Heparan sulfate (HS) and heparin (sometimes collectively called HS-GAG) consist of the disaccharide unit GlcNAc-GlcA (N-acetylglucosamine-glucuronic acid) connected by a beta1,4 linkage. Heparin is exclusively made in mast cells whereas HS is made by virtually every type of cell in the body. As the chain length increases, the polysaccharides can undergo modifcations such as epimerisation of glucuronic acid to iduronic acid and deacetylation and sulfation of GlcNAc to form sulfated glucosamine (Stringer & Gallagher 1997, Sasisekharan & Venkataraman 2000).