MSH: A progressive, malignant disease of the blood-forming organs, characterized by distorted proliferation and development of leukocytes and their precursors in the blood and bone marrow. Leukemias were originally termed acute or chronic based on life expectancy but now are classified according to cellular maturity. Acute leukemias consist of predominately immature cells; chronic leukemias are composed of more mature cells. (From The Merck Manual, 2006),CSP: progressive, malignant disease of the blood-forming organs, characterized by distorted proliferation and development of leukocytes and their precursors in the blood and bone marrow; classified according to degree of cell differentiation as acute or chronic, and according to predominant type of cell involved as myelogenous or lymphocytic.,NCI: Cancer that starts in blood-forming tissue such as the bone marrow and causes large numbers of blood cells to be produced and enter the bloodstream.,MEDLINEPLUS: Leukemia is cancer of the white blood cells. White