4-Methylumbelliferyl β-D-glucuronide dihydrate is a fluorogenic substrate that fluoresces upon treatment with glucuronidase (GUS). MUG dihydrate’s fluorescent properties allow for identification of transformed plants due to the presence of encoded GUS within the plants. 4-Methylumbelliferyl β-D-glucuronide dihydrate is also used to test the presence of coliform bacteria in water.