Although we cannot entirely exclude the possibility that megakaryocytes are altered and affect CAR cells in the mutants, these results suggest that megakaryocytes and their progenitors are not responsible for myelofibrosis in the mutants, supporting the idea that effects of loss of function of Runx1/2 in CAR cells on the marrow fibrotic phenotype are cell autonomous. Here, RUNX1 is linked to myelofibrosis.