Myeloproliferative disorders are abnormal proliferations of bone marrow stem cells, which can manifest as increased platelets, RBCs, or WBCs in the circulation and sometimes as increased fibrosis in the bone marrow with consequent extramedullary hematopoiesis (cell production outside the marrow). Based on these abnormalities, they are classified as

Essential thrombocythemia, primary myelofibrosis, and polycythemia vera are Philadelphia chromosome–negative myeloproliferative disorders. Myeloproliferative disorders sometimes lead to acute leukemia.

Less common myeloproliferative disorders include the hypereosinophilic syndromes and mastocytosis. There are also rare myeloproliferative disorders that overlap with myelodysplastic syndrome.

Each disorder is identified according to its predominant feature or site of proliferation (see Table: Classification of Myeloproliferative Disorders). Despite overlap, each disorder has a somewhat typical constellation of clinical features, laboratory findings, and course. Although proliferation of one cell line may dominate the clinical picture, each disorder is typically caused by clonal proliferation of a pluripotent stem cell, causing varying degrees of abnormal proliferation of RBC, WBC, and platelet progenitors in the bone marrow. This abnormal clone does not, however, produce bone marrow fibroblasts, which can proliferate in a polyclonal reactive fashion.

Mutations of the Janus kinase 2 (JAK2) gene are responsible for polycythemia vera and a high proportion of cases of essential thrombocythemia and primary myelofibrosis. Janus kinase 2 is a member of the tyrosine kinase family of enzymes and is involved in signal transduction for erythropoietin, thrombopoietin, and granulocyte colony-stimulating factor (G-CSF) among other entities. The thrombopoietin receptor gene (MPL), or the calreticulin (CALR) gene is mutated in a significant proportion of essential thrombocythemia and primary myelofibrosis patients.