M5 glycan (Man5), 2-AB labelled (Mannose-5 N-linked oligosaccharide, 2-AB labelled; Oligomannose 5 glycan, 2-AB labelled) is a N-polysaccharide protein and a multifunctional fluorescent linker. The resulting conjugates exhibit high sensitivity and specificity by mimicking the antennal elements of N-glycans.