It is possible to observe gene expression within single cells using a tetracycline inducible promoter for activation. Transcription can be observed by using a fluorescent fusion protein to bind nascent RNA. Ultimately, it is desirable to activate a reporter gene within a single cell with only photons. This is achieved by preparing a chemically altered transcription factor that is functionally unable to activate a reporter gene until it is exposed to photon excitation. We apply two-photon imaging to visualize tumor cells expressing a transgene and ultimately this approach will provide the means to activate a specific gene within a single cell within any tissue to ultimately observe its functional significance in situ.
Robert H. Singer,
"Imaging of gene expression in living cells and tissues," Journal of Biomedical Optics 10(5), 051406 (1 September 2005). https://doi.org/10.1117/1.2103032