Proteoglycans (PGs) are macromolecules composed of glycosaminoglycan (GAG) chains covalently bound to a protein core. The GAG components consist of hexosamine [o-glucosamine (GleN) or o-galactosamine (GaiN)] and either hexuronic acid [o-glucuronic acid (GleA) or L-iduronic acid (IdoA)] or galactose units (in keratan sulfate) that are arranged in alternating, unbranched sequence, and carry sulfate substituents in various positions. The common GAGs [in addition to hyaluronan (HA), which does not occur as a PG but in free form] include the galactosaminoglycans, chondroitin sulfate