• 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 categorized JMML as a mixed myelodysplastic and myeloproliferative disorder.
• 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.


• JMML accounts for 1-2% of childhood leukemias each year; in the United States, there are an estimated 1-2 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.


Approximately 90% of JMML patients have some sort of genetic abnormality in their leukemia cells that can be identified with laboratory testing. This includes:
• 10% of patients with inborn mutations in the CBL gene
• 15% 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)
• Some infants with Noonan syndrome (also caused by mutation in PTPN11) develop a blood disease that can be indistinguishable from JMML but is self-limiting

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

The information on this page was last reviewed in January 2013 by Dr. Christian Flotho, Division of Pediatric Hematology and Oncology, University of Freiburg, Freiburg, Germany.

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