D-luciferin is the natural substrate of the enzyme luciferase (Luc) that catalyzes the production of the typical yellowgreen light of fireflies. The 560 nm chemiluminescence from this reaction peaks within seconds, with light output that is proportional to luciferase concentration when the substrate luciferin is present in excess. The luciferase (luc) gene is a popular reporter gene for research and agent screening. Chemiluminescent techniques are virtually background-free, making the luc reporter gene ideal for detecting low-level gene expression. As little as 0.02 pg of luciferase can be reliably measured in a standard scintillation counter. In addition to its role as a reporter of gene expression, luciferase is commonly used in an extremely sensitive assay for ATP. We of er the firefly luciferase (HY-P1004), luciferin free acid (HY-12591A), as well as its water-soluble sodium salts (HY-12591) and potassium salts (HY-12591B) .