Mannosidase I creates a Man5GlcNAc2-structure on the protein, but note that this has a different structure than the one made on LLO. Next, a GlcNAc residue forms GlcNAc1Man5GlcNAc2, the substrate for a-mannosidase II (aManII). aManII then removes two Man residues, creating the substrate for GlcNAc transferase II, which adds a GlcNAc to the second Man branch. This structure serves as substrate for additional galactosylation, fucosylation and sialylation reactions. Additionally, substitution with more GlcNAc residues can yield tri- and tetra-antennary molecules.Not all structures are fully modified, some remain as high-mannose structures, others as hybrids (one unmodified Man branch and one modified), but the majority become fully modified complex type oligosaccharides.In addition to glycosidase I, mutations have been found:
in MGAT2, in GlcNAc transferase II (CDG-IIa)
in SLC35C1, the GDP-Fuc transporter (CDG-IIc)
in B4GALT1, a galactosyltransferase (CDG-IId)
in COG7, the conserved oligomeric Golgi complex-7 (CDG-IIe)
in SLC35A1, the CMP-sialic acid (NeuAc) transporter (CDG-IIf)However, the use of >100 genes in this process, presumably means that many more defects are to be found. Diagnosis
Classification
Historically, CDGs are classified as Types I and II (CDG-I and CDG-II), depending on the nature and location of the biochemical defect in the metabolic pathway relative to the action of oligosaccharyltransferase. The most commonly used screening method for CDG, analysis of transferrin glycosylation status by isoelectric focusing, ESI-MS, or other techniques, distinguish between these subtypes in so called Type I and Type II patterns.Currently, twenty-two CDG Type-I and fourteen Type-II subtypes of CDG have been described.Since 2009, most researchers use a different nomenclature based on the gene defect (e.g.
CDG-Ia = PMM2-CDG, CDG-Ib = PMI-CDG, CDG-Ic = ALG6-CDG etc.). The reason for the new nomenclature was the fact that proteins not directly involved in glycan synthesis (such as members of the COG-family and vesicular H+-ATPase) were found to be causing the glycosylation defect in some CDG patients. Also, defects disturbing other glycosylation pathways than the N-linked one are included in this classification. Examples are the α-dystroglycanopathies (e.g. POMT1/POMT2-CDG (Walker-Warburg syndrome and Muscle-Eye-Brain syndrome)) with deficiencies in O-mannosylation of proteins; O-xylosylglycan synthesis defects (EXT1/EXT2-CDG (hereditary multiple exostoses) and B4GALT7-CDG (Ehlers-Danlos syndrome, progeroid variant)); O-fucosylglycan synthesis (B3GALTL-CDG (Peters plus syndrome) and LFNG-CDG (spondylocostal dysostosis III)). Type I
Type I disorders involve disrupted synthesis of the lipid-linked oligosaccharide precursor (LLO) or its transfer to the protein.Types include:

Type II
Type II disorders involve malfunctioning trimming/processing of the protein-bound oligosaccharide chain.Types include:

Disorders of O-mannosylation
Disorders with deficient α-dystroglycan O-mannosylation.Mutations in several genes have been associated with the traditional clinical syndromes, termed muscular dystrophy-dystroglycanopathies (MDDG). A new nomenclature based on clinical severity and genetic cause was recently proposed by OMIM. The severity classifications are A (severe), B (intermediate), and C (mild). The subtypes are numbered one to six according to the genetic cause, in the following order: (1) POMT1, (2) POMT2, (3) POMGNT1, (4) FKTN, (5) FKRP, and (6) LARGE.Most common severe types include:

Treatment
No treatment is available for most of these disorders. Mannose supplementation relieves the symptoms in MPI-CDG for the most part, even though the hepatic fibrosis may persist. Fucose supplementation has had a partial effect on some SLC35C1-CDG patients.