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The methylation process is initiated by addition of a methyl group to one strand of the DNA by DNMT3A or DNMT3B. The resultant hemimethylated DNA becomes fully methylated by the action of DNMT1. Removal of methyl groups from DNA may be active (involving DNA demethylases) or passive (in the absence of maintenance – see also Figure 24). Abbreviation: m, methyl (CH3) group.
