Gremlin 1 (coded by Grem1) is an antagonist of bone morphogenetic proteins and required for normal limb, kidney, and lung development during embryogenesis. These Grem1-2A-tdTomato-2A-CreERT2 knock-in (KI) mice express a red fluorescent protein (tdTomato) and an iCre/ERT2 fusion protein from the Grem1 promoter/enhancer elements. As the P2AtdTomato-T2A-CreERT2 sequences were inserted into the 5' end of the endogenous stop codon, endogenous Grem1 expression is retained. Homozygous Grem1-2A-tdTomato-2A-CreERT2 KI mice are viable and fertile with no overt phenotype. tdTomato fluorescence is observed in an subset of lamina propria and submucosal stromal cells and is strongly expressed by desmin+ muscularis mucosa and muscularis propria cells. iCre-ERT2 fusion gene activity is observed in the same cells following tamoxifen administration. The donating laboratory indicates that fluorescence is detectable by flow cytometry and by immunohistochemistry/immunofluorescence. The iCre-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 (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, iCre-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 co-administered. As such, when Grem1-2A-tdTomato-2A-CreERT2 KI mice are bred with mice containing loxP-flanked sequence of interest, tamoxifen-inducible, Cre-mediated recombination will result in deletion of the floxed sequences in the Grem1-expressing cells of the offspring.

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Grem1-2A-tdTomato-2A-CreERT2 knock-in mice express a red fluorescent protein (tdTomato) and an iCreERT2 fusion protein driven by the Grem1 endogenous promoter. Endogenous Grem1 expression is retained. When these mice are bred with mice containing a loxP-flanked sequence of interest, tamoxifen-inducible, Cre-mediated recombination will result in deletion of the floxed sequences in the Grem1-expressing cells of the offspring. These mice may be useful for lineage-tracing or marking Grem1-expressing cells during early organogenesis.

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