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.