A 4-methylumbelliferyl α-glycoside of a GlcNAcβ1-4Man disaccharide, mimicking the inner N-glycan substructure released by endo-β-mannosidase activity. Used as a fluorogenic substrate for α-mannosidases and N-glycan-processing enzymes acting on this linkage; enzymatic cleavage releases 4-methylumbelliferone. 4-Methylumbelliferyl (MU) glycosides are fluorogenic enzyme substrates: cleavage of the glycosidic bond by the target enzyme releases free 4-methylumbelliferone, which is strongly fluorescent under basic conditions (excitation ~365 nm, emission ~450 nm), enabling sensitive, quantitative enzyme-activity assays in solution, in tissue homogenates, and in high-throughput screening formats.