A loxP site was inserted upstream of exon 3 and an FRT flanked neo cassette and loxP site were inserted downstream of exon 4 via homologous recombination. Flp-mediated recombination removed the neo cassette. Exons 3 and 4 were deleted through cre-mediated recombination. Western blot analysis confirms absence of protein expression in homozygous mice. (J:182150)