A clonal hematopoietic disorder characterized by dysplasia and ineffective hematopoiesis in one or more of the hematopoietic cell lines. The dysplasia may be accompanied by an increase in myeloblasts, but the number is less than 20%, which, according to the WHO guidelines, is the requisite threshold for the diagnosis of acute myeloid leukemia. It may occur de novo or as a result of exposure to alkylating agents and/or radiotherapy. (WHO, 2001)