A glycosaminoglycan is any polysaccharide containing a substantial proportion of aminomonosaccharide residues. Glycosaminoglycan (GAG) are long unbranched polysaccharides consisting of a repeating disaccharide unit. Among the most common GAGs are chondroitin sulfate, dermatan sulfate, chondroitin sulfate, keratan sulfate, heparan sulfate, and hyaluronan.