FIGURE 1.

DNA methylation can silence gene expression. Methylation of cytosine located in cytosine-guanosine groupings in gene promoter regions (called 5′-CpG-3′ islands) attracts capping proteins that hinder access to the gene for the transcription factors that normally turn on gene expression and formation of messenger RNA (mRNA). When the transcription factor does not bind to the promoter area of the gene, transcription of mRNA does not occur, and the gene is silenced.