The developmentally restricted myeloid to lymphoid transition seen in the fetal IL-7R progenitor compartment offers an attractive explanation as to why ETV6-RUNX1 ALL is overwhelmingly a disease of childhood and why ETV6-RUNX1 B-ALL frequently co-expresses B and myeloid surface markers (Abdelhaleem, 2007). Here, RUNX1 is linked to acute lymphoblastic leukemia.