In this strain, the Cldn6CIHV allele replaces the entire coding region of the claudin 6 (Cldn6) locus with a CreERT2 fusion protein, an internal ribosome entry site (IRES), and a histone H2B-Venus fluorescent protein. This abolishes gene expression. Homozygotes are viable, fertile, normal in size, and do not display any gross physical or behavioral abnormalities. CLDN6 is a structural protein involved in tight junction formation, with a functional role in the epidermal permeability barrier. In this strain endogenous Cldn6 promoter/enhancer regions drive Cre-ERT2 expression and Venus immunofluorescence in embryonic endoderm during organogenesis. Cre-ERT2 fusion gene activity is inducible and only observed following tamoxifen administration. When Cldn6CIHV mice are bred with mice containing loxP-flanked sequence, tamoxifen-inducible, Cre-mediated recombination will result in deletion of the floxed sequences in the Cldn6-expressing cells of the offspring. These mice may be useful for visualizing endoderm specification, proliferation, and patterning in the developing embryo. 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.