Protein tags are protein or peptide sequences located either on the C- or N- terminal of the target protein, which facilitates one or several of the following characteristics: solubility, detection, purification, localization and expression. Green fluorescence protein(GFP) is a protein composed of 238 amino acid residues(26.9kDa) derived from the Jellyfish Aequorea victoria, which emits green light(emission peak at 509nm) when excited by blue light(excitation peak at 395nm). GFP has become an invaluable tool in cell biology research, since its intrinsic fluorescence can be visualized in living cells. EGFP contains the double-amino-acid substitutions Phe-64 to Leu and Ser-65 to Thr(previously published as GFPmut1). In contrast to wtGFP, EGFP has a single, strong, red-shifted excitation peak at 488nm. GFPmut1 fluoresces 35-fold more intensely than wtGFP when excited at 488nm, due to an increase in its extinction coefficient(Em).