The green fluorescent protein is a 27 kDa polypeptide, which converts the blue chemiluminescence of the Ca2+ -sensitive photoprotein, Inouye and Tsuji (1994a) showed the active chromophore is a tripeptide and is dependent on the presence of oxygen to maturate. Wild-type GFP absorbs blue light at 395 nm and emits green light at 510 nm in bioluminescent organisms and when purified in solution (Ward et al., 1980).