The identification of an acquired somatic mutation in the JAK2 gene, resulting in a valine to phenylalanine substitution at position 617 (JAK2-V617F), has provided new insights into the pathogenesis of BCR-ABL1-negative MPN, which is found in most patients with polycythemia vera (PV) and in about 50% of patients with essential thrombocythemia (ET) and PMF [98, 99]. Here, JAK2 is linked to essential thrombocythemia.