A loxP site was inserted upstream of exon 1, and a loxP site and an FRT-flanked neo cassette were introduced into intron 1 via homologous recombination. Flp mediated recombination removed the neo cassette leaving exon 1 floxed. Cre mediated recombination removed exon 1. Western blot analysis of E12.5 homozygous embryos confirmed the absence of protein expression.