The majority of proteins are posttranslationally modified, and the most significant modification to many secreted and membrane-associated proteins of eukaryotic cells is glycosylation, that is, the attachment of one or more oligosaccharide (glycan) chains. Glycans may be attached to the peptide backbone through different types of linkage but they usually are subdivided into those attached to glycoproteins primarily through an amide linkage to asparagine residues (N-linked glycans), and those attached through an O-glycosidic linkage to serine or threonine residues (O-linked glycans) or where the carbohydrates form part of a glycosylphosphatidyl inositol moiety (GPI) attached to the C-terminus of the peptide. Other types of linkage occur in certain other glycoconjugates such as the linkage to hydroxylysine residues in collagen and β-xylose of glycosaminoglycan chains in proteoglycans to serine residues in the peptide core.