To address this question, we used mice carrying JAK2V617F in myeloid cells, but not in HSCs (LysM-Cre;JAK2V617F mice); these mice develop erythrocytosis, but not overt MPN or myelofibrosis42, consistent with lower levels of inflammatory cytokines, including GM-CSF (supplemental Figure 3A-E). Here, CSF2 is linked to polycythemia.