Three keratan sulfate polymers have been found (KS-I, KS-II, KS-III) bound to core proteins. KS-I is the best characterised keratan sulfate. It is 10 times more abundant in cornea than cartilage. KS-I is elongated by the alternate additions of galactose (Gal) and N-acetylglucosamine (GlcNAc), mediated by glycosyltransferases. Elongation is often terminated by the addition of a single N-acetylneuraminic acid (sialyl) or a fucose residue. KS-I is also sulfated on many Gal and GlcNAc residues by at least two sulfotransferases (reviewed in Caterson & Melrose, 2018; Funderburgh 2000, Funderburgh 2002, Quantock et al. 2010). These reactions are running in parallel which we cannot represent in the diagram. The intermediate reaction products shown are therefore only for demonstration purposes.