In 1992, mass spectrometry (MS) revealed a molecular abnormality derived from a defect in the early step of the N-glycosylation of transferrin, and this observation contributed to establishing CDG as an entity.4) In the last thirty years, nearly 100 different disease types or genes responsible for congenital disorders of N-glycosylation have been identified.5) Since the symptoms and signs of CDG are quite diverse, making it impossible to establish a diagnosis using routine laboratory tests,6,7) MS is an essential approach for accurately diagnosing CDG.8) The gene discussed is TF; the disease is congenital disorder of glycosylation.