A potentially chemiluminescent substrate; the enzymic reaction of an enzyme (a luciferase) converts it to an excited state that decays with emission of visible radiation; e.g. the benzothiazole of fireflies which upon adenylation is decarboxylated and deadenylated with the emission of yellow light. The energy of photon emission derives from an oxidation with molecular oxygen that may be the enzymic or a non-enzymic reaction of the product of the enzymic reaction. There is no structural homology within the luciferins or luciferases, which have a wide biological distribution.