The construct targeting exon 4 consisted of a loxP site in intron 3 and a neo cassette (FRT-flanked pGK-gb2 Neo and a loxP site) in intron 4. Flp-mediated recombination removed the neo cassette, leaving a single FRT site and the loxP site behind. Exon 4 is floxed. (J:232839)