Nonchronic myeloid leukemia (CML) myeloproliferative neoplasms (MPNs), referred to as BCR/ABL1-negative MPNs, have classically been categorized as polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF). Each of these MPNs represents a multipotent hematopoietic stem cell-derived clonal myeloproliferation of one or more of the myeloid lineages with the variably common features of erythrocytosis, granulocytosis, and/or thrombocytosis in peripheral blood (PB) and/or variable bone marrow (BM) fibrosis. It is generally a disease of older individuals; however, ET and PMF have been reported in children. Other than the mentioned clinical characteristics of the PB, this category of disease also possesses a tendency toward organomegaly (i.e., hepatosplenomegaly), thrombosis, and bleeding. The BM is usually hypercellular with a blast count of <10%. Although these diseases are heterogeneous, they are characterized by increased blood cell production related to cytokine hypersensitivity, virtually normal cell maturation, and progressive evolution to BM failure with an end point of fibrosis or leukemia.