A group of related, progressive disorders of the bone marrow characterized by low populations of abnormally developed blood cells. Since myelodysplastic syndrome precedes the development of acute leukemia in approximately one case in four, it is sometimes called preleukemia. The current understanding of this disease is that all blood cells (white, red, and platelets) develop from a single line of precursor cells, and this single line is abnormal. At about the time the abnormal cells are ready to enter the bloodstream, they self-destruct. Thus, the bone marrow is packed tight with abnormally developing blood cells, but the number of cells in the blood is unusually low. The cells that do appear in the bloodstream typically do not perform their functions as well as normal cells.