Exons 3 and 4 were replaced with a IRES-Tau-lacZ-neo cassette via homologous recombination. The gene targeting event results in deletion of most of the paired-like homeodomain and putative DNA-binding domain. RT-PCR analysis of homozygous mutant animals was unable to detect gene transcription using primers to the gene coding region or neomycin resistance gene. Amplification products were obtained using primers to the lacZ gene, but lack of beta-gal activity suggests these transcripts were spliced abnormally.