D-Luciferin sodium salt is used as a substrate which, upon oxidation by the enzyme luciferase, produces bioluminescence. This system is employed as a reporter in plants, bacteria, and mammalian cells. Because chemiluminescent techniques are virtually background-free, this reporter gene system is ideal for detecting low-level gene expression. A substrate that produces bioluminescence upon oxidation by the enzyme luciferase.Substrate for firefly luciferase with a Km of approx 2 μM.