Mutations in the granulocyte colony-stimulating factor (G-CSF) receptor gene are found in a number of patients with severe chronic neutropenia predisposed to acute myeloid leukemia. These mutations result in the absence of the C-terminal domain of the G-CSF-R, a region which has been implicated in differentiation signaling. We generated mice with an equivalent mutation (gcsfr-▵715) by homologous and Cre-mediated recombination in embryonic stem cells. Both wt/▵715 and▵715/▵715 mice have significantly reduced numbers of blood neutrophils compared with their wt/wt littermates. However, under continuous G-CSF administration mutant mice develop peripheral neutrophil counts that significantly exceed those of wild-type littermates. These findings indicate that depending on G-CSF levels in mice, the ▵715 mutation can contribute both to neutropenia and to neutrophilia.