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. It is also a product of S-adenosyl-methionine (SAM)-dependent methylation of biological molecules，including DNA，RNA，and histones and other proteins. SAH is a risk factor for many diseases，including cancer and neurodegenerative diseases.
