A 0.8 kb genomic region between exons 1 and 2 was replaced with a neomycin resistance cassette via homologous recombination resulting in a shift in the downstream reading frame. Northern blot analysis of homozygous mutant embryos did not detect wild-type transcript, but instead detected a larger mutant transcript which contained the neo insert. Western blot of fetal livers did not detect protein product in homozygous mutants.