A 6.6 kb region of the gene encoding zinc fingers 1 and 2 was replaced with a promoter-less lacZ-neo cassette via homologous recombination. The mutation results in deletion of 3 exons, disruption of downstream sequences, and fusion of lacZ in-frame to the Zfr sequence. Immunostaining of E5.5 homozygous mutant embryos did not detect gene product.