38Protein glycosylation events that happen early in the secretory pathway are often dysregulated during 39 tumorigenesis. These events can be probed, in principle, by monosaccharides with bioorthogonal tags that 40 would ideally be specific for distinct glycan subtypes. However, metabolic interconversion into other 41 monosaccharides drastically reduces such specificity in the living cell. Here, we use a structure-based 42 design process to develop the monosaccharide probe GalNAzMe that is specific for cancer-relevant 43