A targeting vector was designed to place a loxP site upstream of exon 4, and a FRT::loxP::Neo::FRT::loxP cassette inserted 211 base pairs downstream of exon 4. Flp-mediated recombination removed the FRT-flanked neo cassette leaving exon 4 floxed. Cre-mediated recombination removed exon 4. (J:229155)