A targeting vector in which the promoter and the first exon of the gene are flanked by loxP sites was constructed. The upstream loxP site with a diagnostic XmaCI restriction site was placed into a BSMI site, and the downstream loxP site was cloned into the SexA1 site. A frt-flanked neomycin cassette was placed between exon 1 and the downstream loxP site. Recombination was confirmed through PCR and Southern blot analysis.