Homologous recombination of a construct containing lacZ and neo resulted in the deletion of a portion of exon 3 and all of exons 4 through 7. The targeted region encoded amino acids 200 through 1380, corresponding to the protein kinase A and leucine zipper domains. Protein was undetected in homozygous mutant mice by Western blot analysis of kidney, liver, and testis tissues. Beta-galactosidase staining showed the expression pattern of lacZ to be similar to that of the endogenous gene.