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 conversion of a DNA base from cytosine (C) to thymine (T) is referred to as transition, of one of two types of base substitution resulting in a point mutation (1). The formation of 5-methylcytosine in DNA can also contribute to epigenetic inheritance.
References
(1) Griffiths, Anthony J.F.; Miller, Jeffrey H.; Suzuki, David T.; Lewontin, Richard C.; & Gelbart, William M. (2000). Chapter 15: Gene Mutation in An Introduction to Genetic Analysis (7th Ed.). New York: W. H. FREEMAN. ISBN 0-7167-3520-2. (available online at the United States National Center for Biotechnology Information)