Coding regions of both Dlx5 and Dlx6 were deleted and replaced with lacZ-floxed-RSV-neo construct via a single homologous recombination event. The gene targeting event also generated a transcriptionally silent genomic duplication spanning both Dlx5 and Dxl6. Absence of gene expression was confirmed by RT-PCR and whole mount in situ hybridization. (J:77244)