= 5-Methylcytosine
<5-Methylcytosine> is a modified <cytosine> with a <methyl group> attached to carbon 5 of its ring. In <DNA>, it retains the usual pairing relationship with <guanine>, so <DNA methylation> can change the chemical information on a sequence without changing its ordinary C/G base-pair identity.
A chemical difference makes the mark experimentally readable. In conventional <bisulfite sequencing>, treatment converts unmethylated <cytosine> to a form read as thymine after amplification, while <5-Methylcytosine> is relatively protected. Comparing the resulting sequences reveals methylation at individual positions. The method introduced by <M. Frommer> and colleagues in 1992 turned a modification invisible in a simple four-letter <DNA> string into a positional readout.<A genomic sequencing protocol that yields a positive display of 5-methylcytosine residues in individual DNA strands by M. Frommer and others>[1992, abstract: bisulfite conversion and strand-level sequencing]{ref} A methylation measurement must still be interpreted in its genomic and cellular context; the mark does not mean universal shutdown of <transcription>.
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