The exon 1 coding sequence was flanked by two loxP sequences inserted in the 5' untranslated region of exon 1 and in intron 1. A FRT-flanked neomycin selection cassette located 3' to the loxP site in intron 1 was deleted via Flp-mediated recombination. Cre-mediated recombination removed the loxP flanked exon 1. RT-PCR, Western blot and immunohistochemical analyses revealed no detectable transcripts or proteins in thoracic tissues (e.g. lung and aorta) from homozygous mutant mice.