The CRE reporting cell lines are used to monitor or confirm the efficiency of CRE recombination in vivo. It is a great method and easy tool to verify the performance of your CRE enzyme(your CRE expression plasmids, or pre-made CRE expression lentivirus, or purified CRE enzyme) in vivo conditions. It is also a control test to verify your CRE-loxP based system. Transformed from HEK293 cells, expressing "LoxP-GFP-stop-LoxP-RFP" cassette under CMV promoter(see Color-Switch cassette below). Cell line also contains a blasticidin antibiotic marker under RSV promoter.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(see the DNA structure scheme above). 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.