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Congenital disorders of glycosylation (CDG) are a rapidly growing disease family with about 40 diseases reported since its first clinical description in 1980 . The large majority of these are diseases of protein hypoglycosylation, but in recent years several defects in lipid glycosylation have also been identified [2,3]. Most protein glycosylation disorders are due to defects in the N-glycosylation pathway, the remaining ones affecting the O-glycosylation pathway or combined N-and O-glycosylation pathways. No defects in C-glycosylation have been detected yet. The first described CDG patients were shown to have an abnormal serum transferrin (Tf) isoelectrofocusing (IEF) pattern with increases in the di-and asialotransferrin fractions . They were found to have deficient phosphomannomutase (PMM) activity  and mutations in the PMM2 gene . PMM-deficient patients were designated as CDG-Ia. Subsequently, a patient was discovered with a serum Tf IEF pattern characterized by increases not only of the even (2 and 0) but also of the uneven (3 and 1) sialoTf bands . Since these patterns were qualitatively different, we called the latter a type 2 pattern as opposed to the type 1 pattern seen in PMM deficiency. In the patient with the type 2 pattern, a deficiency was demonstrated to be in a Golgi glycosyltransferase, namely N-acetylglucosaminyltransferase II . This disease was labeled CDG-IIa. New patients were classified as CDG-I or CDG-II according to the Tf IEF pattern, and each new defect took the next letter of the alphabet.
We presently count 14 CDG-I diseases (CDG-Ia up to CDG-In), and 8 CDG-II diseases (CDG-IIa up to CDG-IIh). Since this nomenclature is based on the Tf IEF pattern, it relates only to N-glycosylation diseases associated with deficient sialylation. Gradually it became clear that CDG-I defects were limited to defects in pre-ER or ER proteins whereas CDG-II defects were caused by defects in Golgi or Golgi-associated proteins. However, some of these disorders also show abnormal O-glycosylation such as the COG defects (review in ) and the V-ATPase defect in cutis laxa type II . Also, it appeared that a patient with an alpha-glucosidase I deficiency in the ER had a normal Tf IEF pattern . Still this patient was labeled as CDG-IIb, which is an inconsistency of this classification. For this reason and for a number of other reasons explained elsewhere , we strongly suggest that this nomenclature should be discontinued in favor of a transparent designation of glycosylation disorders and that it be applied to new and established types of CDG. We propose using only the official gene symbol (not in italics) followed by ‘-CDG’ (list of approved gene names at http://www.genenames.org). A classification of the known types of CDG, along with the traditional and new nomenclature, is shown in Table 1 (adapted from ).