Loss of polyploid megakaryocytes in Jak2V617F animals. (A, top) Unremarkable 2n, 4n, 8n, 16n, 32n, and 64n peaks in megakaryocytes of Jak2wt in gray. (Middle) Normal 2n and 4n peaks in Jak2V617F but loss or decrease of 8n to 64n peaks in the mutant in red. (Bottom) Overlay. Megakaryocytes were distinguished by size and positive staining for CD41, and polyploidy was distinguished by propidium iodide staining. (B) Megacult assay showing an approximately 2-fold decrease in the BM of Jak2V617F-expressing animals (P < .01).