Juvenile myelomonocytic leukemia (JMML) is a deadly pediatric hematologic malignancy characterized by excessive proliferation of monocytes/macrophages, elevated fetal hemoglobin, splenomegaly, and hypersensitivity of leukemia cells to granulocyte–macrophage colony-stimulating factor (GM-CSF) [1,2,3,4,5]. Here, CSF2 is linked to juvenile myelomonocytic leukemia.