Main Page | See live article | Alphabetical index 5-methylcytosine is the methylated form of cytosine. The methyl group is attached to carbon 5, and alters its properties. 5-methylcytosine is formed by the action of DNA methyltransferase on CpG sites. It can undergo spontaneous deamination to form thymine. This article is from Wikipedia. All text is available under the terms of the GNU Free Documentation License.