Over the past few years, clinical studies have identified mutations of CBL, and rarely CBL-B, in a subset of patients with chronic myeloid leukemia, particularly those within the myeloid dysplastic syndrome/myeloproliferative neoplasm (MDS/MPN) group; the incidence is particularly high among patients with juvenile myelomonocytic leukemia (JMML), chronic myelomonocytic leukemia (CMML) and atypical chronic myeloid leukemia (aCML) [3–9]. The gene discussed is CBLB; the disease is juvenile myelomonocytic leukemia.