A glycosaminoglycan(GAG) is a linear heteropolysaccharide possessing a characteristic disaccharide repeat sequence. One monosaccharide of the disaccharide repeat is an amino sugar with D-glucosamine or galactosamine, and the other unit is typically, but not always, a uranic acid residue of either D-glUCUr0nk acid or iduronic acid. Both units are variably N-and O-sulfated, which adds to the heterogeneity of these complex macromolecules. The most common GAG structures are chondroitin sulfate (CS), dermatan sulfate (DS), heparan sulfate (HS), keratan sulfate (KS), hyaluronic acid (HA), and heparin; representative structures for each disaccharide are shown in Figure 1. With the exceptions of heparin and HA, GAG chains are covalently attached at their reducing end through an 0-glycosidic linkage to a serine residue or N-linked to asparagine in a core protein; the resulting macromolecule is termed a proteoglycan. A major function of cell surface proteoglycans is in cell adhesion and migration, dynamic processes that are mediated through interactions between the proteoglycan GAG chains and extracellular matrix (ECM) components, such as laminin, collagen, and fibronectin. Proteoglycans also occur as integral components of basement membranes in probably all mammalian tissues. Interactions of these macromolecules with other ECM constituents contribute to the general architecture and permeability properties of the basement membrane, and thus these GAGS play a structural role. Proteoglycans and GAGS play a critical role in the pathophysiology of basement membrane-related diseases, including diabetes, atherosclerosis, and metastasis. In addition, cellspecific growth factors and enzymes are immobilized in the ECM and at the cell surface are bound to GAGS. As such, GAGS localize proteins and enzymes at their site of action to facilitate their physiological functions and in some cases prevent their proteolytic degradation. Recently, proteoglycans and GAGS have been shown to regulate protein secretion and gene expression in certain tissues by mechanisms involving both membrane and nuclear events, including the binding of GAGS to transcription factors. Limited information is available on the factors that regulate the expression of proteoglycans and their associated GAGS. It is known that in the developing vertebrate marked changes in proteoglycan expression occur, suggesting that these macromolecules play a role in cell differentiation. Although GAGS have a heterogeneous composition, recent findings indicate that specific structural determinants of the polysaccharide units mediate their interaction with GAG-binding proteins. Furthermore, several of these proteins