Approximately 50 children are diagnosed with JMML in the United States every year. JMML is usually found in children younger than age two and is more common in boys than girls. It is not known why children develop this type of cancer, but a strong link exists between JMML and certain genetic syndromes, such as neurofibromatosis type 1 and Noonan syndrome. About 85 to 90% of children with JMML have a mutation in one of the following genes in their leukemia cells:

• PTPN11 (35% of children with JMML)
• NRAS or KRAS (25% of children with JMML)
• CBL (10–15% of children with JMML)
• NF1 (10–15% of children with JMML)

In addition, some children with JMML are found to have other chromosomal abnormalities. For example, about 30% of children with JMML have a deletion on chromosome 7 (called monosomy 7).

Infants with Noonan syndrome sometimes develop a JMML-like disorder during the first year of life that usually disappears before 18 months of age without treatment. It is important to identify children with Noonan syndrome who have temporary JMML from children who have the type of JMML that is caused by PTPN11 mutations in only the leukemia cells. When PTPN11 mutations occur only in the leukemia cells, the disease can be very aggressive and will not go away without treatment.