Myelodysplastic syndrome (MDS) is currently a consensus-based pathologic diagnosis that is determined by partly subjective findings on bone marrow histology and peripheral blood smear. MDS constitutes a stem cell-derived, ineffective clonal myeloproliferation that results in symptomatic cytopenia and will eventually evolve into acute myeloid leukemia. MDS is rare in persons younger than 50 years but is a common hematologic malignancy in the elderly. As such, the overwhelming majority of patients with MDS are not suitable candidates for allogeneic hematopoietic stem cell transplantation. At the same time, insufficient knowledge regarding disease pathogenesis has restricted the development of effective drug therapy.