About one to two percent of children with leukemia have juvenile myelomonocytic leukemia (JMML). This means only 25 to 50 new cases a year in the United States. JMML mainly affects children younger than four years old, with the average age being two years old. In this disease, myelocytic and monocytic blood cells grow excessively. Doctors don't understand why some cells become JMML cells and others don't, however there are a few risk factors that have been associated with the disease.
Neurofibromatosis type 1 (NF1)
There are two conditions that are seemingly related to JMML. Neurofibromatosis type 1 (NF1) is diagnosed in approximately one in 10 patients with JMML. Children with NF1 are at considerable risk for developing JMML.
Infants with Noonan syndrome are possibly at risk for developing JMML or a similar disorder. Noonan syndrome is characterized by distinct facial features, growth retardation and heart anomalies. Children with Noonan syndrome who do develop a JMML-like disorder often get better without aggressive treatment by the time the child turns one year old.