epigenetic alteration means a heritable change that does not affect the DNA sequence but results in a change in gene expression. DNA methylation is a process by which methyl groups are added to the DNA molecule. Methylation can change the activity of a DNA segment without changing the sequence. When located in a gene promoter, DNA methylation typically acts to repress gene transcription. In mammals DNA methylation is essential for normal development and is associated with a number of key processes including genomic imprinting, X-chromosome inactivation, repression of transposable elements, aging, and carcinogenesis. In addition, abnormal DNA methylation, unlike genetic alteration, can be detected in early cancer or precancerous cancer. DNA methylation has the advantages such as high cancer specificity, structural stability over proteins and RNA, and DNA amplification by PCR. DNA methylation is therefore recognized as the best cancer early diagnosis biomarker.