Enhanced green fluorescence protein (EGFP) is a 27 kDa protein derived from the jellyfish Aequorea victoria, which emits green light (emission peak at a wavelenth of 509 n M) when excited by blue light (excitation peak at a wavelenth of 395 n M). EGFP has become an invaluable tool in cell biology research, since its intrinsic fluorescence can be visualized in living cells. EGFP fluorescence is stable under fixation conditions and suitable for a variety of applications.