In this strain a Cre-estrogen receptor (Cre-ERT2) fusion protein is knocked into minichromosome maintenance deficient 2 mitotin (Mcm2) gene downstream of the stop codon. Mcm2 is a component of the DNA replication licensing complex expressed in stem/progenitor cells in a variety of regenerative tissues. Mcm2 plays an essential role in eukaryotic cell division as it marks DNA replication origins during G1 for use in the subsequent S-phase. Mcm2 prevents multiple rounds of replication from a single origin and suppresses endoreduplication. Reduced expression results in highly penetrant early onset cancers and stem cell deficiencies. In these mutant mice, MCM2 expression is reduced to 62% of normal in heterozygotes and 35% in homozygotes. Heterozygotes mice are viable, fertile, and normal in size. Homozygotes do not survive beyond 12 weeks age and exhibit thoracic thymomas. They have a severe stem cell deficiency in the subventricular zone of the brain, skeletal muscle, and the small intestinal crypt. They also display a hunched appearance, rapid shallow respiration, generalized muscle weakness, limited movement, loss of adipose tissue, frailty, and modest hair loss or slight graying. Many homozygous mice also exhibit enlarged spleens, and polyps in both the small intestine and colon. The Cre-ERT2 fusion protein is only active when it binds to the estrogen analog 4-hydroxytamoxifen (OHT). When these mice are bred with mice containing a loxP-flanked sequence, tamoxifen-inducible Cre-mediated recombination is expected to result in deletion of the floxed sequences in the Cre recombinase-expressing highly proliferative stem/progenitor cells of the offspring. For example, when bred to mice carrying the Tg(CAG-Bgeo/GFP)21Lbe transgene, tamoxifen-induced GFP expression is observed in 2% and 10% of stem/progenitor cells in a large number of tissues, including endodermal, mesodermal, and neuroectodermal derivatives. This strain may be useful for studying function and regulation of the core DNA replication machinery.

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.