A myelodysplastic syndrome (MDS) is one of a group of cancers in which immature blood cells in the bone marrow do not mature, and as a result, do not develop into healthy blood cells. Early on, no symptoms typically are seen. Later, symptoms may include feeling tired, shortness of breath, bleeding disorders, anemia, or frequent infections. Some types may develop into acute myeloid leukemia.Risk factors include previous chemotherapy or radiation therapy, exposure to certain chemicals such as tobacco smoke, pesticides, and benzene, and exposure to heavy metals such as mercury or lead. Problems with blood cell formation result in some combination of low red blood cell, platelet, and white blood cell counts. Some types of MDS cause an increase in the production of immature blood cells (called blasts), in the bone marrow or blood. The different types of MDS are identified based on the specific characteristics of the changes in the blood cells and bone marrow.Treatments may include supportive care, drug therapy, and hematopoietic stem cell transplantation. Supportive care may include blood transfusions, medications to increase the making of red blood cells, and antibiotics. Drug therapy may include the medications lenalidomide, antithymocyte globulin, and azacitidine. Some people can be cured by chemotherapy followed by a stem-cell transplant from a donor.About seven per 100,000 people are affected by MDS; about four per 100,000 people newly acquire the condition each year. The typical age of onset is 70 years. The prognosis depends on the type of cells affected, the number of blasts in the bone marrow or blood, and the changes present in the chromosomes of the affected cells. The average survival time following diagnosis is 2.5 years. MDS was first recognized in the early 1900s; it came to be called myelodysplastic syndrome in 1976.