Many different organisms, ranging from bacteria and fungi to fireflies and fish, are endowed with the ability to emit light, but the bioluminescent systems are not evolutionarily conserved: genes coding for the luciferase proteins (Lase) are not homologous, and the luciferins are also different, falling into many unrelated chemical classes. Biochemically, all known Lase are oxygenases that utilize molecular oxygen to oxidize a substrate (a luciferin; literally the "light-bearing' molecule), with formation of a product molecule in an electronically excited state. The color of the light may differ, even though the same luciferin/Lase system underlies the reaction. Filters or differences in Lase structure are responsible in some cases; in others a secondary emitter associated with a second protein is involved. In the coelenterates a green fluorescent protein, whose chromophore is derived from the primary amino-acid sequence, results in a red shift of the emission. In the bacteria accessory proteins causing either blue- or red-shifts have been isolated from different species; the chromophores are noncovalently bound. Although radiationless energy transfer has been implicated in the excitation of such accessory emitters, this may not be so in all cases.