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The classical myeloproliferative neoplasms (MPNs), also called BCR-ABL MPNs, are the most frequent diseases among the myeloproliferative disorders. MPNs are characterized by excessive production of terminally differentiated blood cells that are fully functional. Classical MPNs have been classified into 3 entities: polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF), which have frequent disease-related complications, such as venous and arterial thrombosis, hemorrhages, and transformation to acute myeloid leukemia (AML).1 All MPN entities arise from a single somatically mutated hematopoietic stem cell (HSC) that clonally expands and gives rise to virtually all myeloid cells, and B and natural killer (NK) cells. The clonal expansion of the MPN HSC is accompanied by single or multilineage hyperplasia. PV is characterized not only by an excess of erythrocytes and predominant erythroid lineage involvement, but is also associated with a variable hyperplasia of the megakaryocytic/granulocytic lineages. ET is characterized by an increased platelet count with a megakaryocytic hyperplasia, whereas PMF is a more heterogeneous disorder both by its clinical and biological characteristics, defined by the presence of bone marrow fibrosis (excess of collagen fibers) and megakaryocytic hyperplasia. Myeloproliferation in PMF initially predominates in the bone marrow and later expands to extramedullary sites, such as the spleen.