Nomenclature: C57BL/6-Gt(ROSA)26Sor^{tm9(Cre/ESR1)Arte}

• Inducible general Cre deleter
• Tight and inducible regulation of Cre activity after tamoxifen administration
• Successful ubiquitous induction of Cre activity after tamoxifen administration in all tissues analyzed. Efficiency of deletion varies between different organs and different loxP flanked (floxed) target genes but usually exceeds 70%, except in the brain were deletion efficiency is lower.
• If a strain is maintained by homozygous sibling matings, breeders should be handled carefully and quietly to avoid poor breeding, loss of litters, or premature death
• For the generation of the experimental colony which will be used for induction of deletion of a floxed target gene, it is mandatory to always maintain the CreER in a heterozygous state, to diminish noninduced background recombination and possible side effects. In addition, background recombination may be enhanced, if embryos are exposed to CreER(^T2) carrying mothers. It is therefore strongly recommended to breed CreER(^T2) negative females which are heterozygous or homozygous for the floxed allele to CreER(^T2) positive males which are wild type, heterozygous, or homozygous for the floxed allele. To avoid unwanted deletion of the floxed allele, it is strongly recommended to monitor the status of the floxed allele in the offspring from these breedings and to maintain the floxed parental line also in the absence of CreER(^T2).
• When proper induction conditions are used, side effects are only seen in tamoxifen induced homozygous CreER^T2 mice. Side effects include illness and lethality of tamoxifen induced animals, which may occur between 5 and 15 days after induction. Symptoms include: reduced mobility, weight loss, severe sustained diarrhea, and death. To avoid side effects, homozygous CreER^T2 mice should not be induced. Taconic recommends a starting dose of 4.55 mg per dose for mice weighing 23 grams and 3 mg per dose for mice weighing 23 grams. Dosing should be by oral gavage. If side effects are seen, dose reduction should be pursued with monitoring of deletion of the target gene. Taconic recommends a pilot study with the proper experimental and control cohorts to optimize the induction protocol and to monitor the deletion of the floxed allele. Such pilots can be performed by Taconic.
• Ligand-independent Cre activity may occur at a low level and is likely due to proteolysis of the Cre fusion protein and subsequent nuclear transport. Our experience with breeding CreERT2 to a variety of floxed target genes is that approximately 25% of floxed alleles displaying some degree of ligand-independent Cre recombination. Among those floxed alleles affected, the percent of animals examined containing one or more copies of the recombined allele varied from below 1% to as high as 98%. Learn more by reading the related Taconic Insight, "Cre/loxP Recombination System: Applications, Best Practices, and Breeding Strategies".

Origin