Recent studies have shown that PTPN11 gain-of-function mutations induce cytokine hypersensitivity in myeloid progenitors [16, 34, 35] and myeloproliferative disease with some similarity to JMML in mice [32, 36–38], establishing the causal role of PTPN11 mutations in the pathogenesis of JMML. The gene discussed is PTPN11; the disease is juvenile myelomonocytic leukemia.