These transgenic mice express tamoxifen-inducible Cre recombinase, cre/ERT2, and enhanced green fluorescent protein (EGFP) under the control of the 2.4 kb mouse Prrx1, paired related homeobox 1, gene promoter. Transgene expression is detected in developing limbs by fluorescence, as early as E10.5, and in craniofacial areas and limbs by E17.5. Tamoxifen treatment of pregnant females when embryos are E9.0 results in recombination at E17.0 in chondrocytes and osteoblasts in the developing limb. Tamoxifen treatment of transgenic mice, postnatal day 19, results in recombination in the perichondrium and periosteum. Mice that are homozygous for the transgene are viable and fertile. The Donating Investigator reports there is no difference between the phenotype exhibited by homozygotes compared to hemizygotes. During backcrossing, the Y chromosome may not have been fixed to the C57BL/6 genetic background.
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 (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, 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.
