Juvenile myelomonocytic leukaemia (JMML)

Juvenile myelomonocytic leukaemia (JMML) is a very rare type of slowly developing (chronic) blood cancer that occurs in young children. It used to be called:

  • juvenile chronic myelogenous leukaemia (JCML)

  • chronic myelomonocytic leukaemia of childhood

Leukaemia means a cancer of the blood forming system. The blood forming system is the bone marrow, the soft inner part of your bones. Although JMML has leukaemia as part of its name, the World Health Organisation (WHO) does not classify it as a leukaemia. It's now included in a group of blood cancers called myeloproliferative and myelodysplastic disorders.

About the bone marrow and blood cells

Bone marrow is a spongy material that fills the bones.

Diagram of bone marrow

It contains early blood cells, called stem cells. These develop into the 3 different types of blood cells.

Diagram of three different types of blood cell

Normally stem cells in your bone marrow keep dividing to make new blood cells. This keeps the numbers of cells circulating in your blood within a normal range.

Doctors can measure the number of blood cells your child has from a blood test. They call this their blood count.

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 JMML 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.

Problems with infections

The abnormal white blood cells are made in the bone marrow, enter the bloodstream and circulate around the body. They don't work normally, so children with JMML don't have the same protection against infection as they should.

This means your child might have a lot of infections. Or find it more difficult to recover from infections they do have.

Who gets JMML?

Children of any age can be diagnosed with JMML. Over half of those diagnosed (over 50%) are under 2 years old. It’s slightly more common in boys than girls. But we don’t know why.

We don’t know what causes, or how to prevent, JMML. It’s not something you or anyone has done, and no one is to blame.

Children with a genetic disorder called neurofibromatosis 1 (NF1) Open a glossary item are more at risk of developing JMML. But this only happens in between 10 to 15 out of every 100 cases (between 10 to 15%).

Symptoms of JMML

Some of the symptoms of JMML are similar to those of other childhood illnesses. And they vary between children. Remember JMML is rare. And it develops fairly slowly. This means that symptoms might develop over weeks or months.

In JMML, as the abnormal blood cells multiply in the bone marrow, fewer normal blood cells are made. If there are not enough normal blood cells, the body cannot work normally.

Because of this, there are a number of symptoms in children with JMML. Your child might have a combination of these. Symptoms include:

  • being tired, lethargic and generally feeling unwell
  • being irritable – it might take a young child longer to settle
  • bruising easily
  • nosebleeds and bleeding gums
  • looking pale
  • fever
  • getting lots of infections
  • an enlarged liver and spleen Open a glossary item – you might have noticed your child has jumped a nappy or trouser size quickly
  • pain in the tummy – if the monocytes have collected in this area
  • skin rashes
  • cough and wheezing

Diagnosing JMML

Your child will need a number of tests if their doctor suspects they have JMML. Your child might have more than one test on the same day. And they only have the tests their team think are needed.

These tests aren’t generally painful. But, your child will have to keep still for some of them.

Main tests

The main tests doctors use to diagnose JMML are:

Blood tests can:

  • check your child’s general health, including how well their liver and kidneys are working
  • check the numbers of blood cells
How do they take blood?

A doctor, nurse or person specialising in taking blood (a phlebotomist) takes a small amount of blood from your child. This is usually from the back of the hand, the inside of the elbow or wrist area.

Most blood tests take a couple of minutes. 

The specially trained staff will help your child feel as comfortable as possible. Things that can make it easier for your child include:

Local anaesthetic

The phlebotomist might use a local anaesthetic. This means that they apply a numbing spray or cream to your child's skin before they do the test. This should mean it’s less painful for your child, although they might still feel the needle go in.


Distraction could be singing, counting, watching something on your mobile phone or a DVD.

Sitting with your child

Holding your child while they are having their blood test can make them feel safe and secure. You can give them a cuddle afterwards if you’re unable to hold them during their test.

After their blood test

Your child can usually eat, drink and play as normal after a blood test.

Your child has this test to check whether there are immature cells in the bone marrow.


A doctor or specialist nurse removes a sample of bone marrow cells or an area of bone marrow in one piece. Doctors can then look at the cells or tissue under a microscope.

This is usually from your child's lower back.

Diagram of a baby's bone marrow test

Your child might have the sample taken from a bone in their leg if your child is very young.

Diagram showing a baby bone marrow aspirate on the leg

Your child usually has this test in the outpatient department of the hospital. Most children and young people have this test under general anaesthetic. Or some children might have a medicine (sedative) to make them feel sleepy and relaxed. This helps them to lie still on the bed while they have the test.


Your child stays on the day unit or children’s ward for a couple of hours after the test. This is so the team can be sure they have recovered and are awake enough from the sedation or anaesthetic. The nurses on the unit usually check that your child has:

  • had something to drink
  • had something to eat or some milk
  • had a wee
  • a clean and dry dressing over the site of the test

A bone marrow biopsy is a safe test. Your child’s doctor or nurse talks with you about any possible problems they might have after it. They give you a phone number to call if you have any problems after. If in doubt, give the number a call.

