The JAK1/2 inhibitor ruxolitinib decreases WBC count and reduces splenomegaly in CSF3R^T618Imice. A second cohort of mice was transplanted with bone marrow expressing CSF3R^T618I (n = 18); treatment with 90 mg/kg ruxolitinib twice per day (n = 9) or vehicle (n = 9) was initiated at day 12 post transplant. (A) Treatment with ruxolitinib results in prompt reduction in WBC count. Peripheral blood was collected on all mice 1 to 2 times per week. (B) Ruxolitinib decreases spleen weight. Mice were sacrificed on day 21 or 24 to evaluate spleen size. (C) Representative images of spleens from ruxolitinib- or vehicle-treated mice at time of necropsy. (D) Ruxolitinib-treated mice have significantly increased body weight compared with vehicle-control mice. Body weight was recorded on day 24 post transplant.