Myelodysplastic syndromes are a heterogeneous group of disorders characterized by clonal expansion of hematopoietic stem cells and ineffective hematopoiesis. Although all 3 cell lineages in myeloid hematopoiesis can be involved, the erythroid dysplasia and ineffective erythropoiesis of MDS are usually the most severe, and often precede the development of other bone marrow lineage defects.