An FRT-flanked neomycin resistance cassette was inserted downstream of exon 2. Exon 3 was replaced with a floxed exon 3-GFP fusion. Flp-mediated recombination removed the selection cassette. Cre-mediated recombination removed the modified exon 3. Western blot analysis confirmed the absence of protein expression in E18.5 lungs and kidneys.