The targeting construct is designed to contain from 5' to 3'; an FRT site, a loxP site, a PGK-neomycin resistance cassette, an FRT site, a loxP site, inserted into intron 2. A third loxP site is inserted in intron 3. Flp-mediated recombination removed the FRT-flanked neo cassette leaving exon 3 floxed. (J:305949)