Luciferase is the general term given to a class of oxidative enzymes that catalyze reactions that give off light, a process known as bioluminescence (Fig. 1). In biology, researchers can take advantage of this reaction and use it as a readout for various biological processes. This has perhaps been exploited most in luciferase reporter cell lines where a promoter region from a gene of interest is placed immediately upstream of the coding sequence for luciferase. In this system, transcriptional activation of the gene of interest leads to a level of luciferase expression that is proportional to the level of gene activation. The level of activation is then assessed by lysing the cells and adding the luciferase substrate, D-luciferin. Using the One-Step Luciferase Assay System, this is accomplished in a single step as the lysis buffer has been formulated with D-luciferin. After a brief incubation period, bioluminescence is read on a luminometer.