N-Glycosylation is one of the most common co- and posttranslational modifications of eukaryotic proteins occurring in the ER lumen. N-glycosylation plays pivotal roles in protein folding and intra- or inter-cellular trafficking of N-glycosylated proteins. Quality control mechanisms in the ER sift out incorrectly-folded proteins from correctly-folded proteins, the former then destined for degradation. Incorrectly-folded N-glycans are exported to the cytosol where the process of degradation begins. Once the unfolded protein is cleaved from the oligosaccharide (forming free oligosaccharides, fOS), step-wise degradation of mannose moieties, both in the cytosol (Suzuki & Harada 2014) and then in the lysosome (Aronson & Kuranda 1989, Winchester 2005), results in complete degradation. Breakdown must be complete to avoid lysosomal storage diseases that occur when fragments as small as dimers are left undigested.