A loxP site was inserted upstream of exon 2. A floxed neomycin resistance cassette with a 3' loxP site was inserted downstream of exon 2. Flp-mediated recombination removed the selection cassette. Cre-mediated recombination removed exon 2, which encodes a lysine zipper motif, and results in a truncated protein. Western blot analysis confirmed the absence of protein expression in brain and lung. (J:219243)