Firefly luciferase is widely used as a reporter for studying gene regulation and function, and for pharmaceutical screening. It is a very sensitive genetic reporter due to the absence of endogenous luciferase activity in mammalian cells or tissues. The enzyme is a 62,000 Dalton protein, which is active as a monomer and does not require subsequent processing for its activity. Firefly luciferase catalyzes ATP-dependent D-luciferin oxidation to oxyluciferin, producing light emission centered at 560 nm. This firefly luciferase assay kit is designed for simple and efficient quantitation of firefly luciferase reporter enzyme activity from cultured cells with high sensitivity and linearity. The kit is suitable for plate sizes ranging from 96-well to 24-well and has reagents sufficient for 150 assays.