S-(5′-Adenosyl)-L-homocysteine is a key intermediate in methionine and S-adenosylmethionine (SAM) metabolism. It acts as a potent feedback inhibitor of SAM-dependent methyltransferase enzymes, which are involved in numerous biological methylation reactions, including DNA, RNA, protein, and phospholipid methylation. Due to this inhibitory role, it plays an important part in regulating cellular methylation processes. In research, it is used to study epigenetic regulation, as modulation of methyltransferase activity can affect gene expression patterns. It also serves as a biomarker for impaired methylation capacity in certain disease states, such as liver disorders and cardiovascular diseases. Additionally, because of its role in metabolic pathways, it is relevant in the development of therapeutic agents targeting methylation-related diseases, including cancer and neurological disorders.