TF and hyperinsulinemic hypoglycemia, familial, 4: The first CDG classification system (sub)classified the N-glycosylation defects alphabetically (e.g., CDG-Ia, CDG-IIa, etc.)and was based on the serum transferrin pattern obtained by IEF, the gold standard screening technique for N-glycosylation defects with sialic acid deficiency [6, 7].