From glycosylation disorders back to glycosylation: What have we learned?

Diseases of glycosylation have long remained confined to the rare hematological disorders, the Tn-syndrome [1] and paroxysmal nocturnal hemoglobinuria [2]. This rarity was often interpreted as a sign that defects of glycosylation are either lethal, or remain asymptomatic because of the large redundancy found in glycosylation pathways. The description of multiple glycosylation disorders over the last years has definitively settled the issue and demonstrated the broad range of biological processes relying on proper glycosylation. However, beyond establishing the developmental and physiological roles of glycosylation how did glycosylation disorders provided new insights to the field of glycobiology?