Hmga1 is required for progression of chronic MPN to MF in JAK^V617F-mutant mice. (A) Red blood cell (RBC) count and hemoglobin (Hb) (mean ± standard deviation [SD]) in PB from 10- to 12-week-old WT, JAK2^V617F, and JAK2^V617F/Hmga1^+/− mice. (B) Representative spleens (left panel) and graph showing relative spleen sizes (right panel; mean ± SD) in WT, JAK2^V617F, and JAK2^V617F/Hmga1^+/− mice. (C) Platelet (PLT) count from 40- to 60-week-old JAK2^V617F and JAK2^V617F/Hmga1^+/− mice. (D) Megakaryocytes per high-power field (Mks/HPF) in BM from JAK2^V617F and JAK2^V617F/Hmga1^+/− mice (left panel). MF scores in BM (middle panel) and spleen (right panel) from JAK2^V617F and JAK2^V617F/Hmga1^+/− mice. (E) Representative images from mouse models stained with hematoxylin and eosin (H&E of BM from femurs; left panels), CD61 by immunohistochemistry (BM from femurs; left middle panels), fibrosis (BM reticulin stain; right middle panels) and splenic fibrosis (reticulin; right panels)). Scale bar, 50 μm. (F) Representative images of BM from the femurs of WT, JAK2^V617F, and JAK2^V617F/Hmga1^+/− mice (H&E stain). Scale bars, 250 μm. Data in (A-D) are mean ± standard deviation. *P < .05, **P < .01, ***P < .001, 1-way ANOVA, followed by Tukey’s multiple-comparison test (A [right panel]), Kruskal-Wallis test, followed by Dunn’s multiple-comparison test (A [left panel], B), Mann-Whitney U test (C), 2-tailed Student t test (D). ns, not significant.