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Myelodysplastic syndromes (MDS) represent a group of myeloid (bone marrow) stem cell disorders that gradually affect the ability of a person's bone marrow to produce normal red blood cells, white blood cells, and platelets.

Bone marrow is a semi-liquid tissue that is inside many bones such as the backbones, shoulder blades, ribs, pelvis, and stomach. The hematopoietic (blood-forming) stem cells in the bone marrow are responsible for producing and forming new blood cells. Erythrocytes, or mature red blood cells, help transport oxygen. White blood cells, (leukocytes) are one of the most critical elements of the circulatory system because they help protect the body against infections caused by foreign microorganisms such as bacteria and viruses. Platelets are the smallest cells in the blood and are formed in the red bone marrow. They help control bleeding and bruising.

People with myelodysplastic syndromes have a risk of the disease progressing to acute myeloid leukemia (AML), which is a bone marrow malignancy. Some studies suggest that AML is a natural progression of MDS and not a separate disease. In some people, MDS may gradually progress over a period of many years while in others it progresses rapidly to AML.

The risk of myelodysplastic syndromes increases with age as the disease commonly affects older people between the ages of 58 and 75. It is estimated that MDS affects 15 to 50 people per 100,000 Americans who are over the age of 70. Up to 20,000 new cases of myelodysplastic syndromes are diagnosed each year. However, since there are no actual registries listing the incidence of myelodysplastic syndromes, some researchers estimate that the numbers may actually be much higher. For example, anemia in older individuals may be ascribed to "old age" without consideration of the possibility of myelodysplastic syndrome.

The incidence of myelodysplastic syndromes in children is only 5% to 7% of all pediatric hematologic malignancies. It has been reported that up to 17% of childhood AML may result from a prior myelodysplastic phase. About 2% to 3% of all cases of juvenile leukemia are associated with juvenile myelomonocytic leukemia.

The exact causes of MDS remain unknown. Myelodysplastic syndromes are referred to as "clonal disorders" and it is believed that a genetic progression or evolution occurs in patients with MDS. The first step, called initiation, involves an "attack" on hematopoietic stem cells (actively dividing cells that are the source of blood cells). The second step, called tumor promotion or clonal expansion, is characterized by ineffective hematopoiesis (the hampering of normal formation and development of blood cells in the bone marrow) and typically a high rate of cell death. The third step, called malignant transformation, is characterized by the increase in leukemia blast cells and the progression to AML. These steps may also be characterized as pre-MDS phase, early-MDS phase, and late-MDS phase.

Certain gene mutations, specifically RAS, PTPN11, FLT3, and genes involved in DNA repair, have also been linked to the development of MDS. However, MDS is not caused by a virus nor is it contagious. In childhood MDS, it is believed that some conditions such as Fanconi's anemia, Shwachman's syndrome, and Down's syndrome increase the child's risk of developing MDS. About 30% of children with myelodysplastic syndromes are reported to have an inherited genetic disorder.