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MPL W515l/K mutated Essential Thrombocythemia (ET) usually present with increased platelet counts around 1000 x 10 9 /L as the only abnormal laboratory finding with normal values for hemoglobin and leukocytes and no or minor splenomegaly on palpation. Early stage MPL 515 mutated ET show the presence of clustered small and giant megakaryocytes with pronounced deeply lobulated nuclei, which are not seen in JAK2V617F positive ET, prodromal PV, and classical PV. MPL 515 mutated ET has no clinical, laboratory and bone marrow features of prodromal PV. Clustering of large mature large to giant megakaryocytes with pronounced hyper lobulated nuclei in a normocellular bone marrow is the hallmark of JAK2-wild type MPL 515 mutated thrombocythemia. Bone marrow histology in MPL 515 mutant patients revealed isolated megakaryocytic proliferation in a normocellular bone marrow at diagnosis with a reduction of erythropoiesis during follow-up. MPL 515 Thrombocythemia do not have PV features at diagnosis, do not evolve into PV, have normal LAP score, serum EPO and ferritin levels. In contrast to JAK2 V617F positive ET, no spontaneous Endogenous Erythroid Colonies (EEC) was found in none of evaluated MPLW5 15 L cases. Spontaneous megakaryocyte growth in culture with an overall normal response to ThromboPoietin (TPO) has been demonstrated in two MPL 515 mutated cases.

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