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Juvenile Myelomonocytic Leukemia (JMML)

Reviewed by: Larissa Hirsch, MD
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What Is Leukemia?

Leukemia is a cancer that mostly affects white blood cells. White blood cells (also called leukocytes) fight infections. 

Bone marrow is a spongy material inside the bones that makes white blood cells, red blood cells, and platelets. With leukemia (loo-KEE-mee-uh), the bone marrow makes white blood cells that don’t work. These abnormal cells can’t protect the body from germs. They crowd the bone marrow, enter the bloodstream, and can spread to other parts of the body, like the lymph nodes, brain, or liver.

Leukemia is the most common type of cancer in children. It can be acute (fast-growing) or chronic (slow growing). Most kids and teens treated for leukemia are cured of the disease.

What Is Juvenile Myelomonocytic Leukemia?

Juvenile myelomonocytic (mye-uh-low-mon-uh-SIT-ik) leukemia (JMML) happens when immature blood cells (called blasts) make too many myelocytes and monocytes (two types of white blood cells.) The myelocytes, monocytes, and blasts crowd the normal cells in the bone marrow and other organs in the body and cause problems.

JMML tends to happen in kids younger than age 4. Many kids have a change in their genes (a mutation) that affects the monocytes.

Some medical conditions (like neurofibromatosis type 1 and Noonan syndrome) make a child more likely to develop the disease.

What Are the Signs & Symptoms of Juvenile Myelomonocytic Leukemia?

JMML develops over time, taking weeks to months. At first, kids can have very few symptoms. Eventually they may develop anemia, which is when the body has too few red blood cells. This happens because the bone marrow stops making the usual amount of red blood cells.

Kids with anemia may:

  • look pale
  • feel very tired or weak
  • get short of breath while playing

When they don’t have enough platelets (PLATE-lits), kids with leukemia may bruise easily, get nosebleeds, or bleed for a long time after even a minor cut.

Other symptoms of leukemia can include:

  • pain in the bones or joints, sometimes causing a limp
  • swollen (swollen glands) in the neck, groin, or elsewhere
  • poor appetite and weight loss
  • fevers
  • belly pain

Because their white blood cells can't fight infections, kids with leukemia are more likely to get viral or bacterial infections.

Sometimes leukemia can spread, or metastasize (meh-TASS-tuh-size). If it spreads to the brain, symptoms may include headaches, seizures, balance problems, or vision problems. If it spreads to the lymph nodes in the chest, symptoms may include breathing problems and chest pain.

How Is Juvenile Myelomonocytic Leukemia Diagnosed?

Doctors check for leukemia with special tests. These include:

  • Blood tests. Tests such as a complete blood count, liver function and kidney function panels, and blood chemistry tests can give important information about the number of normal blood cells in the body and how well the organs are working. The shapes and sizes of the blood cells are checked with a microscope.
  • Imaging studies. These may include an X-ray, CT scan, MRI, or ultrasound. Doctors use these to rule out other causes of symptoms, or look for a mass of leukemia cells in the chest that can affect breathing or blood circulation.
  • Bone marrow aspiration and biopsy. For this procedure, a child gets medicine to sleep and be comfortable. The doctor then puts a needle into a large bone, usually the hip, and removes a small amount of bone marrow. A lab does these tests on the bone marrow sample:
    • Flow cytometry tests. Doctors carefully look at the cancer cells and figure out the type and subtype of the leukemia. This is important because treatment varies among different types of leukemia.
    • Genetic tests. By looking carefully at the blood or bone marrow, doctors check for changes in the genes. This can help them figure out the best treatment.
    • Tissue typing or HLA (human leukocyte antigen) typing. If a child needs a stem cell transplant (sometimes called a bone marrow transplant), this test helps find a suitable stem cell donor. It compares the proteins on the surface of the child's blood cells with the proteins on a potential donor's cells. The more HLA markers a child and donor share, the greater the chances that a transplant will go well.
  • Spinal tap (lumbar puncture). The doctor uses a hollow needle to remove a small amount of cerebrospinal fluid (the fluid surrounding the brain and spinal cord). The fluid is checked for cancer cells. This helps doctors plan treatment.

How Is Juvenile Myelomonocytic Leukemia Treated?

Chemotherapy (the use of drugs to kill cancer cells) may be given to control JMML at first. But the most effective treatment is a stem cell (bone marrow) transplant. To do this, doctors:

  1. Give high-dose chemotherapy to destroy cancer cells and normal bone marrow and immune system cells.
  2. Put healthy donor stem cells into the body. The new stem cells can rebuild a healthy blood supply and immune system.

Researchers are looking into other treatments, such as:

  • Molecular-targeted therapies: These medicines slow the growth of cancer cells by blocking certain molecules or proteins that help cancer cells grow.
  • Immunotherapy: These medicines mark cancer cells so that the body's immune system can find them and remove them from the body.

What Else Should I Know?

Having a child being treated for cancer can feel overwhelming for any family. But you're not alone. To find support, talk to anyone on your child’s care team or a hospital social worker. Many resources are available to help you get through this difficult time.

You also can find information and support online at:

Reviewed by: Larissa Hirsch, MD
Date reviewed: October 2021