Figure 1: Heparan sulfate structure. Heparan sulfate glycosaminoglycan is linked to specific serine residues on heparan sulfate proteoglycans by a tetrasaccharide sequence of glucuronic acid, galactose, galactose, and xylose. The HS chains contain clusters of N-acetylated unmodified domains and N-sulfated modified domains. Specific sequences in the N-domains bind to different growth factors and their receptors, for example, fibroblast growth factor-2 (FGF-2) and its receptor (FGFR).