Glycoproteins are biomolecules in which sugars and proteins are covalently bonded. The binding modes of glycans on proteins are generally classified as N-linked glycans (N-glycans) and O-linked glycans (O-glycans). N-glycans modify GlcNAc as a reducing end to the amide group (–CONH2) of asparagine (Asn) in the Asn-X-Ser/Thr motif on polypeptides. Based on structural features, they are categorized into high mannose, hybrid, and complex types (Fig. 40.1). In contrast, O-glycans refer to glycans that are covalently bonded to the hydroxyl group (–OH) of Ser or Thr residues on polypeptides, which include O-GalNAc, O-Fuc, O-Man, O-GlcNAc, O-Gal, and O-Glc (Fig. 40.1). It is known that the glycan structure of the same glycoprotein varies depending on the time of development, tissue, and cell [1]. Additionally, the functions of glycans in glycoproteins have been clarified through the analysis of knockout (KO) and tissue-specific KO mice of glycan genes [2]. Therefore, glycans play a significant role in the functional expression of glycoproteins.