Myeloproliferative neoplasms are heterogeneous group of clonal hematopoietic stem cell neoplasms with excessive proliferation of one or more of the erythroid, megakaryocytic, or myeloid lineages and relatively normal maturation resulting in increased numbers of red cells, platelets, and/or granulocytes in the peripheral blood. Constitutive tyrosine kinase activation appears to be a common pathogenetic mechanism.