A 2.7 Kb region containing the first exon (including the translation start codon) and the 5' end of intron 1 was replaced with a floxed neomycin selection cassette. Mice derived from correctly targeted ES cells were crossed to Tg(ELLa-cre)C5379Lmgd mice to delete the selection cassette. No mRNA could be detected in these mice by either RT-PCR or by Northern blot analysis.