The classic myeloproliferative neoplasms - essential thrombocytosis, polycythemia vera, and primary myelofibrosis - are acquired, clonal hematopoietic stem cell disorders characterized by an overproduction of mature blood cells, bone marrow hypercellularity, extramedullary hematopoiesis, a tendency for thrombosis, and, rarely, leukemic transformation. Despite being classified as neoplastic diseases, the myeloproliferative neoplasms are often characterized by longevity, with survival measured in decades, even in the absence of treatment. Primary myelofibrosis is the rarest of the myeloproliferative neoplasms, is the most obscure with regard to its pathophysiology, and carries the least favorable although highly variable natural history. The identification of molecular lesions specific to the myeloproliferative neoplasms, in particular JAK2 V617F, has broadened understanding of the common features within these disorders and has advanced diagnostic, prognostic, and therapeutic tools. This article highlights the challenges inherent in the management of primary myelofibrosis and presents an opportunity to address the basis of individual variation within a rare and complex disorder.
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