The ImaGene Green™ GUS Gene Expression Kit contains a fluorescein-based galactosidase substrate that has been covalently modified to include a 12-carbon lipophilic moiety, C₁₂FDGlcU. Once inside the cell, the substrates are cleaved by ß-glucoronidase, producing a fluorescent product that is well retained by the cells, probably by incorporation of the lipophilic tail within the cellular membrane. In addition to the C₁₂FDGlcU, the kits also include the broad-spectrum ß-glucoronidase inhibitor, D-glucaric acid-1,4-lactone.