JMML patients present with splenomegaly, for which they often undergo splenectomy; thus, the spleen provides a rich source of leukemia cells, as evident from the high number of CD34+ cells we recovered from most JMML patients (2–12% in JMML spleens compared to 0.3–3.5% in healthy donor umbilical cord blood (HD UCBs) (Figure 1B). Here, CD34 is linked to juvenile myelomonocytic leukemia.