CAT assay is an enzyme assay. CAT stands for the chloramphenicol acetyl transferase, a bacterial enzyme which inactivates the chloramphenicol by acetylating it. CAT assays are many times performed to test the function of a promoter. The gene coding for CAT is linked onto the promoter (transcription control area) from another gene, and the construct is "transfected" into cultured cells. The quantity of CAT enzyme produced is taken to indicate transcriptional activity of the promoter (relative to other promoters which should be tested in parallel). It is simple to perform a CAT assay than it is to do a Northern blot, so CAT assays were common techniques for testing the effects of sequence changes on the promoter function. It is largely supplanted by the reporter gene luciferase.