Part of exon 3, all of intron 3, and part of exon 4 were replaced with a lacZ-neo derived cassette via homologous recombination, fusing lacZ in-frame with exon 3. The gene targeting event deletes most of the DNA-binding domain, the leucine zipper dimerization domain, and the last splice acceptor site. Homozygous mutant embryos were detected up to E11.5 by Southern blot analysis. (J:65292)