A fluorescent protein that develops an acceptor of fluorescence resonance energy transfer (FRET) in a manner dependent upon stimulus light to thereby enable multi-color labeling of any arbitrary subcellular organelle, cell or tissue. There is provided a fluorescent protein characterized in that it consists of a fused protein of donor fluorescent protein and acceptor fluorescent protein so that before stimulus light irradiation, the donor protein can emit fluorescence by irradiation with excitation light for the donor protein, and so that after stimulus light irradiation, intramolecular FRET between the donor fluorescent protein and the acceptor fluorescent protein is induced by irradiation with excitation light for the donor protein to thereby effect emission of fluorescence from the acceptor protein, the fluorescence of the donor protein and the fluorescence of the acceptor protein being those of wavelengths different from each other.
FLUORESCENT PROTEIN