Stock No. 019016 is a C57BL/6NJ-congenic R26^FlpoER strain. The phenotype for R26^FlpoER mice on a mixed genetic background (mostly Swiss Webster; Stock No. 018906) is described below. In an attempt to offer alleles on well-characterized or multiple genetic backgrounds, alleles are frequently moved to a genetic background different from that on which an allele was first characterized. It should be noted that the phenotype of C57BL/6NJ-congenic R26^FlpoER mice could vary from that originally described for R26^FlpoER mice on a mixed genetic background. We may modify the strain description if necessary as published results become available.

Heterozygous R26^FlpoER mice are viable and fertile with no reported phenotypic abnormalities. Homozygous R26^FlpoER mice are viable and fertile with no expected phenotypic abnormalities (the phenotype of homozygotes has not been fully characterized to date [September 2012]). The R26^FlpoER allele is designed with the CMV enhancer/chicken beta-actin core promoter and tamoxifen-inducible FlpoER fusion protein inserted into the Gt(ROSA)26Sor locus. The FlpoER fusion protein is an optimized FLPe variant (Flpo) fused to the G400V/M543A/L544A triple mutation of the human estrogen receptor ligand binding domain (ER^T2) by a linker sequence that promotes FLP recombination efficiency. Under control of the endogenous Gt(ROSA)26Sor promoter/enhancer regions and the CMV enhancer/chicken beta-actin core promoter, widespread expression of FlpoER is observed in all embryonic and adult tissues, but translocation to the nucleus / FLP-mediated recombination is rare prior to tamoxifen treatment. Following tamoxifen administration, FLP-mediated recombination is observed in all embryonic and adult tissues. Importantly, recombination is consistent between animals within each stage and organ (and the extent of recombination is specific to each organ and stage of induction). When R26^FlpoER mice are bred with mice containing frt-flanked sequences, tamoxifen-inducible FLP-mediated recombination will result in deletion of frt-flanked sequences in the FLP-expressing cells of the offspring.

These mice are useful for "Mosaic mutant Analysis with Spatial and Temporal control of Recombination" (MASTR). By breeding these ubiquitous inducible R26^FlpoER mice with FLP-inducible eGFPcre mice (R26^MASTR; Stock No. 018903), the double mutant line can be bred to mice harboring a floxed gene of interest for mosaic mutant analysis of deleted genes in any tissue at any time (embryonic or adult). For example, mice harboring the R26^FlpoER, R26^MASTR, and the Smo^lox conditional knockout allele (Stock No. 004526) exhibit sonic hedgehog receptor ablation following tamoxifen administration, and allow one to test the cell autonomous function of Smo in granule cell precursor proliferation.

The FlpoER fusion protein consists of the mouse codon-optimized FLPe recombinase variant (FLPo) fused to the G400V/M543A/L544A triple mutation of the human estrogen receptor ligand binding domain (ER^T2) by a linker sequence that promotes FLP recombination efficiency. The ER^T2 does not bind its natural ligand (17β-estradiol) at physiological concentrations but will bind the synthetic estrogen receptor ligands 4-hydroxytamoxifen (OHT or tamoxifen) and, with lesser sensitivity, ICI 182780. Restricted to the cytoplasm, FlpoER can only gain access to the nuclear compartment after exposure to tamoxifen. To counteract the mixed estrogen agonist effects of tamoxifen injections, which can result in late fetal abortions in pregnant mice, progesterone may be coadministered.