The RUNX1 gene encodes an alpha subunit of the core-binding factor (CBF), an important heterodimeric transcription factor in hematopoietic ontogeny and development, and is one of the most frequently disrupted genes in acute leukemia. In addition to its involvement in several translocations, the RUNX1 gene is often subject to deletions or point mutations in acute myelogenous leukemia (AML). Interestingly, in addition to complete loss-of-function mutations, many of the alterations involve missense point mutations within the Runt domain that disrupt DNA binding activity (DB-mutants). In vitro assays have suggested that these DB mutants have a dominant-negative (DN) activity, presumably due to their ability to bind and sequester CBF beta but inability to bind DNA. A strict correlation between the type of mutation and its monoallelic or biallelic incidence is not apparent even though a DN mutant should only affect one allele while a loss of function mutation should affect both alleles. It has been hypothesized that loss of one allele (haploinsufficiency) is sufficient for loss of tumor suppressor activity but the relative high incidence of specific DB mutations suggests a more complex scenario. We thus sought to determine if expression of DB mutants in murine bone marrow (BM) resulted in a similar phenotype as the loss of Runx1, or if these mutations are associated with a gain-of-function. Two RUNX1-DB mutants were thus evaluated using the established retroviral transduction/transplantation mouse model. Between 3 and 6 months after transplantation, peripheral blood, spleen and BM cells were analyzed. Long-term repopulating cells expressing RUNX1 DB-mutants were able to contribute normally to both myeloid and lymphoid compartments, although a disproportionate increase in the B-cell compartment was observed in 3 out of 10 mice. Surprisingly, and inconsistent with a DN activity, disruption of normal T-cell or megakaryocytic development was not observed in the mice, in contrast to Runx1−/+ mice. Significantly, however, replating assays in vitro demonstrated that RUNX1-DB mutants lead to a significant increase in self-renewal activity, in contrast to BM cells of floxed Runx1 mice expressing the Cre recombinase, which showed a less dramatic effect on self-renewal. Colonies derived from CFU-Cs expressing RUNX1-DB mutants were composed of dysplastic granulocytic and monocytic cells, with an increasing number of immature blasts after multiple replatings (>7), whereas residual colonies from Runx1fl/− BM receiving CRE showed a different morphology with more mature cells. Thus our data suggest that RUNX1-DB mutants do not act in a dominant negative fashion to inhibit normal RUNX1 function, but impart a gain-of-function that results in impaired myeloid differentiation and increased self-renewal potential, consistent with its association with AML.