A loxP site, FRT site, neomycin resistance cassette, lox2272 site, polyadenylation sequence and FRT site replaced exon 1 of Ptf1a and exons 1 and 2 of Ptf1aos. RT-PCR confirmed the abscence of transcript expression of both transcripts in E9.5 embryos. This allele was generated in ES cells containing Etn2Sd.