Activated K-Ras-induced increase in splenic hematopoietic progenitor cells requires STAT3 S727. Animals reconstituted with bone marrow of the indicated genotypes were injected with poly(I:C) and allowed to develop early (cohort 1, 1.5-fold increase in peripheral white blood cell counts) and later signs of MPNs (cohort 2, more than twofold increase). Bone marrow (A,C) and splenic leukocytes (B,D) were harvested and the proportions and total numbers of MPs, LSK, CMPs (Lin⁻, cKit⁺, FCγR⁻, CD34⁺), GMPs (Lin⁻, cKit⁺, FCγR⁺, CD34⁺), and MEPs (Lin⁻, cKit⁺, FCγR⁻, CD34^lo) were assessed by flow cytometry. Representative flow cytometry dot plots are shown in panels A and B. Proportion and total numbers of cells per organ are shown for the early (1) and later cohorts (2) in panels C and D. Number of animals per cohort were: 2 Mx:F/+ (cohort 1); 2 Mx:F/+ (cohort 2); 6 Mx:K-Ras:F/+ (cohort 1), 4 Mx:K-Ras:F/+ (cohort 2); 5 Mx:K-Ras:F/SA (cohort 1); 4 Mx:K-Ras:F/SA (cohort 2). Statistical significance was calculated using the Student t test. *P < .05.