A somatic mutation of JAK2, coding for constitutively activated Jak2-V617F, is frequently observed in BCR/ABL1-negative myeloproliferative neoplasms (MPNs): 96% in polycythemia vera (PV), 55% in essential thrombocythemia (ET), and 65% in primary myelofibrosis (PMF) [3]. Here, JAK2 is linked to primary myelofibrosis.