luciferin

thumb|right|300px|Space-filling model of [[firefly luciferin(color coding: black=carbon, white=hydrogen, blue=nitrogen, red=oxygen, yellow=sulfur)]]

Luciferin () is a generic term for the light-emitting compound found in organisms that generate bioluminescence. Luciferins typically undergo an enzyme-catalyzed reaction with molecular oxygen. The resulting transformation, which usually involves breaking off a molecular fragment, produces an excited state intermediate that emits light upon decaying to its ground state. The term may refer to molecules that are substrates for both luciferases and photoproteins. Luciferins and luciferases are usually specific to particular species or taxonomic groups.