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Chronic myelomonocytic leukaemia (CMML)

Chronic myelomonocytic leukaemia (CMML) is a specific type of leukaemia which affects the myeloid-cell-producing stem cells.

Blood cells are formed in the bone marrow, which is a spongy tissue found inside the bones. Blood-forming stem cells divide to produce either more stem cells or immature cells that become mature blood cells over time. A blood stem cell may become a myeloid stem cell or a lymphoid stem cell. In CMML, the myeloid stem cells are effected.

A myeloid stem cell becomes one of three types of mature blood cells:

The characteristic feature of CMML is the presence of large numbers of a type of white blood cell called a monocyte.

CMML has some similarities with a condition called myelodysplastic syndrome (MDS) and some resemblance to a group of conditions called myeloproliferative neoplasms (MPN). It is classified as being a mixture of these two conditions. About half of all cases have a high number of white cells at diagnosis and are most like MPN, while the remainder are more like MDS with low or normal white cell numbers at diagnosis. It is not clear whether these are separate diseases or different stages of the same disease.

CMML can be divided into two types, CMML-1 and CMML-2, based on the numbers of blasts in the blood and bone marrow and on the appearance of the blast cells. Treatment planning takes account of which type of CMML a patient has been diagnosed as having.

In a minority of cases CMML may develop into another form of leukaemia called acute myeloid leukaemia (AML). The distinction is based on the percentage of blast cells in the blood. This happens in between 15 and 30 in very 100 patients with CMML, and can happen after months or after years or even decades.

Myelomonocytic leukaemia rarely affects children, when this occurs it is regarded as a different disease to CMML, and is known as juvenile myelomonocytic leukaemia (JMML). This factsheet describes CMML, which affects adults only.

  • Published: Aug 2016
  • Next planned review: Aug 2018