Heterogeneity in structure of pericellular and cell-associated heparan sulfate proteoglycans with pro- and antiangiogenic activity. Syndecans are transmembrane proteoglycans, while glypicans are bound to the plasma membrane via a phosphoinositol linkage. Type XVIII collagen, a hybrid collagen/proteoglycan molecule, is a member of the multiplexin gene family characterized by alternating collagenous (thin lines) and noncollagenous domains (rectangles, not drawn in scale). Perlecan is a modular proteoglycan with five distinct domains. Both type XVIII collagen and perlecan are secreted products but are in close association with the cell surface and are intrinsic components of most basement membranes. Agrin, another HSPG with similarities to perlecan, is not shown. Glycosaminoglycan chains are shown as fuchsia-colored beaded strings and are not drawn to scale relative to their respective protein cores. The precise location and number of the heparan sulfate chains on type XVIII collagen are not known.