These targeted mutation mice have a tamoxifen inducible Cre-mediated recombination system ("MerCreMer") driven by the endogenous Pou5f1 (POU domain, class 5, transcription factor 1) promoter. Mice that are heterozygous for the targeted mutation are viable, fertile, normal in size and do not display any gross physical or behavioral abnormalities. The viability and fertility of homozygotes is not yet known. Inducible Cre mediated recombination occurs in Pou5f1 (Oct4) expressing cells in embryos and in iPS cells derived from these mice. Of note, this double fusion protein has substantially greater Cre recombinase activity with less promiscuity compared with a single CreMer fusion protein. As the cre is flanked on each end with a mutated murine estrogen receptor ligand binding domain (amino acids 281-599, G525R); Cre expression is tamoxifen inducible yet estrogen insensitive. These Oct4-Inducible Cre mice may be useful for generating conditional mutations for studying lineage tracing, and production of cell lines and targeted mutations (knockouts).

 The cre/Esr1* fusion protein consists of Cre recombinase fused to a G525R mutant form of the mouse estrogen receptor; which does not bind its natural ligand (17&beta;-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/Esr1* 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.

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These knock-in mice (also referred to as Oct4-MerCreMer mice) express a tamoxifen-inducible cre recombinase from the endogenous promoter/enhancer elements of the Pou5f1 gene. When induced, cre activity is observed in Pou5f1 expressing cells in embryos and in Induced pluripotent stem (iPS) cells derived from these mice. These mice may be useful for generating conditional mutations for studying lineage tracing, and production of cell lines and targeted knock-out mutations.

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