Green fluorescent protein (GFP) is responsible for the green bioluminescence from the jellyfish Aequorea victoria. The intense fluorescence of GFP is due to the nature of a chromophore composed of modified amino acids within the polypeptide. Formation of the fluorescent chromophore is species independent and apparently does not require any additional factors. Hence, because the gene product is easily detectable by its intense fluorescence, the gfp cDNA has become a unique reporter system. The advantages of GFP are being exploited in a variety of experimental systems and this is illustrated in the accompanying articles on Dictyostelium, plants, Drosophila and mammalian cells.