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 NtsR1 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. Mice were bred to a FlpO deleter line to remove neo cessette to make constitutive cre expression throughout life.