Keratan sulfate (KSI) is the best characterised keratan sulfate. It is 10 times more abundant in cornea than cartilage. KSI is attached to an asparagine (Asn) residue on the core protein via an N-linked branched oligosaccharide (an N-glycan core structure used as a precursor in N-glycan biosynthesis). KSI is elongated by the alternate additions of galactose (Gal) and N-acetylglucosamine (GlcNAc), mediated by glycosyltransferases. Elongation is terminated by the addition of a single N-acetylneuraminic acid (sialyl) residue. KSI is also sulfated on Gal and GlcNAc residues by at least two sulfotransferases (Funderburgh 2000, Funderburgh 2002, Quantock et al. 2010). KSI can be attached to asparagine residues on core proteins, creating so called proteoglycans (PGs). Seven common core proteins found in corneal and skeletal tissues are used as examples here.