A targeting vector was constructed where an upstream loxP site was cloned into a blunted NsiI site in intron 7 and a floxed neomycin resistance cassette was cloned into a BglII site in intron 9. Targeting took place in PC3 ES cells homozygous for the cre transgene Tg(Prm-cre)70Og, which expresses Cre recombinase in male germ cells. Cre mediated recombination subsequently removed exons 8 and 9 upon transmission from male chimeric mice. Offspring from these male chimeras were selected for further breeding. This is considered a null allele. (J:149811)