D-Luciferin is a substrate for the enzyme firefly luciferase. Chemiluminescence at 560 nm is generated when luciferin is oxidized by an ATP-dependent process, catalyzed by luciferase. Luciferase is encoded by the luc gene, which is widely used as a reporter gene in a variety of cells. Because of the intrinsic low background of chemiluminescence technique, detection of the luc gene expression can be made at very low level. In addition, luciferin/luciferase has been used to measure 10-15 molar quantity of ATP. D-luciferin, sodium salt is a water-soluble form of D-luciferin.