J. Clin. Invest. 108: 175–180 (2001). DOI: 10.1172/JCI200113561. a molecular design in which clusters of N-and O-sulfated sugar residues (red or green in Figure 1) are separated by nonsulfated regions (blue in Figure 1). Depending on the enzyme chosen for gene targeting, the change in HS structure may be more or less dramatic (Figure 1). If enzymes involved in polymerization of the HS chain are knocked out (EXT in Figure 1), only short stubs containing the HS linkage region will be attached to the core protein. If NDST is chosen, the polysaccharide chain will be formed but will remain nonsulfated (Figure 1). Gene targeting of any of the other modifying enzymes will result in a more subtle alteration of the structure of the HS chain (Figure 1). Since several of the enzymes involved in HS biosynthesis occur in isoforms (see Esko and Lindahl, this Perspective series, ref. 1), the outcome of a gene knockout may be more complex. For example, four NDSTs have been identified, several of which may be contained in the same cell (see ref. 8 and references therein). As demonstrated for NDST-1–deficient cells, the HS synthesized by these cells is still sulfated, but to a lesser extent than control cells (9). So far mouse strains have been described that carry mutations in an HS polymerase (10), in two NDST isoforms (9, 11–13), and in a 2-O-sulfotransferase (14), respectively. In addition, work is in progress in several laboratories to create mouse strains lacking other enzymes or combinations of enzymes, as well as generating mice carrying alleles that allow for conditional gene inactivation. It can be anticipated that only the most obvious phenotypes in these mouse strains have so far been identified and that detailed analyses will reveal further physiological alterations. It will be a challenge to explain the cause of several of the observed defects on a molecular level.