A PGK-Neo expression cassette was inserted in the "sense" orientation into exon 2. Homologous recombination was verified by Southern blot analysis. Two weak bands representing aberrent transcripts were seen in northern blots of mRNA from embryos homozygous from the targeted gene. RT-PCR analysis confirmed that none of these transcripts represented wild-type mRNA and affinity labelling of embryo fibroblasts with TGFbetaI indicated that there was no binding to TGFbetaRIII.