Exon 2, encoding the runt domain, was disrupted by the insertion of a cassette containing IRES-lacZ and neo. RT-PCR analysis of dorsal root ganglia RNA extracts showed an absence of transcript in homozygous mutant mice. Protein was undetected by Western blot analysis of thymus tissue and immunostaining of dorsal root ganglia. Expression of beta-galactosidase was visualized in mutant embryos via X-gal staining.