1.2 kb of genomic sequence, including regions encoding the conserved serine threonine kinase catalytic subdomains 8 through 10, was replaced with a neomycin selection cassette by homologous recombination. Western blot analysis of homozygous mutant mice using two antibodies directed to the amino and carboxy termini showed an absence of protein. A kinase phosphorylation assay showed no activity in homozygous mutant mice verifying a complete functional ablation.