4-Methylumbelliferyl 3-O-(?-L-fucopyranosyl)-?-D-galactopyranoside is an organic compound that is used as a radioactive marker for DNA. It can be synthesized in the laboratory and its use allows for the tracking of DNA molecules. The production of this marker is achieved by recombinant expression of the gene encoding the enzyme ?-D-galactosidase, which hydrolyzes 4-methylumbelliferyl ?-D-galactopyranoside to release 4-methylumbelliferone and ?-L-fucopyranose. The enzyme uses UDP glucose as a cofactor to catalyse the reaction. This product has been isolated from Corynebacterium glutamicum, with recombinant expression vectors used to produce it in host cells such as Escherichia coli or Corynebacterium glutamicum.