Green fluorescent variants of wild type GFP (wtGFP) with improved thermostability and fluorescence emission, including enhanced green fluorescent protein (EGFP, [4]), and mMGFP [5], have gained popularity for use in mice. Recently however, several additional mutants of wtGFP, with altered excitation and emission spectral profiles (i.e. fluoresce in colors other than green), as well as improved thermostability and fluorescence, have been described [4,6].