The cell line demonstrates strong GFP fluorescent signal in normal culture condition as the constitutive CMV promoter drives the GFP high expression to the stop codon. The downstream RFP ORF was not expressed because of the stop codon after the GFP ORF. Once the CRE protein was present in nuclear, the CRE excises / deletes the DNA fragment between two loxP sites, which removes the stop codon. As a result, the RFP ORF is then expressed under the CMV promoter, and the cell line switches to RFP fluorescent. The RFP signal can be easily monitored via fluorescent cell sorting (for the ratio between GFP and RFP cells), or visualized under microscope, or measured the fluorescent intensity by a meter or reader with RFP filter set.