Juvenile myelomonocytic leukemia (JMML) is a rare type of blood cancer that usually affects young children. It occurs when monocytes, a type of white blood cell, grow uncontrollably.

JMML accounts for around 1–2% of all childhood leukemias.

This article examines the symptoms, causes, treatment, and outlook for JMML.

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In JMML, the excess monocytes collect in the bone marrow and crowd out healthy cells, such as:

  • white blood cells, which help to fight infections
  • red blood cells, which supply oxygen throughout the body
  • platelets, which allow blood to clot to prevent bleeding

This means people with JMML have lower than usual levels of these healthy cells, which affects the functions they perform in the body.

The abnormal JMML cells can also collect in other areas, such as the lungs, liver, intestines, and skin.

JMML usually occurs in children under 4 years of age and is more common in males.

Although the exact cause of JMML is unknown, the condition occurs due to DNA damage in developing stem cells within the bone marrow.

Stem cells help to form white blood cells, red blood cells, and platelets.

The genetic mutation affects the RAS genes. These abnormal changes affect monocytes, which causes them to grow uncontrollably.

People with certain conditions may have a predisposition for developing JMML, including:

To diagnose JMML, doctors may order a range of different tests, which may include:

  • blood tests to measure blood cell amounts, organ function, and any biomarkers of cancer
  • bone marrow biopsy, in which a doctor removes a small sample of bone marrow cells to examine under a microscope for abnormal cell changes
  • cytogenetic and molecular tests, which use blood or bone marrow samples to look for gene mutations and chromosome changes in cancer cells

People with JMML usually have specific genetic mutations or abnormal chromosome changes.

In over 95% of people with JMML, doctors will identify a genetic mutation in the RAS pathway. Around 25–30% of people with JMML will have abnormalities in chromosome 7.

Doctors also use certain diagnostic criteria to diagnose JMML. This includes specific levels of monocytes, genetic changes, and factors such as an enlarged spleen.

In most cases, allogeneic hematopoietic stem cell transplantation (HSCT) is the only effective treatment option for JMML.

Allogeneic hematopoietic stem cell transplantation (HSCT)

According to the Leukemia and Lymphoma Society (LLS), HSCT is the only known cure for JMML.

HSCT uses healthy stem cells from a donor and transplants them into the person with JMML. People may receive stem cells from a donor’s blood or bone marrow or from umbilical cord blood.

A catheter into a blood vessel will supply the stem cells to the person receiving them.

The healthy donor stem cells will create new white blood cells, red blood cells, and platelets in the bone marrow. The newly created white blood cells will help to fight any remaining cancer in the body.

Possible complications of stem cell transplant include:

  • the immune system rejects and destroys the donor stem cells before they reach the bone marrow
  • graft-versus-host-disease (GVHD), where immune cells from the donor stem cells attack healthy cells in the person with JMML, which can be serious

Learn more about HSCT.


People may also have strong chemotherapy in the lead-up to a stem cell transplant. This helps wipe clean the bone marrow and primes the body to receive stem cells.

Chemotherapy helps manage JMML while people are waiting for a transplant, which may take 1–2 months.

Anyone aged 1 month or above may have the chemotherapy drug azacitidine (Vidaza) as part of their JMML treatment.

People may want to talk with a doctor about the possible side effects of chemotherapy.

Chemotherapy also helps to weaken the immune system, which reduces the chance of the body rejecting the donor stem cells. Chemotherapy alone will usually result in JMML returning after treatment.

Clinical trials

People may also want to discuss the option of clinical trials with a healthcare professional.

The LLS suggests that participating in a clinical trial may be a good option for people looking to try newer drug therapies for JMML, as the condition is challenging to treat with current medications.

JMML is an aggressive form of leukemia, which means it can be difficult to treat. Treatment with HSCT may be effective in curing around 50% of JMML cases.

After receiving a stem cell transplant, relapse may occur in up to 30–40% of people.

Relapse usually occurs within the first year following treatment, although a second stem cell transplant may cure JMML.

This section answers some common questions about JMML.

What age does juvenile myelomonocytic leukemia occur?

It is most common for doctors to diagnose JMML in children under the age of 4 years.

Can childhood leukemia run in families?

According to the American Cancer Society, people usually inherit genetic risk factors for childhood leukemia from their parents. In many cases, there is no known genetic cause for leukemia.

Having a sibling with childhood leukemia may slightly increase the risk of developing leukemia, but the risk of this is low.

In identical twins, this risk is higher. If one twin develops childhood leukemia, there may be a 1 in 5 chance of the other twin also developing leukemia. This risk is higher if leukemia occurs in the first year after birth.

There appears to be no increased risk of childhood leukemia in a child with a parent who develops leukemia in adulthood.

JMML gene mutations may be an acquired mutation, which means people do not inherit the genetic mutations that lead to JMML.

JMML is a rare type of blood cancer that usually affects children under the age of 4 years.

JMML occurs when a type of white blood cell grows out of control and crowds out healthy blood cells in the bone marrow.

The most effective treatment for JMML is stem cell transplant, which delivers healthy stem cells from a donor to a person with JMML to create new blood cells.

In around half of JMML cases, stem cell transplant may cure JMML.