A portion of the CDC25-H domain, a highly conserved region of the gene, from amino acids 1005-1070 was replaced with a PGK-neo cassette via homologous recombination. Absence of gene expression in homozygous mutant animals was confirmed by RT-PCR analysis of brain RNA and absence of protein product was demonstrated by Western blot of brain and lung extracts using an antibody directed against a region of the protein upstream of the disruption.