CALR and essential thrombocythemia: MPNs have been categorized by the WHO as: chronic myeloid leukaemia (CML), polycythaemia vera (PV), primary myelofibrosis(PMF) and essential thrombocythemia (ET) and are characterized by the presence of the BCR-ABL gene, JAK2, MPL or CALR mutations, excessive erythrocyte production, fibrotic bone marrow degeneration, and excessive megakaryocyte lineage proliferation, respectively (3).