In this study, we report the results of a screening for mutations in SETBP1 and JAK3 of a cohort of seventy Italian patients with JMML, identifying 11.4% of them harboring secondary mutations in these two genes and discovering two new mutations in the SKI domain of SETBP1.JMML xenotransplantation and colony assay provide an initial understanding of the secondary nature of these events occurring in early precursor cells and suggest a different propagating capacity of clones harboring particular mutations.