It’s likely your child can go home the same day, unless they are staying in hospital for other tests.

Other tests

Your child might also have other tests to see if any organs of the body are affected by the JMML. And to see how well they are generally.

These other tests might include:

  • x-rays

  • CT scans

  • MRI scans

  • spinal fluid test (lumbar puncture)

My child has just been diagnosed with JMML

Finding out that your child has a rare condition like JMML can be devastating. Even if you have been told that it is a possibility, hearing this confirmed by a doctor can be shocking. You might feel numb and that it isn’t real.

There are a range of emotions you might feel including anger, disbelief and sadness. Sometimes a diagnosis can feel like a relief, especially if your child has been unwell for some time and you have been worrying about what’s wrong.

We have information written for parents of newly diagnosed children.

Gene changes in JMML

More than 95 in 100 children with JMML (more than 95%) have changes in particular genes Open a glossary item.

These include:

  • KRAS
  • NRAS
  • RRAS
  • CBL - children with CBL changes also often have poor growth, developmental delay and inflammation of the blood vessels
  • NF1 (neurofibromatosis type 1) - some children with NF1 gene changes have learning problems and are at increased risk of tumours that are cancerous or non cancerous (benign)
  • PTPN 11 - this is related to a genetic syndrome called Noonan syndrome

Doctors test for these gene changes as it helps decide which treatment is best. And researchers hope that learning more about JMML will improve treatment for all children with JMML in the future.

Treatment for JMML

Watch and wait (CBL gene)

A small number of children with changes in the CBL gene do not need treatment straightaway. This is because there is a chance the JMML will go away without treatment. Your child will be followed up closely by their specialist team. This is called watch and wait.

Watch and wait or low dose chemotherapy (NRAS gene)

Some children with the NRAS gene change might also be followed closely without treatment. This is because there is a chance the JMML will go away without treatment. Or they might have low dose chemotherapy. This is because their JMML might not require intensive treatment.

Stem cell or bone marrow transplant

The treatment for all other children with JMML is a stem cell or bone marrow transplant. Some children need chemotherapy before this treatment. This is to get the JMML under control enough for transplant. They will then have more intensive chemotherapy as part of the transplant process.

A transplant involves having intensive cancer drug treatment, which has a good chance of destroying the immature white blood cells. But it also destroys the healthy stem cells in the bone marrow. The child then has a drip of new stem cells into their bloodstream after their high dose treatment. These are most commonly from:

  • family member, most often a brother or sister or
  • a donor from the transplant register

Tissue typing is a set of blood tests that show how closely a possible bone marrow donor’s tissue matches the person needing a transplant. The results help doctors decide who could be a possible donor. A brother or sister is most likely to be a match.

At the moment, this is the only type of treatment that can possibly cure JMML.

We have information about having a stem cell transplant. This information is written for adults with cancer, but you might find some of it useful.

We have some information about supporting your other children. It includes tips from parents. It includes some information about sibling donors for a stem cell or bone marrow transplant on the page.

Unfortunately, a transplant is only suitable for some children. Doctors and scientists are trying to improve treatment for this disease, but currently it is still difficult to cure.


Coping with your child’s diagnosis of JMML can be difficult, both practically and emotionally. Being well informed about your child's condition and their treatment can make it easier to make decisions and cope with what happens.

Children’s Cancer Information

You might find some of our information on childhood cancer useful to read. There is a list of support organisations, including those who provide support and information for families with children with any serious illness.

Cancer Research UK nurses

For support and information, you can call the Cancer Research UK information nurses on freephone 0808 800 4040, from 9am to 5pm, Monday to Friday. They can give advice about who can help you and what kind of support is available.

Leukaemia Care

Leukaemia Care provide support to anyone affected by blood cancer. They have information on their website about JMML.

  • International Consensus Classification of Myeloid Neoplasms and Acute Leukemias: integrating morphologic, clinical, and genomic data
    D Arber and others
    Blood, September 2022. Volume 140, Issue 11, Pages 1200 to 1228

  • Oxford Textbook of Cancer in Children (7th Edition)
    H N Caron and others
    Oxford University Press, 2020

  • JMML genomics and decisions
    C M Niemeyer
    Haematology, ASH Education Program, November 2018. Volume 1, Pages 307 to 312

  • Juvenile myelomonocytic leukemia-A comprehensive review and recent advances in management
    A K Gupta and others
    American Journal of Blood Research, February 2021. Volume 11, Issue 1, Pages 1 to 21

  • International Classification of Diseases 11th Revision International Classification of Diseases for Mortality and Morbidity Statistics
    World Health Organization (WHO), 2022

  • The information on this page is based on literature searches and specialist checking. We used many references and there are too many to list here. Please contact patientinformation@cancer.org.uk with details of the particular issue you are interested in if you need additional references for this information.

Last reviewed: 
12 Dec 2023
Next review due: 
12 Dec 2026

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