The targeting construct contained loxP sites flanking exon 4 and a FRT-flanked PGK-neomycin selection cassette inserted between exons 3 and 4. Flp-mediated recombination removed the neomycin cassette. Cre-mediated recombination removed exon 4. In the absence of exon 4, the protein is not expected to fold properly and should be unstable. (J:239395)