A targeting vector was designed to insert a loxP site upstream of exon 1 and a second loxP site downstream of exon 1 followed by an FRT-flanked neomycin resistance (neo) cassette which included a third loxP site, downstream of exon 1. Flp-mediated recombination removed the FRT-flanked neo cassette leaving exon 1 floxed. (J:225860)