Mice hemizygous for this enhancer trap transgene are viable and fertile, with expression of the Cre-ERT2 fusion gene under control of the promoter/enhancer regions surrounding the site where the enhancer trap transgene randomly inserted. Cre recombinase activity is observed in brain tissues following tamoxifen administration (although some cre activity is reported prior to tamoxifen administration). The donating investigators may not have assessed expression in tissues other than brain. When these enhancer trap transgenic mice are bred with mice containing a loxP-flanked sequence of interest, Cre-mediated recombination will result in deletion of the floxed sequence(s) in cre-expressing tissues in the offspring.
For characterization information, see images at the Allen Institute for Brain Science website (Et(cre/ERT2)398Rdav images).
The Cre-ERT2 fusion protein (Cre-ERT2) 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.
