The entire coding region, except for sequences encoding the 52 N-terminal amino acids, was deleted and replaced with a lacZ-neomyin resistance cassette. The mutation removes all zinc finger domains from the gene and creates a lacZ fusion protein. Absence of gene expression in homozygous mutant animals was verified by RT-PCR and in situ hybridization. Expression of the lacZ fusion protein was observed by X-gal staining of mutant embryos.