Most proteins, including immunoglobulins, human virus receptors, and viral-coded proteins, are post-translationally modified with sugars or sugar chains that are generically referred to as glycans. Glycans are primarily classified as N-linked or O-linked oligosaccharides, depending on whether they are bound to the amide group of asparagine (N-linked) or the hydroxyl group of serine or threonine (O-linked). Glycans are associated with protein conformation, folding, solubility, stability, half-life, and antigenicity and are the moieties recognized by glycan-binding proteins. The congenital disorders of glycosylation (CDGs) are genetic disorders affecting the N-glycosylation process. CDGs are divided into defects in the synthesis of N-glycans (CDG-I) and defects in the processing of N-glycans (CDG-II). CDG-IIb (Online Mendelian Inheritance in Man database number, 606056) is caused by mutations in the gene encoding MOGS (also known as glucosidase 1). MOGS is an enzyme that is expressed in the endoplasmic reticulum and is involved in the trimming of N-glycans.1 A single case of CDG-IIb has been reported; the patient died at the age of 74 days from severe neurologic complications.2 In this study, scientists evaluated the immune system and susceptibility to viral diseases in two siblings with CDG-IIb who presented with severe hypogammaglobulinemia but not many infections.