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Finding of the deletion phenomenon of certain oligosaccharides in human milk and its correlation to the blood types of the donors opened a way to elucidate the biochemical basis of blood types in man. This success led to the idea of establishing reliable techniques to elucidate the structures and functions of the N-linked sugar chains of glycoproteins. N-Linked sugar chains were first released quantitatively as oligosaccharides by enzymatic and chemical means, and labelled by reduction with NaB3H4. After fractionation, structures of the radioactive oligosaccharides were determined by a series of methods developed for the studies of milk oligosaccharides. By using such techniques, structural rules hidden in the N-linked sugar chains, and organ- and species-specific N-glycosylation of glycoproteins, which afforded a firm basis to the development of glycobiology, were elucidated. Finding of galactose deficiency in the N-linked sugar chains of serum IgG from patients with rheumatoid arthritis, and malignant alteration of N-glycosylation in various tumors opened a new research world called glycopathology. However, recent studies revealed that several structural exceptions occur in the sugar chains of particular glycoproteins. Finding of the occurrence of the Galbeta1-4Fucalpha1- group linked at the C-6 position of the proximal N-acetylglucosamine residue of the hybrid type sugar chains of octopus rhodopsin is one of such examples. This finding indicated that the fucosyl residue of the fucosylated trimannosyl core should no more be considered as a stop signal as has long been believed. Furthermore, recent studies on dystroglycan revealed that the sugar chains, which do not fall into the current classification of N and O-linked sugar chains, are essential for the expression of the functional role of this glycoprotein. It was found that expression of many glycoproteins is altered by aging. Among the alterations of the glycoprotein patterns found in the brain nervous system, the most prominent evidence was found in P0. This protein is produced in non-glycosylated form in the spinal cord of young mammals. However, it starts to be N-glycosylated in the spinal cord of aged animals. These evidences indicate that various unusual sugar chains occur as minor components in mammals, and play important roles in particular tissues.