MSH: A de novo myeloproliferation arising from an abnormal stem cell. It is characterized by the replacement of bone marrow by fibrous tissue, a process that is mediated by CYTOKINES arising from the abnormal clone.,NCI: A chronic myeloproliferative neoplasm characterized by bone marrow fibrosis, proliferation of atypical megakaryocytes and granulocytes in the bone marrow, anemia, splenomegaly, and extramedullary hematopoiesis. The cause is unknown. The median survival time is 3-5 years from diagnosis. (WHO, 2001),NCI: A progressive, chronic disease in which the bone marrow is replaced by fibrous tissue and blood is made in organs such as the liver and the spleen, instead of in the bone marrow. This disease is marked by an enlarged spleen and progressive anemia.