Monosomy 7 is the most frequent aberration [33,53,65], occurring in combination with PTPN11 and KRAS mutations, but rarely with NF1, NRAS, or CBL. A traditional hallmark of clonogenic JMML cells is their hypersensitivity to GM-CSF in vitro [66,67]. This evidence concerns the gene NF1 and juvenile myelomonocytic leukemia.