Protein glycosylation can be classified according to their respective invariant core structures and the linkage to amino acids of the proteins. The main types of protein glycosylation are N-linked glycosylation, O-linked glycosylation, and C-linked glycosylation resulting in N-glycans, O-glycans, and C-glycans, respectively. The core structures are further elongated with the exception of C-linked glycosylation and O-linked GlcNAcylation.