A loxP site and a FRT-flanked neo cassette were inserted on the 3' side of exon 1 and the second loxP site was inserted approximately 1.7 kb 5' of the start of exon 1. Flp-mediated recombination removed the neo cassette, leaving 3,198 bp, including exon 1 and about 1.7 kb of upstream sequence, floxed. (J:234808)