Table 1.

Mechanisms of epigenetic regulation

	Site(s) of action	Mechanism(s) of action	Effect(s)
DNA methylation	CpG islands (stretches of DNA >200 bp with >50% GC content) and other CpG regions (<200 bp)	Addition of methyl group to the 5′ position of cytosines by DNMTs	Turn off gene transcription
Histone modification	Histone proteins (H2A, H2B, H3, and H4) that form the nucleosome core	Chromatin remodeling by methylation, acetylation (via HAT, HDAC), phosphorylation, or ubiquitylation of histone tails	Regulate accessibility of DNA to RNA polymerase II and transcription factors
Noncoding RNAs	miRNAs (21–23 nucleotide long regulatory RNAs)	Regulatory transcripts: binding of miRNA to the 3′ untranslated regions of mRNA	Induce degradation of target mRNA

CpG, cytosine-phosphate-guanine, DNMT, DNA methyltransferase; HAT, histone acetyltransferases, HDAC, histone deacetylases; miRNA, microRNA