Cells, genetically modified with optical marker

The present invention concerns a method for obtaining mammalian cells, genetically modified (stably transfected) with a fluorescent marker, preferably green fluorescent protein (GFP), as well as mammalian cells with especially high fluorescence. Fluorescent protein expressing cells with very high fluorescence are described as well as a method to obtain such cells. The fluorescence of such cells can be as high as about 1000 times the auto-fluorescence of the respective host cell. Due to the high fluorescence, said cells are very suitable tools in the development, surveillance and/or optimization of production processes, for selection of culture media and process parameters and for the determination of cell growth and the detection, quantification and monitoring of biomass.