Glycoproteins are proteins that contain oligosaccharide chains (glycans) covalently attached to polypeptide side-chains. Secreted extracellular proteins are often glycosylated. In proteins that have segments extending extracellularly, the extracellular segments are also glycosylated. Glycoproteins are often important integral membrane proteins, where they play a role in cell-cell interactions. Glycoproteins are also formed in the cytosol, but their functions and the pathways producing these modifications in this compartment are less well understood. One example of glycoproteins found in the body is mucins, which are secreted in the mucus of the respiratory and digestive tracts. The sugars attached to mucins give them a considerable water-holding capacity and also make them resistant to proteolysis by digestive enzymes. Glycoproteins are important for white blood cell recognition, especially in mammals [1][2].