A C to T mutation at nucleotide 2403 of the cDNA was introduced via site directed mutatgenesis and homologous recombination. The mutation generates a stop codon that truncates 96 carboxy terminal residues and thereby recapitulates a mutation observed in a human patient with both congenital neutropenia and acute myeloid leukemia. RT-PCR and restriction enzyme analyses showed that homozygous mutant mice lacked normal transcript and expressed mutant transcript at normal levels.