It was previously reported that hematopoietic cells derived from the fetal livers of Nf1-deficient mice are hyperproliferative and cause a JMML-like myeloid proliferative disorder when transplanted into irradiated recipients (Birnbaum et al., 2000; Largaespada et al., 1996; Zhang et al., 2001; Zhang et al., 1998). Here, NF1 is linked to juvenile myelomonocytic leukemia.