S-Adenosyl-L-homocysteine is an amino acid derivative used in several metabolic pathways in most organisms. S-Adenosyl-L-homocysteine (SAH) is an amino acid derivative and an intermediate, by-product, or modulator of several metabolic pathways, including the activated methyl cycle and cysteine biosynthesis. SAH is formed by the demethylation of S-adenosyl-L-methionine. It is also a product of S-adenosyl-methionine (SAM)-dependent methylation of biological molecules, including DNA, RNA, and histones and other proteins. Inhibitors that block its hydrolysis are being developed as anti-viral, anti-parasitic, anti-arthritic and immunosuppressive agents.