Homozygous mice die upon birth with multiple abnormalities of the heart and urogenital system (among other tissues). Heterozygous mice are viable and fertile. The Osr1eGFPCreERt2 (Osr1GCE) "knock-in" allele both abolishes Osr1 gene function and expresses an eGFPCreERt2 fusion protein (EGFP and creERT2 fusion protein from the Osr1 promoter/enhancer elements). While EGFP immunofluorescence is observed in intermediate mesoderm and dorsal lateral plate mesoderm of developing embryos (~E8.5), Cre-ERT2 fusion gene activity is inducible; observed in the same cells only following tamoxifen administration. As such, when Osr1GCE mice are bred with mice containing loxP-flanked sequence of interest, tamoxifen-inducible, Cre-mediated recombination will result in deletion of the floxed sequences in the Osr1-expressing cells of the offspring. Of note, removal of the frt-flanked neo cassette is reported to result in ectopic Cre recombinase expression in the absence of tamoxifen. The Cre-ERT2 fusion protein consists of Cre recombinase fused to a triple mutant form of the human estrogen receptor; which does not bind its natural ligand (17_-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, Cre-ERT2 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. The Jackson Laboratory 2024, used with permission.