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"Good Morning America's" Robin Roberts announced this morning that she has myelodysplastic syndrome, or MDS. The syndrome is also known as preleukemia.

MDS can be broken down by its name: Myeloid refers to a type of blood cell; dysplasia means a problem with the development of those cells.

The condition occurs when "something goes wrong in your bone marrow -- the spongy material inside your bones where blood cells are made," according to the Mayo Clinic.

A healthy person's bone marrow produces stem cells that mature into blood cells. But the bone marrow of a person with MDS produces abnormal stem cells that turn into defective blood cells.

Deformed cells get into the bloodstream and eventually outnumber healthy blood cells, according to the National Cancer Institute. Often the deformed blood cells don't live as long as they should, producing a shortage in the body.

There are several types of MDS, depending on the kind of myeloid cells - red blood cells, white blood cells or platelets - that are being affected. Having too few red blood cells results in anemia; having too few white blood cells can result in frequent infections.

The term "preleukemia" is a bit misleading, as most MDS cases do not become cancerous. Certain types of MDS can progress to acute myeloid leukemia, however.

MDS can be caused by exposure to chemotherapy and radiation, common cancer treatments. (Roberts is a breast cancer survivor.)

Symptoms are rare during the early stage of the disease, but can include tiredness, shortness of breath and easy bruising/bleeding. Doctors generally diagnose through a blood test and a bone marrow biopsy.

More here:
What is preleukemia or MDS?