DEFINITION

• Juvenile Myelomonocytic Leukemia (JMML) is a serious chronic leukemia (cancer of the blood) that affects children mostly aged 4 and under.
• The average age of patients at diagnosis is 2 years old.
• The World Health Organization has included JMML in the category of Myelodysplastic and Myeloproliferative disorders.
• The name JMML now encompasses all diagnoses formerly referred to as Juvenile Chronic Myeloid Leukemia (JCML), Chronic Myelomonocytic Leukemia of Infancy, and Infantile Monosomy 7 Syndrome.

FREQUENCY

• JMML accounts for 1-2% of childhood leukemias each year; in the United States, an estimated 25-50 new cases are diagnosed each year, which also equates to about 3 cases per million children.
• There is no known environmental cause for JMML.
• Since about 10% of patients are diagnosed before 3 months of age, it is thought that JMML is a congenital condition in these infants.

GENETICS

• About 80% of JMML patients have some sort of genetic abnormality in their leukemia cells that can be identified with laboratory testing. This includes:
  • 15-20% of patients with neurofibromatosis 1 (NF1)
  • 25% of patients with mutations in one of the RAS family of oncogenes (only in their leukemia cells)
  • another 35% of patients with a mutation in a gene called PTPN11 (again, only in their leukemia cells).

If you need help with some of the medical terms included in these pages, please use the NIH Medical Glossary.