4-Methylumbelliferyl-N-acetyl-alpha-D-neuraminic acid sodium salt (4-MU-Neu5Ac) is a fluorogenic substrate specifically designed for the detection and assay of neuraminidase (sialidase) activity. It consists of the fluorophore 4-methylumbelliferone (4-MU) covalently linked to N-acetyl-alpha-D-neuraminic acid (Neu5Ac, a sialic acid) via an alpha-glycosidic bond. The primary application of this compound is as a diagnostic and research reagent in enzymology and microbiology. Neuraminidases are enzymes that cleave terminal sialic acid residues from glycoproteins, glycolipids, and oligosaccharides. When 4-MU-Neu5Ac is hydrolyzed by neuraminidase, it releases the highly fluorescent 4-methylumbelliferone, enabling sensitive, continuous, and quantitative measurement of enzyme activity. This makes it a critical tool for studying viral neuraminidases (e.g., from influenza viruses), bacterial sialidases, and mammalian sialidases involved in various physiological and pathological processes. It is widely used in high-throughput screening for antiviral drug discovery (particularly against influenza), in clinical diagnostics for detecting microbial neuraminidase activity, and in basic biochemical research on glycosylation and carbohydrate metabolism. The compound is typically supplied as a ready-to-use substrate in assay buffers.