Myelodysplastic syndromes (MDS) are a group of cancers in which immature blood cells in the bone marrow do not mature and therefore do not become healthy blood cells. Early on, there are typically no symptoms. Later symptoms may include feeling tired, shortness of breath, easy bleeding, or frequent infections. Some types may develop into acute myeloid leukemia. It causes when exposure to certain chemicals such as tobacco smoke, pesticides, and benzene, and exposure to heavy metals such as mercury or lead.