β-Galactosidase (β-Gal) is derived from the bacterial lacZ gene and is widely used as an intracellular reporter gene. There are a number of detection options depending on which substrate is used. However, β-Gal is affected by endogenous enzymatic activity in most mammalian cells [4] . Therefore, it is important to distinguish between the endogenous and exogenous (reporter) enzymes in the assay system. This usually requires the pH of the assay mixture to be adjusted, although this may not completely eliminate endogenous enzyme activity. Therefore, the utility of β-Gal for HTS assays is limited.