RUNX1/AML1 mutations have been identified in myelodysplastic syndromes (MDSs). In a mouse bone marrow transplantation model, a RUNX1 mutant, D171N, was shown to collaborate with Evi1 in the development of MDSs; however, this is rare in humans. Using enforced expression in human CD34(+) cells, we showed that the D171N mutant, the most frequent target of mutation in the RUNX1 gene, had an increased self-renewal capacity, blocked differentiation, dysplasia in all 3 lineages, and tendency for immaturity, but no proliferation ability. BMI1 overexpression was observed in CD34(+) cells from the majority of MDS patients with RUNX1 mutations, but not in D171N-transduced human CD34(+) cells. Cotransduction of D171N and BMI1 demonstrated that BMI1 overexpression conferred proliferation ability to D171N-transduced cells in both human CD34(+) cells and a mouse bone marrow transplantation model. Stepwise transduction of D171N followed by BMI1 in human CD34(+) cells resulted in long-term proliferation with a retained CD34(+) cell fraction, which is quite similar to the phenotype in patients with higher-risk MDSs. Our results indicate that BMI1 overexpression is one of the second hit partner genes of RUNX1 mutations that contribute to the development of MDSs.