S-(5’-Adenosyl)-L-methionine chloride, commonly abbreviated as SAM or AdoMet, is a crucial sulfonium compound formed by the enzymatic linkage of the amino acid L-methionine to the 5'-carbon of adenosine triphosphate (ATP). It serves as the primary biological methyl donor in a vast array of transmethylation reactions, transferring its methyl group to substrates such as nucleic acids, proteins, phospholipids, and small molecules. SAM is also a key precursor in polyamine biosynthesis and serves as a propylamino donor in the synthesis of spermidine and spermine. This molecule is fundamental to cellular metabolism and gene expression regulation. In research and pharmaceutical contexts, SAM chloride is widely used as a biochemical reagent to study methyltransferase enzymes, epigenetic mechanisms (including DNA and histone methylation), and metabolic pathways. It is also investigated for its potential therapeutic applications related to liver function, mood disorders, and osteoarthritis, often formulated as a dietary supplement. Due to its instability, it is typically handled and stored as a stable salt form, such as the chloride or tosylate salt, under controlled conditions.