Myelodysplastic syndrome comprises a group of clonal hematologic disorders characterized clinically and morphologically by ineffective hematopoiesis. It is viewed by most hematologists as encompassing stages of neoplastic hematopoiesis associated with cytopenias and as excluding nonneoplastic conditions. Neoplastic transformation of hematopoietic cells can occur at various levels of stem-cell development. The initial hematopoietic stem cell injury can result from cytotoxic chemotherapy, radiation exposure, viral infection, chemical exposure, or genetic predisposition. A clonal mutation...