The green fluorescent protein (GFP) is a protein composed of 238 amino acid residues (26.9kD) that exhibits bright green fluorescence when exposed to light in the blue to ultraviolet range. Although many other marine organisms have similar green fluorescent proteins| GFP traditionally refers to the protein first isolated from the jellyfish. The GFP has a major excitation peak at a wavelength of 395 nm and a minor one at 475 nm. Its emission peak is at 509 nm| which is in the lower green portion of the visible spectrum.