Firefly is the luciferin , or light emitting compound, found in many Lampyridae  species. It is the substrate of luciferase; responsible for the characteristic yellow light emission from many firefly species. As with all other luciferins, oxygen is required to elicit light; however, it has also been found that ATP and magnesium are required for light emission. Firefly luciferinis used in a luciferin-luciferase system that requires ATP as a cofactor. Because of this, it can be used as a bio-indicator of the presence of energy or "life". Luciferin is found in both d and l isomers and it is the d-luciferin molecule that is active.D-Luciferin is commonly used in pre-clinical molecular (in vivo) imaging to image tumors in rodents.