The exon encoding a portion of the ATP-binding domain, which is common to both known isoforms, was replaced by a neomycin selection cassette. Northern blot analysis showed an absence of transcript in homozygous mutant uterine tissue. Western blot analysis of platelets, aorta, and duodenum from homozygous mutant mice showed that while Prkg1 protein was absent, levels of Prkg2 protein were unaffected. cGMP-stimulated kinase activity was undtected in lung extracts from homozygous mutant mice.