The targeting vector was generated by inserting an IRES-Cre between the stop codon and the polyadenylation site of the sequence encoding the 3' end of the mouse NtsR2 gene, with an frt-flanked NEO cassette placed upstream of the IRES-Cre. Correctly recombined ES clones via homologous recombination were expanded, and injected into mouse blastocystes to generate chimeric males. Presence of frt-flanked NEO cassette prevents cre expression until exposure to FlpO. (J:263745)