Cx3cr1CreER knock-in/knock-out mice express a Cre-ER fusion protein from endogenous Cx3cr1 promoter/enhancer elements. Insertion of the Cre-ER fusion protein knocks out endogenous CX3CR1 expression. CX3CR1 is a chemokine receptor expressed in the mononuclear phagocyte system, as well as microglia. Homozygous mice are viable and fertile. When Cx3cr1CreER mice are bred with mice containing loxP-flanked sequence, tamoxifen-inducible, Cre-mediated recombination will result in deletion of the floxed sequences.
For example, when bred to B6.129X1-Gt(ROSA)26Sortm1(EYFP)Cos/J mice (Stock No. 006148), cre-mediated recombination results in yellow fluorescent protein expression in microglia, and intestinal and kidney macrophages.
Additionally, very low levels of Cre activity may be present prior to introduction of tamoxifen - but the Cre activity after tamoxifen are significantly greater than those baseline levels. As such, it is recommended that researchers include tamoxifen-negative controls to establish any baseline levels of Cre activity in their experiments. For example, when Cx3cr1CreER are bred to the dual fluorescent reporter line mT/mG (Stock No. 007676), the resulting mice had some basal Cre activity (GFP expression) in a fraction of microglia prior to tamoxifen treatment, but the Cre activity upon tamoxifen treatment was significantly greater (almost completely restricted to microglia).
Of note, The Jackson Laboratory also has available Cx3cr1CreER mice with EYFP expression (Stock No. 021160).
