What is chronic myelomonocytic leukaemia (CMML)?

Chronic myelomonocytic leukaemia (CMML) is a rare type of blood cancer. In CMML there are too many monocytes in the blood. Monocytes are a type of white blood cell that fights infection.

The World Health Organisation (WHO) has included CMML in a group of blood cancers called:

  • myeloproliferative neoplasms (MPN) and
  • myelodysplastic syndromes (MDS)

CMML has different treatment options. This is because people with CMML can have features of both MPN and MDS.

These conditions are also known as myeloproliferative and myelodysplastic disorders.

The bone marrow and blood cells

To understand how and why leukaemia affects you, it helps to know how you make blood cells.

The bone marrow is the soft inner part of our bones that makes the blood cells.

All blood cells start from the same type of cell called a stem cell. The stem cell makes immature blood cells. These immature cells go through various stages of development before they become fully developed blood cells. 

The bone marrow makes different types of blood cells, including:

  • red blood cells to carry oxygen around our bodies
  • white blood cells to fight infection
  • platelets to help the blood clot

The diagram shows how the different types of cells develop from a single blood stem cell. 

A simplified diagram showing how blood cells are made

What are myeloproliferative neoplasms and myelodysplastic syndromes?

They are described as:

  •  myeloproliferative neoplasms (MPN) - a condition where there are too many blood cells made
  • myelodysplastic syndromes (MDS) - a condition where the blood cells are abnormal and not fully mature
  • myeloproliferative and myelodysplastic overlap disorders - this has features of both. CMML is the most common type

What happens in CMML 

In CMML it is a specific type of white blood cells called monocytes that are increased and abnormal. Monocytes are part of the immune system and help the body to fight infection. Too many are produced and they are not developed enough to work properly. It is also more difficult for the bone marrow to produce other good quality mature blood cells such as:

  • red blood cells

  • platelets

  • other white blood cells

As the number of abnormal blood cells increase, the levels of normal blood cells decrease.

Signs and symptoms of CMML

CMML usually develops slowly and doesn’t cause symptoms at first. When it does start to cause symptoms they might include:

  • tiredness and sometimes breathlessness because of a low red blood cell count (anaemia)
  • infections that don’t get better
  • bleeding (such as nosebleeds) or bruising easily because of a low platelet count
  • tummy (abdominal) discomfort from a swollen spleen
  • weight loss
  • loss of appetite
  • sweating
  • skin rashes or lumps
  • aches and pains in your bones and muscles

Risks and causes of CMML

We don't know the cause of most cases of CMML. But there are some risk factors Open a glossary item that can increase your risk of developing it.

The risk of developing CMML increases with age. The average age at diagnosis is between 65 and 74 years old. It is slightly more common in men than women.

Sometimes, CMML is caused by radiotherapy Open a glossary item or chemotherapy Open a glossary item treatment for cancer. This is called therapy or treatment related CMML.

Gene changes

Research has shown a number of gene Open a glossary item changes and chromosome Open a glossary item abnormalities that are important in CMML. A change in a gene is also known as a mutation. 

Doctors have identified genes where changes might lead to CMML, these include:

  • TET2

  • SRSF2

  • ASXL1

  • RUNX1

  • RAS pathway

Many people with CMML have more than one gene change. To look for gene changes your doctor tests samples of your blood or bone marrow.

This is quite complicated. Your doctor will give you more information about your situation. But knowing what the gene changes are will help your doctor plan your treatment. 

Types of CMML

The World Health Organisation (WHO) has split CMML into 2 types. They are CMML-1 and CMML-2. They do this by looking at the number of abnormal myeloid cells (blasts) in blood and bone marrow samples.

Doctors describe the number of blast cells as a percentage. This is the number of blast cells in every 100 white cells. 

  • CMML -1 means you have less than 5 % blast cells in your blood or less than 10 % blasts in your bone marrow.

  • CMML - 2 means you have 5 -19% blast cells in your blood and 10-19% in your bone marrow.

Knowing your type of CMML, along with other factors, helps your doctor to decide on your risk group. And can help them decide on the best treatment for you.

Risk groups

Doctors use risk groups to try and predict how well the CMML would respond to standard treatment. There are different risk groups doctors use for CMML.

Generally, doctors use the following to find out your risk group:

  • your type of CMML - which includes how many blast cells are in your blood and bone marrow

  • your white blood cell count

  • any genetic changes in the CMML cells

  • if you have a low number of red blood cells and need red blood cell transfusions

You can ask your doctor any questions you have about your risk group and what this means. They can explain how your risk group helps them choose the best treatment for you.

Change (transformation) to acute myeloid leukaemia

CMML can develop into acute myeloid leukaemia (AML). Doctors call this transformation.

Transformation can happen in around 30 out of every 100 people with CMML (around 30%). This can vary and depends on which type of CMML you have. CMML-2 has a higher risk of transformation. Transformation to AML might happen after a few months or after several years.


We have information on the treatments available for CMML.

  • Chronic myelomonocytic leukemia: 2022 update on diagnosis, risk stratification, and management 

    M. M Patnaik and A Tefferi

    American Journal of Haematology, 2022

  • Therapy related-chronic myelomonocytic leukemia (CMML): Molecular, cytogenetic, and clinical distinctions from de novo CMML

    M. M Patnaik and others

    American Journal of Hematology, 2018, Volume 93, Issue 1, Pages 1-154

  • The 5th edition of the World Health Organization Classification of Haematolymphoid Tumours: Myeloid and Histiocytic/Dendritic Neoplasms

    J D Khoury and others

    Leukemia, 2022. Volume 36, pages 1703–1719

  • Diagnosis and Treatment of Chronic Myelomonocytic Leukemias in Adults

    J Kwon

    Blood Research, 2021. Volume 56, Supplement 1, Pages 5–16.

Last reviewed: 
26 Feb 2024
Next review due: 
26 Feb 2027

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