Exon 3 was flanked by loxP sites with a FRT-flanked neomycin resistance cassette placed downstream. Before microinjection into blastocystes, the neomycin cassette was excised by transient expression of Flp-recombinase. Mice were crossed with EIIa-cre transgenic mice to remove the floxed allele. No protein was detected by immunoblot analysis of homozygote embryonic fibroblast lysates.