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 by blue 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. This lentivirus expresses GFP under the control of the Tet promoter.