A model compound, 4-(4-hydroxyphenyl)methylideneimidazol-5-one undergoes a reversible redox reaction identical to that of the Aequorea green fluorescent protein (GFP), strongly suggesting that the GFP chromophore is derived via the autoxidation of a nonfluorescent dihydro precursor in dihydro-GFP. (C) 1997 Elsevier Science Ltd.