A 70bp fragment of the per promoter (from -563 to -494 upstream of the transcription start site, contains a consensus E-box) is fused upstream of a transcription initiation site, an ATG translation start codon, an FRT cassette that contains mCherry and tandem translational stop codons, and finally the pLUC coding region without its normal start codon. In the absence of FLP recombinase, the mCherry open reading frame, but not the FLP coding sequence, can be expressed. Excision of the FRT cassette using FLP recombinase removes the mCherry sequence, allowing expression of the FLP coding sequence.

A 70bp fragment of the per promoter (from -563 to -494 upstream of the transcription start site, contains a consensus E-box) is fused upstream of a transcription initiation site, an ATG translation start codon, an FRT cassette that contains mCherry and tandem translational stop codons, and finally the pLUC coding region without its normal start codon. In the absence of FLP recombinase, the mCherry open reading frame, but not the FLP coding sequence, can be expressed. Excision of the FRT cassette using FLP recombinase removes the mCherry sequence, allowing expression of the FLP coding sequence.