A targeting vector was designed to insert an internal ribosome entry site (IRES) fused to a CreERT2 fusion gene (Cre-ERT2; Cre recombinase fused to a G400V/M543A/L544A triple mutation of the human estrogen receptor ligand binding domain), followed by a frt-flanked neo cassette, upstream of the stop codon. (J:235123)