JAK2 and essential thrombocythemia: The discovery by different groups in 2005 of a JAK2V617F mutation present in 50% of patients with essential thrombocythemia (ET), 60% of patients with myelofibrosis (MF) and over 90% of patients with polycythemia vera (PV) emphasized the importance of the JAK2 signaling pathway in MK growth and the production of platelets.