The targeting vector was designed to insert a loxP site upstream of exon 2, a loxP site downstream of exon 2, followed by an FRT-flanked neomycin resistance (neo) cassette, and a third loxP site. Exon 2 is the first coding exon of the gene. Flp-mediated recombination removed the FRT-flanked neo cassette. Cre-mediated recombination removed exon 2.