Heparan sulfate (HS) is a negatively charged cell surface polysaccharide required for the biologic activities of circulating extracellular ligands. GLCE is responsible for epimerization of D-glucuronic acid (GlcA) to L-iduronic acid (IdoA) of HS, which endows the nascent polysaccharide chain with the ability to bind growth factors and cytokines (Ghiselli and Agrawal, 2005 [PubMed 15853773])