A loxP site was inserted about 200 bp upstream of the ATG start site and an FRT flanked neo cassette and loxP site were inserted downstream of exon 1 via homologous recombination. Recombinase mediated recombination removed exon 1 and the neo cassette. Immunohistochemical analysis confirmed the absence of protein expression in brains from homozygous mice. (J:266702)