CSF3R^T618Icauses a fatal disorder reminiscent of neutrophilic leukemia. Mice were transplanted with bone marrow expressing CSF3R^WT (n = 5) or CSF3R^T618I (n = 5). (A) CSF3R^T618I causes leukocytosis. Total WBCs were measured over time in both the WT and T618I groups using an animal blood counter (scil Vet ABC). (B) Leukocytosis of CSF3R^T618I is caused by granulocytic expansion. The percentage of granulocytes was measured using an animal blood counter and plotted over time. (C) CSF3R^T618I results in an expansion of the granulocyte lineage at the expense of lymphocytes. Flow cytometry analysis was performed on peripheral blood from CSF3R^WT and CSF3R^T618I mice over time. The percentage of granulocytes relative to B-cell and T-cell lineages was increased in CSF3R^T618I mice relative to CSF3R^WT mice. Gating schemes used for analysis are shown in supplemental Figure 1. (D) CSF3R^T618I mutation is uniformly lethal. For CSF3R^T618I mice, events represent mouse death (n = 3) or euthanization as a result of moribund appearance (n = 2). The CSF3R^WT mice were sacrificed at day 90 for histologic analysis; all mice appeared healthy. (E) CSF3R^T618I mice have increased levels of phospho-STAT3. Blood from CSF3R^WT or CSF3R^T618I mice was pooled and pSTAT3 levels were measured by phospho-flow cytometry. Mean fluorescence intensity (MFI) of pSTAT3 was 604.5 for CSF3R^WT and 1354.3 for CSF3R^T618I.