Epigenetics: Figure 1 Search Clinical Trials Find a Researcher Order a Test Figure 1. Three types of epigenetic modification that regulate gene expression in mammalian cells. (A) Methylation of DNA cytosine residues at the carbon 5 position (5meC) is a common epigenetic mark in many eukaryotes and is most often found in CpG sequence context. When located at gene promoters, DNA methylation is usually association with repression of gene expression. (B) Histone proteins are subject to a number of covalent modifications, primarily at their N-terminal tails, including methylation, acetylation, phosphorylation, ubiquitylation and ADP-ribosylation. These modifications lead to either repression or enhancement of gene expression. (C) Gene regulation by transcription factors and microRNAs. One or more transcription factors activate transcription by binding to cis-regulatory sites, which are commonly situated upstream of protein-coding genes. After transcription, one or more microRNAs bind to cis-regulatory sites, usually in the 3' UTR of the mRNA, and repress protein translation4.