Genetic reporter systems are widely used to study eukaryotic gene expression and cellular physiology. Applications include the study of receptor activity, transcription factors, intracellular signaling, mRNA processing and protein folding. Dual reporters are commonly used to improve experimental accuracy. The term “dual reporter” refers to the simultaneous expression and measurement of two individual reporter enzymes within a single system. Typically, the “experimental” reporter is correlated with the effect of specific experimental conditions, while the activity of the co-transfected “control” reporter provides an internal control that serves as the baseline response. Normalizing the activity of the experimental reporter to the activity of the internal control minimizes experimental variability caused by differences in cell viability or transfection efficiency. Other sources of variability, such as differences in pipetting volumes, cell lysis efficiency and assay efficiency, can be effectively eliminated. Thus, dual-reporter assays often allow more reliable interpretation of the experimental data by reducing extraneous influences. The Dual-Luciferase® Reporter (DLR™) Assay System(a–c) provides an efficient means of performing dual-reporter assays. In the DLR™ Assay, the activities of firefly (Photinus pyralis) and Renilla (Renilla reniformis, also known as sea pansy) luciferases are measured sequentially from a single sample. The firefly luciferase reporter is measured first by adding Luciferase Assay Reagent II (LAR II) to generate a stabilized luminescent signal. After quantifying the firefly luminescence, this reaction is quenched, and the Renilla luciferase reaction is simultaneously initiated by adding Stop & Glo® Reagent to the same tube. The Stop & Glo® Reagent also produces a stabilized signal from the Renilla luciferase, which decays slowly over the course of the measurement. In the DLR™ Assay System, both reporters yield linear assays with subattomole sensitivities and no endogenous activity of either reporter in the experimental host cells. Furthermore, the integrated format of the DLR™ Assay provides rapid quantitation of both reporters either in transfected cells or in cell-free transcription/translation reactions. Promega offers the pGL4 series of firefly and Renilla luciferase vectors designed for use with the DLR™ Assay Systems. These vectors may be used to co-transfect mammalian cells with experimental and control reporter genes.