JMML is a rare clonal myelodysplastic-myeloproliferative disorder typical of infancy and early childhood, characterized by spontaneous in vitro proliferation of bone marrow and peripheral blood hematopoietic progenitors in the absence of exogenous growth factors, due to selective hypersensitivity to granulocyte-macrophage colony-stimulating factor (GM-CSF) (7). Here, CSF2 is linked to juvenile myelomonocytic leukemia.