Chloramphenicol O-acetyltransferase (CAT) has, until recently, been widely used as a reporter protein [5] . CAT is a bacterial enzyme that is not expressed in eukaryotes, and it therefore provides a highly specific detection system. CAT catalyzes the reaction between acetyl-CoA and chloramphenicol to form CoA. The CAT assay has been widely used but is difficult to adapt to HTS platforms because its activity is not easy to detect directly in cells.