As the sulfate content rises, so does the iduronic acid:glucuronic acid ratio. Once glucosamine is sulfated, glucuronic acid (GlcA) is epimerised to iduronic acid (IdoA). The enzyme glucuronyl C5-epimerase (GLCE) mediates this reaction, evidence of function and cellular location coming from mouse studies (Li et al. 2001, Crawford et al. 2001). The distinction between HS and heparin is fairly arbritary but generally, low-sulfated and GlcA-rich polysaccharides are called HS and high-sulfated and IdoA-rich polysaccharides are called heparin. It can be argued that this structure can now be called either heparan sulfate-PG or heparin-PG.