The t(8;21) RUNX1-ETO translocation is one of the most frequent cytogenetic abnormalities in acute myeloid leukemia (AML). In RUNX1-ETO + patient samples, differing classes of activating c-KIT receptor tyrosine kinase mutations have been observed. The most common (12%-48%) involves mutations, such as D816V, which occur in the tyrosine kinase domain, whereas another involves mutations within exon 8 in a region mediating receptor dimerization (2%-13% of cases). To test whether distinct subtypes of activating c-KIT mutations differ in their leukemogenic potential in association with RUNX1- ETO, we used a retroviral transduction/ transplantation model to coexpress RUNX1-ETO with either c-Kit D814V or c-Kit T417IΔ418-419 in murine hematopoietic stem/progenitor cells used to reconstitute lethally irradiated mice. Analysis of reconstituted animals showed that RUNX1-ETO;c-Kit D814V coexpression resulted in 3 nonoverlapping phenotypes. In 45% of animals, a transplantable AML of relatively short latency and frequent granulocytic sarcoma was noted. Other mice exhibited a rapidly fatal myeloproliferative phenotype (35%) or a lethal, shortlatency pre-B-cell leukemia (20%). In contrast, RUNX1-ETO;c-Kit T417IΔ418-419 coexpression promoted exclusively AML in a fraction (51%) of reconstituted mice. These observations indicate that c-KitD814V promotes a more varied and aggressive leukemic phenotype than c-Kit T417IΔ418-419, which may be the result of differing potencies of the activating c-Kit alleles. © 2012 by The American Society of Hematology.