Methylation is a common but generally minor pathway of xenobiotic biotransformation. Unlike most other conjugative reactions, methylation often does not dramatically alter the solubility of substrates and results either in inactive or active compounds. Methylation reactions are primarily involved in the metabolism of small endogenous compounds such as neurotransmitters but also play a role in the metabolism of macromolecules for example nucleic acids and in the biotransformation of certain drugs. A large number of both endogenous and exogenous compounds can undergo N– (Fig. 6a), O– (Fig. 6b), S– (Fig. 6c) and arsenic–methylation during their metabolism (Feng et al., 2010). The co–factor required to form methyl conjugates is S–adenosylmethionine (SAM), which is primarily formed by the condensation of ATP and L–methionine.