(MDS) - A heterogeneous group of closely related clonal hematopoietic disorders. All are characterized by a hypercellular or hypocellular bone marrow with impaired morphology and maturation, dysplasia of the myeloid, megakaryocytic and|or erythroid lineages, and peripheral blood cytopenias resulting from ineffective blood cell production. Included diseases are - refractory anemia (RA), refractory anemia with ringed sideroblasts (RARS), refractory anemia with excess blasts (RAEB), refractory cytopenia with multilineage dysplasia and ringed sideroblasts (RCMD-RS) - chronic myelomonocytic leukemia (CMML) is a myelodysplastic|myeloproliferative disease. MDS is considered a premalignant condition in a subgroup of patients that often progresses to acute myeloid leukemia (AML).