Interestingly, polycythemia and reticulocytosis were efficiently resurrected in secondary mice by transplantation of BM and/or spleen cells from primary JAK2 V617F recipients, and this was equally true for donors harvested in the early, polycythemic phase of the disease as well as the later, myelofibrotic phase (Figure 5C). Here, JAK2 is linked to polycythemia.