PTPN11 and myelodysplastic syndrome: Juvenile myelomonocytic leukemia (JMML) is a rare and aggressive childhood myelodysplastic/myeloproliferative neoplasm (MDS/MPN) thought to be initiated by the activation of the RAS signal transduction pathway due to germline or somatic mutations in genes encoding RAS (NRAS, KRAS) or RAS-pathway regulators (PTPN11 encoding the SHP2 cytoplasmic phosphatase, and less frequently NF1 or CBL) [1, 2].