Green fluorescent protein (GFP), a 27 kDa protein derived from the jellyfish Aequorea victoria, emits green light (emission peak at a wavelength of 509 nm) when excited byblue light (excitation peak at a wavelength of 395 nm). Green Fluorescent Protein (GFP) has become an invaluable tool in cell biology research, since its intrinsic fluorescence can be visualized in living cells.