Juvenile myelomonocytic leukemia (JMML) is an aggressive overproduction of cells in the myeloid lineage, affecting young children with a median age of 2 years, and is characterized by hypersensitivity to the cytokine granulocyte macrophage colony-stimulating factor (GM-CSF). This evidence concerns the gene CSF2 and juvenile myelomonocytic leukemia.