C12FDGlcU, a lipophilic analog of fluorescein di-β-D-glucuronic acid, is valuable for detecting β-glucuronidase (GUS) gene expression. This compound penetrates cells and is subsequently cleaved by β-glucuronidase, releasing a yellow-colored, green-fluorescent fluorescein (Abs Em of the reaction product: 495 518 nm) [1] [2].