The green fluorescent protein (GFP) from the Pacific Northwest jellyfish Aequorea victoria has generated intense interest as a marker for gene expression and localization of gene products, The chromophore, resulting from the spontaneous cyclization and oxidation of the sequence -Ser(65) (or Thr(65))-Tyr(66)-Gly(67)-, requires the native protein fold for both formation and fluorescence emission. The structure of Thr(65) GFP has been determined at 1.9 angstrom resolution, The protein fold consists of an 11-stranded beta barrel with a coaxial helix, with the chromophore forming from the central helix. Directed mutagenesis of one residue adjacent to the chromophore, Thr(203), to Tyr or His results in significantly red-shifted excitation and emission maxima.