Based on the concept of genetics，epigenetics is neither simply the methylation and acetyl change caused by environmental factors in vitro, nor simply regulation by genes like transcription factor and miRNA, but a 'heritable' phenotypic alteration due to Non-DNA variation. Till now, much of so-called epigenetics research has not really extricated itself from the conception of classical genetics.
(1) Epigenetics refers to the variation of gene expression levels due to the alteration of non-gene sequence, including DNA methylation, histone modification, chromatin remodeling and non-coding RNA regulation, which affect its function and properties mainly through regulation of the process of gene transcription or translation. The expected role of such markers are as following:
a) To predict drug response or prognose individual differences, to guide the treatment by optimizing and selecting a suitable medicine dose and pattern for the individual patient;
b) As a molecular target, by reversing the abnormal epigenetic modifications, to inaugurate a new way to treat cancer.
(2) The so-called DNA methylation means the junction of a methyl group onto the 5 'carbon of Genomic CpG dinucleotide cytosine with covalent bonds. Under normal circumstances, the CpG dinucleotidesthe of human genome "junk" sequences are relatively rare and always under the state of methylation. In contrast, the CpG islands rich in CpG dinucleotides and about the size of 100-1000bp in the human genome are always unmethylated, which are associated with 56% of the encode genes in the human genome. Human genome sequence draft analysis reveals that the number of CpG islands in the human genome is about 28,890, most of chromosomes contain 5-15 CpG islands per Mb with an average number of 10.5 CpG islands per Mb, CpG islands good correspondence. There is a good correspondence between gene number and the of density CpG islands. Because of the close relationship between DNA methylation and human development as well as tumor diseases, especially the fact of transcriptional inactivation of tumor-suppressor genes on account of the methylation of CpG island, DNA methylation has become a significant research task in epigenetics and epigenomics.
(3) Abnormal "epigenetic”: epigenetics believes allergic disease is the result of the interaction of genetic and environmental factors; environmental factors can cause errors in gene expression or functional abnormalities, hence result in abnormal "epigenetic”, also known as epimutation. Environmental factors includes infections, food, passive smoking and allergens, which all can affect the differentiation of Th1 / Th2 cell by adjusting the acetylation / deacetylation and methylation / demethylation of histone on IFN-γ and IL-4 gene locus.