The WHO classification of the myelodysplastic syndromes.

For more than two decades the French-American-British (FAB) classification has provided a framework for the diagnosis and classification of the myelodysplastic syndromes. However, with widespread use of this classification, it became clear that prognostic differences existed within single categories. The FAB classification has therefore been modified by a World Health Organization (WHO) expert group to take account of these prognostic differences. In addition, therapy-related myelodysplasia and the 5q- syndrome have been recognized as distinct types of disorder and the existence of mixed myelodysplastic/myeloproliferative disorders has been recognized; chronic myelomonocytic leukaemia, previously regarded as one of the myelodysplastic syndromes, was re-assigned to this latter group of disorders.