In particular, variants in the PMM2 gene have been proved to contribute to defects in the protein glycosylation pathway (11, 13–16), manifesting as carbohydrate-deficient glycoprotein syndrome type I (congenital disorders of glycosylation, type 1a, CDG1a), with CDG1a the most common CDG disorder. Here, PMM2 is linked to PMM2-congenital disorder of glycosylation.