The development of molecularly targeted treatments of adult leukemias warrants investigation of these targets in similar pediatric leukemias. The NF1 tumor suppressor gene, which encodes a GTPase activating protein for p21^ras, is frequently inactivated in juvenile myelomonocytic leukemia (JMML). Other patients with JMML acquire activating RAS gene mutations. Recipient mice reconstituted with Nf1^−/− fetal hematopoietic cells develop a myeloproliferative disease (MPD) that models the human disease. JMML arises from clonal expansion of a hematopoietic stem cell, and JMML cells and murineNf1^−/− hematopoietic cells are hypersensitive to granulocyte macrophage-colony stimulating factor and KitL, the ligand for c-kit. We generated embryos doubly mutant for theW^v allele of c-kit and Nf1 to ask if reduction of c-kit activity would delay or prevent the development of MPD. Despite a reduction in c-kit activity to approximately 10% of wild-type levels,Nf1^−/−;W^v/W^vcells induced MPD in recipient mice.