glycoprotein is a compound containing carbohydrate (or glycan) covalently linked to protein. The carbohydrate may be in a form of monosaccharide, disaccharide(s), oligosaccharide(s), polysaccharide(s), or their derivatives (e.g. sulfo- or phos-pho-substituted). One, a few, or many carbohydrate units may be present. There are two most common types of glycoproteins: O- and N-glycoproteins. In eukaryotes, the most prevalent type of O-linked glycosylation is the mucin-type glycosylation, where N-acetyl-D-galactosamine (GalNAc) is linked in an ?-anomeric configuration to the ?-hydroxyl group of either a serine or a threonine residue of the polypeptide. Other types of O-linked glycosylation include glycosaminoglycans, such as heparin and chondrotin sulfate, which are attached to the polypeptide chain through ?-linked xylose residues. An older and more prevalent type of glycosylation, N-linked glycosylation, is found in a wide range of organisms ranging from Archae to mammals and other eukaryotes. N-Glycosylation is a modification performed cotranslationally (during the translation of mRNA to protein) and is available to any secreted or membrane-bound protein containing the triplet amino acid sequence AsnXxxSer/Thr (where Xxx is any amino acid except Pro). An oligosaccharide is transferred to the amide side chain of Asn from a dolichol phosphate glycosyl donor, by the action of mem-brane-bound oligosaccharyl transferase in the endoplasmic reticulum (ER). The fully translated glycoprotein is then subject to glycan trimming and processing which is further elaborated in the ER and Golgi apparatus.