Mice homozygous for the R26R-Confetti conditional allele are viable and fertile, with a CAG promoter, loxP site, and STOP cassette preventing transcription of the downstream Brainbow 2.1 sequences. The Brainbow 2.1 region contains two loxP-flanked dimers, each uniquely positioned in head-to-tail tandem. One dimer has nuclear-localized green fluorescent protein (hrGFPII) and a reverse-oriented cytoplasmic yellow fluorescent protein (mYFP). The other dimer has cytoplasmic red fluorescent protein (tdimer2(12)) and a reverse-oriented membrane-tethered cyan fluorescent protein (mCerulean). The Brainbow2.1 region may be written as loxP-STOP-loxP-GFP-PFY-Pxol-loxP-RFP-PFC-Pxol to show the transcriptional direction of each part. When bred to mice that express Cre recombinase, the resulting offspring may have a recombination event that stochastically places one of the four fluorescent proteins into position directly downstream of the CAG promoter within the cre-expressing tissues. Because this CAG promoter-driven Brainbow 2.1 reporter construct was targeted for insertion into the Gt(ROSA)26Sor locus, fluorescent protein expression is determined by which tissues express Cre recombinase. The donating investigator reports that mice do not express any fluorescent cells prior to introduction of Cre recombinase. The donating investigator confirms fluorescent protein expression following exposure to cre can be detected by direct fluorescence (and presumably also via mRNA (in situ hybridization) and antibody staining (immunohistochemistry)) Initial Cre recombination outcomes may recombine the loxP-flanked STOP cassette (green), invert the loxP-STOP-loxP-GFP-PFY-Pxol region (yellow), recombine the loxP-STOP-loxP-GFP-PFY-Pxol-loxP region (red), or invert the entire loxP-STOP-loxP-GFP-PFY-Pxol-loxP-RFP-PFC-Pxol region (blue). Other recombination outcomes may not remove the STOP cassette and result in no fluorescent reporter labeling in cre-expressing cells. In addition, sequential recombination outcomes may reduce the construct to a single invertible dimer segment that can continue to invert as long as Cre recombinase is present. The donating investigator also reports that weaker cre expression favors inverting the loxP-STOP-loxP-GFP-PFY-Pxol region rather than removal of the loxP-STOP-loxP region: this results in less green-fluorescing cells / more non-fluorescing cells than is expected if using a strong cre-expressing line. The Jackson Laboratory 2024, used with permission.