5-methylcytosine

5-methylcytosine is a modified form of the DNA base cytosine, where a methyl group is added. This modification is important in the regulation of gene expression—it helps control whether a gene is turned on or off. Found throughout the genomes of many organisms, including humans, 5-methylcytosine plays a critical role in processes like cell differentiation and development, and is also involved in some diseases, including cancer. Essentially, it acts like a chemical marker that influences how genes function without changing the DNA sequence itself.