The somatic valine-to-phenylalanine mutation in the pseudokinase domain of Jak2 (Jak2-V617F) has been found in the majority of patients with polycythemia vera and in about 50% of patients with the other myeloproliferative neoplasms, essential thrombocythemia and primary myelofibrosis [1], [8]. Here, JAK2 is linked to essential thrombocythemia.