Shcherbo, D., Merzlyak, E.M., Chepurnykh, T.V., Fradkov, A.F., Ermakova, G.V., Solovieva, E.A., Lukyanov, K.A., Bogdanova, E.A., Zaraisky, A.G., Lukyanov, S., and Chudakov, D.M.
Bright far-red fluorescent protein for whole-body imaging.  Nature Methods 4: 741-6 (2007).

For deep imaging of animal tissues, the optical window favorable for light penetration is in near-infrared wavelengths, which requires proteins with emission spectra in the far-red wavelengths. Here we report a far-red fluorescent protein, named Katushka, which is seven- to tenfold brighter compared to the spectrally close HcRed or mPlum, and is characterized by fast maturation as well as a high pH-stability and photostability. These unique characteristics make Katushka the protein of choice for visualization in living tissues. We demonstrate superiority of Katushka for whole-body imaging by direct comparison with other red and far-red fluorescent proteins. We also describe a monomeric version of Katushka, named mKate, which is characterized by high brightness and photostability, and should be an excellent fluorescent label for protein tagging in the far-red part of the spectrum.