Schematic representation of the five HSPG prototypes. A total of thirteen distinct genes encompass nearly all the full-time HSPGs. These include four syndecans, six glypicans, and one each of perlecan, agrin, and the hybrid HSPG/collagen type XVIII. In addition, splice variants have been reported for perlecan, agrin, and type XVIII collagen. The size is roughly proportional to the number of amino acid residues (aa). The linear schemes do not take into account globular domains or other types of protein folding. The key to the various color-coded modules is provided in the bottom panel. The heparan sulfate chains are shown as wavy lines.