are a group of diseases of the blood and bone marrow in which the bone marrow makes too many white blood cells. These disease have features of both myelodysplastic syndromes and myeloproliferative disorders. In myelodysplastic diseases, the blood stem cells do not mature into healthy red blood cells, white blood cells, or platelets and as a result, there are fewer of these healthy cells. In myeloproliferative diseases, a greater than normal number of blood stem cells develop into one or more types of blood cells and the total number of blood cells slowly increases. The 3 main types of myelodysplastic/myeloproliferative diseases include chronic myelomonocytic leukemia (CMML); juvenile myelomonocytic leukemia (JMML); and atypical chronic myelogenous leukemia (aCML). When a myelodysplastic/myeloproliferative disease does not match any of these types, it is called myelodysplastic/myeloproliferative neoplasm, unclassifiable (MDS/MPN-UC). Symptoms of CMML and JMML may include fever, feeling tired and weight loss. Symptoms of aCML may include easy bruising or bleeding and feeling tired or weak. Myelodysplastic/myeloproliferative diseases may progress to acute leukemia. There are different types of treatment for individuals with one of these diseases, which may include chemotherapy, another drug therapy, stem cell transplant and/or supportive care. Source: Genetic and Rare Diseases Information Center (GARD), supported by ORDR-NCATS and NHGRI.