Genetic mutations in specific Ras pathway components (PTPN11, NRAS, KRAS, NF1, or CBL), resulting in net hyperactivation of the Ras-GTP-GDP loop, are present in hematopoietic cells of >90% of children diagnosed with JMML [4,21,22,23,24,25,26]. Here, NF1 is linked to juvenile myelomonocytic leukemia.