The Fgd5 gene encodes a protein that upregulates CDC42 activity and is a marker for
immunophenotypic hematopoietic stem cells. These Fgd5ZsGr•CreERT2 mice carry a knock-in/knock-out mutation of the Fgd5 gene in which ZsGreen and creERT2 cassettes (separated
by a 2A peptide cassette) have been inserted in frame into the first coding exon. Under control of the endogenous upstream promoter/enhancer elements, tamoxifen-inducible cre activity is observed in ZsGreen+ LSKCD150+ hematopoietic stem cells. Homozygotes are not viable, homozygous embryos die around E11.5–E12.0.
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.
