MUG is a fluorogenic MU substrate for detecting galactosidase in cell extracts and purified enzyme preparations using a fluorescence microplate reader or fluorometer. The organic and inorganic esters and glycosides of 7-hydroxy-4-methylcoumarin (4-methylumbelliferone, MU) are among the most commonly used fluorogenic substrates. These substrates yield water-soluble blue fluorescent coumarin fluorophores. MU develops maximum fluorescence above pH 10.