4-Methylumbelliferyl N-acetyl-4,7,8,9-tetra-O-acetyl-a-D-neuraminic acid methyl ester (4MU) is a chemiluminescence substrate. It is used in the detection of an enzyme involved in the synthesis of glycoproteins and glycolipids. The enzyme conjugates 4MU with a sugar molecule to form a fluorescent product. This product can be detected by its emission of light at 560 nm when illuminated with light at 450 nm.