The Cre recombinase coding sequence, preceded by an internal ribosomal entry site (IRES) and followed by a frt-flanked neomycin resistance cassette, interrupts exon 1 so as to disrupt codon 63. RT-PCR using primers to sequences upstream of the insertion site and in exon 2 detects the expected Fgf16 transcripts in E9.0 wild-type and heterozygous female embryos, but none in hemizygous male embryos. Cre recombinase is not expressed from this allele. Crossing mice bearing this allele with mice expressing FLPe recombinase yields progeny in which the neo cassette has been excised and Cre recombinase is expressed under control of the endogenous Fgf16 promoter.