Glycosaminoglycans (GAGs) are classified into several types, including hyaluronic acid, chondroitin sulfate, dermatan sulfate, heparan sulfate, heparin, and keratan sulfate. These GAGs are characterized by the composition of their constituent monosaccharides, the structure of their glycosidic linkages, and their complex patterns of sulfation. Among them, heparin is a well-known anticoagulant that has been used clinically for nearly a century. Heparin shares the same basic sugar chain structure as heparan sulfate, consisting of disaccharide units containing α-L-iduronic acid or β-D-glucuronic acid and α-D-glucosamine (Fig. 108.1). However, the degree of sulfation per disaccharide differs between heparin and heparan sulfate, and only highly sulfated heparin exhibits anticoagulant activity. The main sugar chain units of heparin include sulfated trisaccharides (TriS) and disaccharides (GlcNS6S-IdoA2S). Heparin is primarily stored in the secretory granules of mast cells, whereas heparan sulfate is widely expressed on the cell surface and in the extracellular matrix. Currently, pharmaceutical-grade heparin is predominantly derived from animal tissues.