The glycans on proteins are either N-linked (to the amide group of asparagine via an N-acetylglucosamine residue) or O-linked (to the hydroxyl group of serine or threonine via an N-acetylgalactosamine or a xylose residue).

The first patients with a CDG were reported in 1980 (78). Since then, investigators have discovered a list of deficiencies in N-glycan assembly, N-glycan processing and O-glycan assembly. A combination of N- and O-glycosylation defect is also possible. Most glycosylation disorders are severe, multisystem disease, underlining the extremely important biological roles of glycans for example in recognition, signal transduction and immunity.