S-adenosylmethionine (AdoMet) is a metabolically activated form of methionine involved in donating methyl groups. Transfer of a methyl group from AdoMet to a target molecule converts AdoMet to S-Adenosylhomocysteine (AdoHcy) (see here). Table 21.1 lists some important AdoMet-dependent transmethylations. Substrates range from small metabolites, such as norepinephrine (see reaction here) to polymers, such as DNA (see here), RNA, or proteins.