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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. The World Health Organisation (WHO) has included CMML in a group of blood cancers called myeloproliferative and myelodysplastic disorders.

CMML is a separate condition with different treatment options because people with CMML can have features of both myeloproliferative disorders (MPN) and myelodysplastic disorders (MDS).

The bone marrow and 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 cell. 

Diagram showing how blood cells are made

What are myeloproliferative and myelodysplastic disorders?

A myeloproliferative disorder is a condition where there are too many blood cells made. A myelodysplastic disorder is where the blood cells made are abnormal and not fully mature. In reality, the two disorders often overlap, which is why the WHO has put them together in the same category here.

In CMML it is a specific type of white blood cells called monocytes that are 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 blood cells such as:

  • red blood cells
  • platelets
  • other white blood cells

This is because the monocytes take up a lot of space in the bone marrow.

What happens in CMML?

White blood cells called monocytes help the body to fight infection. In CMML the bone marrow produces abnormal monocytes. They are not fully developed and can’t work normally. These abnormal cells are also called blast cells. 

These abnormal blood cells either stay in the bone marrow or are destroyed before they get into the bloodstream. As the CMML develops, the bone marrow becomes full of the abnormal monocytes. These abnormal blood cells then spill out into the bloodstream.

Because the bone marrow is full of the abnormal cells, it can’t make enough of the other types of blood cells. The low numbers of normal blood cells in the bloodstream eventually cause symptoms.

Risks and causes of CMML

We don't know the cause of most cases of CMML but there are some risk factors that can increase your risk of developing it. A risk factor is something that might make you more likely to develop a particular condition or disease.

Your risk of developing CMML increases with age. The average age at diagnosis is between 71 and 74 years old. And it is more common in men than women.

Sometimes, CMML is caused by radiothereapy or chemotherapy treatment for cancer. This is called secondary or treatment related CMML.

Gene changes

Researchers have found that around half of the people who have CMML (around 50%) have a change in a gene  Open a glossary itemcalled TET2. The TET2 gene makes a protein that controls how many monocytes the stem cells make. Up to 30 out of 100 people (up to 30%) have a change in a gene called RAS. The change makes the cells multiply uncontrollably.

We need more research to find out what causes the changes in these genes. These genetic changes aren’t inherited, so they can’t be passed on to your children. These gene changes in your bone marrow cells are picked up over the course of your lifetime. 

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

Types of CMML

The World Health Organisation (WHO) has split CMML into 3 types. They are called type 0, type 1 and type 2. The type tells you how many abnormal myeloid cells (blasts) you have in your 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. 

  • Type 0 CMML means you have less than 2% blast cells in your blood and less than 5% blast cells in your bone marrow.
  • Type 1 CMML means you have 2-4% blast cells in your blood or 5-9% blasts in your bone marrow. Some people have both.
  • Type 2 CMML means you have 5-19% blast cells in your blood and 10-19% in your bone marrow.

Having Auer rods in the samples means you have type 2 CMML. Auer rods are material inside the cells that look like long needles. They can only be seen under the microscope.

Knowing your type of CMML helps your doctor to decide on the best treatment for you.

Change (transformation) to acute myeloid leukaemia

CMML can develop into an acute myeloid leukaemia (AML) if the number of blast cells in your blood rises above 20%. Doctors call this transformation.

Transformation happens in between 15 and 30 out of every 100 people with CMML (between 15 to 30%). This might happen after a few months or after several years.


We have information on the treatments available for CMLL.

Last reviewed: 
20 Aug 2020
Next review due: 
20 Aug 2023
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