J. Clin. Invest. 108: 165–167 (2001). DOI: 10.1172/JCI200113560. away. Thus, heparan sulfate could easily cross a fullthickness basement membrane and touch an adjacent cell. The protein cores vary in size from about 32 to nearly 500 kDa (Figure 1). However, relatively few modules are used for building the protein core of various HSPGs, and even fewer for the covalent binding of the heparan sulfate chains. Diversity in protein core structure is, thus, a fraction of that expressed by heparan sulfate. Most, if not all, HSPGs can be hybrid molecules, carrying both heparan and chondroitin sulfate side chains. We do not know what tissue-and cell-specific constraints regulate the addition of these glycosaminoglycans or other aspects of the fate of a given HSPG, such as its targeting to a specific compartment within the membrane or its ability to be displaced by added heparin.