THPO and primary myelofibrosis: In the absence of THPO in JAK2V617F/THPOdel/del mice, erythrocytosis was partially suppressed at 6–12 weeks, the neutrophil count was suppressed until after 15–18 weeks, and the platelet count was reduced to normal at all-time points (Fig 1C); marrow (Fig 2A–2D) and spleen (Fig 3A–3D) CFU-GEMM, CFU-GM, and CFU-Mk numbers were reduced but not marrow BFU-E, there was a reduction in spleen weight, though not to normal, (Fig 4) and, most strikingly, reversal of the osteomyelofibrosis and splenic EMH (Fig 5).