The targeting vector was designed to insert a loxP site upstream of exon 3 and an FRT-flanked neomycin resistance (neo) cassette, followed by a second loxP site, downstream of exon 3. Germline Flp- and Cre- mediated recombination removed the FRT-flanked neo cassette and exon 3 sequences. Quantitative real-time PCR analysis confirmed the absence of exon 3 genomic DNA in several soft tissues and tibial bone from homozygous mutant mice.