= Methyltransferase
A <methyltransferase> is an <enzyme> transferring a <methyl group> from a donor to an acceptor. The acceptor can be a small molecule, <DNA>, <RNA> or a <protein>; the family name specifies a type of chemical transfer, not a universal biological function. Many methyltransferases use S-adenosylmethionine as donor. <Molecular Biology of the Cell, 4th edition by Bruce Alberts and others>[Chapter 7, Control of Gene Expression: DNA methylation and chromatin modifications]{ref}
For example, <DNA methylation> changes selected bases, while <histone methylation> modifies <amino acid> side chains in a different macromolecule. Different acceptor positions can have different consequences. A specific activity assay therefore needs a defined donor, acceptor and product measurement: disappearance of donor alone does not establish the intended site or exclude another methyl-transfer reaction.
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