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Mutation of a cytosine (C) DNA base to a thymine (T) base is more likely to occur if the C is followed by a guanine (G) and if the C is chemically modified by methylation. This was thought to be mainly due to the spontaneous deamination of methylated C to T. A new study in DNA Repair by Tomkova and colleagues from the labs of Benjamin Schuster-Böckler and Skirmantas Kriaucionis demonstrates that errors introduced during replication may also have a role in the increased mutability of methylated cytosine to thymine.