The relationship between the myelodysplastic syndromes and the myeloproliferative disorders.

As a result of clonal evolution typical cases of one of the myelodysplastic syndromes may develop myeloproliferative features. Similarly, typical cases of one of the myeloproliferative disorders may develop dysplastic features, either as part of the natural history of the disease or as a result of exposure to mutagenic drugs or isotopes. There is also an important group of "overlap syndromes" in which cases, at presentation, have both dysplastic and proliferative features. Chronic myelomonocytic leukaemia and many cases of atypical chronic myeloid leukaemia, juvenile chronic myeloid leukaemia and the childhood monosomy 7 syndrome are "overlap syndromes". In addition, a significant minority of cases which fit the generally agreed criteria for a diagnosis of one of the myelodysplastic syndromes (refractory anaemia, refractory anaemia with ring sideroblasts or refractory anaemia with excess of blasts) also have thrombocytosis, neutrophilia, monocytosis, eosinophilia or basophilia.