Mixed myeloproliferative and myelodysplastic disorders.

When the French-American-British (FAB) classification system was first devised in the 1970s for the myelodysplastic syndromes (MDS), chronic myelomonocytic leukemia (CMML) was included in the FAB MDS classification schema. It always appeared out of place, however, at least when compared with the other FAB MDS categories, as CMML clearly has some features of a myeloproliferative disorder. Yet CMML also did not fit well with the traditional four myeloproliferative disorders. In the 1990s there was ongoing discussion about distinguishing a proliferative type of CMML versus a dysplastic type of CMML, based primarily on the total white blood cell count. In 2001 the World Health Organization reclassified most of the hematologic malignancies, creating a new category of mixed myelodysplastic/myeloproliferative diseases. Finally, CMML has an appropriate home, along with other disorders that seem to fit quite well in this "mixed" category.