MDS (myelodysplastic syndromes) is a group of disorders in which your blood-forming stem cells are defective and fail to make healthy, mature blood cells. Doctors consider MDS to be a slow growing form of blood and bone marrow cancer. With MDS, patients have low blood counts for at least one blood cell type and blood cells look abnormal under a microscope. Doctors must do a bone marrow biopsy to be certain of the diagnosis. Symptoms and the course of MDS vary for each patient depending on which blood cells are affected.