Green fluorescent protein (GFP) here refers to the protein first purified from jellyfish Aequorea victoria, though many other organisms have similar proteins. It is a 26.9 kDa protein (composed of 238 a.a. residues) that shows green fluorescence in short-wave light (blue to ultraviolet). Despite of wild-type GFP, many mutants of GFP have been engineered for wider usage in research. Enhanced GFP (eGFP) has S65T and F64L mutations, which make GFP show increased fluorescence and fold more efficiently under 37�? respectively. eGFP allows the use of GFP in mammalian cells. In A. Victoria, GFP plays roles as an energy transfer acceptor. It has long been used in cell and molecular biology as a reporter of gene expression. GFP can also been applied as a molecular thermometer to measure temperature accurately in fluids.