The region from the translational start codon to the start of the second intron 3 of the splice consensus site was replaced with the lacZ reporter and a frt-flanked neomycin resistance positive selection cassette by recombineering. Flp-mediated recombination removed the neo cassette. RT-PCR confirmed no products indicating transcription and splicing of exon 3 and 4 in homozygous E12.5 head, and E18.5 and adult lung tissue. However, a product in adult testis RNA at low levels indicated unusual transcription and spicing of exon 3 and 4 in this tissue. Northern blot analysis from adult testis did not detect mRNA in homozygotes but did detect the reporter transcript. (J:249115)