CSF2 and juvenile myelomonocytic leukemia: Juvenile myelomonocytic leukemia (JMML) is a rare myeloproliferative disorder that occurs in early childhood, often characterized by peripheral blood monocytosis, monosomy 7, dysplastic bone marrow, granulocyte-macrophage colony-stimulating factor (GM-CSF) hypersensitivity, and increased fetal hemoglobin.1,2 The standard treatment for JMML is chemotherapy followed by hematopoietic stem cell transplantation (HSCT), but the relapse rate for children with JMML status post-HSCT approaches 50% within 5 years1,3,4 and remains the main cause of therapy failure and mortality.