Analysis of gene expression is most commonly assayed by transient transfection. These systems are usually based on the use of fusion genes which are inserted into cells, and expression of the gene is assayed within 48 hours after introduction of DNA. Usually the fusion consists of the promoter binding site or enhancer sequence under study which is attached to a reporter gene. The amount of the reporter protein synthesized under the experimental conditions, is presumed to reflect the ability of the sequences studied to direct or promote transcription. Several enzymes are commonly used as reporter proteins, among them are chloramphenicol acetyl transferase (CAT), â-galactosidase, human growth hormone (hGH), and luciferase. Luciferase has become one of the more widely used reporter enzymes. The reporter plasmid contains the gene from the firefly Photinus pyralis. The enzyme catalyzes a bioluminescent reaction which requires the substrate luciferin as well as Mg+2 and ATP. Mixing these reagents with the cell extract containing luciferase, results in a flash of light that decays rapidly. This light can be detected by a luminometer. The total light emission is proportional to the luciferase activity of the sample. The luciferase assay is fast and sensitive and does not require a radioactive substrate as is in the CAT assay. A disadvantage of the luciferase assay is that it requires a rather expensive instrument, the luminometer, to measure enzyme activity. In addition, this assay lacks reproducibility between samples, largely due to the rapid kinetics of the emission.