Exons 6 to 8, shared by all spicing variants of the expressed transcripts, were replaced with a targeting vector containing a beta-gal reporter and a neomycin selection cassette. RT-PCR analysis revealed absence of mRNA transcripts in lung, liver, fore- and hindlimb tissues from newborn (P0) homozygous mutant mice. Immunoblot analysis confirmed the absence of encoded protein in E14 homozygous mutant embryos.