Diagnostic approach to suspected JMML. *Approximately 7% to 10% of JMML patients will not present with splenomegaly, although virtually all will develop splenomegaly within weeks to months of initial presentation. The mean age at diagnosis of JMML is 1.8 years, so that when encountering adolescents, alternative diagnoses such as chronic myelomonocytic leukemia should be considered. **GM-CSF in methylcellulose assays is not currently available in a Clinical Laboratory Improvement Amendments-approved setting. WAS, Wiskott-Aldrich syndrome; CML, chronic myelogenous leukemia; Ph+ ALL, Philadelphia-positive acute lymphoblastic leukemia; NF1, neurofibromatosis type 1.