A lacZ gene with a nuclear localization signal followed by a frt-flanked neomycin selection cassette in the reverse transcriptional orientation was inserted into the starting codon of the gene locus. Founder mice were crossed with a FLP-deleter strain to remove the neomycin selection cassette. Gene inactivation was confirmed by immunoblot analysis of skin lysates. LacZ expression was detected in the skin, kidney, and a subpopulation of cells in the apical layer of the vomeronasal organ.