Fig. 1

Diagrammatic representation of three main epigenetic models. The gene expression could be modulated at multiple levels, including histone modifications, DNA methylation, and non-coding RNAs. Briefly, the histone post-translational modifications are categorized as two types. Repressive histone modifications including H3K27me3 and H3K9me3 mainly distributed in the heterochromatin region, where the chromatin structure is tight. Active histone modifications are divided into H3K4me1, H3K4me3, and H3K27ac. They are mainly distributed in the autochromatin region, which is more conducive to the gene transcription. DNA methylation on CpG islands plays different roles in gene expression depending on the number of methyl as well as the modification sites. Various non-coding RNAs generated by transcription of non-coding regions also regulate gene expression in the nucleus or cytoplasm at transcriptional or post-transcriptional levels