Juvenile myelomonocytic leukemia (JMML) is a rare type of blood cancer that occurs when bone marrow production of white blood cells becomes severely disregulated. In children with JMML, the bone marrow — the soft, spongy center of the bones responsible for creating blood cells — produces immature white blood cells that grow too rapidly and keep reproducing even when they run out of space. This crowds out healthy bone marrow cells and can lead to symptoms such as infections, anemia, and bleeding. Currently, the only cure for JMML is stem cell transplant.