The AML1 gene encodes a DNA-binding protein that contains the runt domain and is the most frequent target of translocations associated with human leukemias. Here, point mutations of the AML1 gene, V105ter (single-letter amino acid code) and R139G, (single-letter amino acid codes) were identified in 2 cases of myelodysplastic syndrome (MDS) by means of the reverse transcriptase–polymerase chain reaction single-strand conformation polymorphism method. Both mutations are present in the region encoding the runt domain of AML1 and cause loss of the DNA-binding ability of the resultant products. Of these mutants, V105ter has also lost the ability to heterodimerize with polyomavirus enhancer binding protein 2/core binding factor β (PEBP2β/CBFβ). On the other hand, the R139G mutant acts as a dominant negative inhibitor by competing with wild-type AML1 for interaction with PEBP2β/CBFβ. This study is the first report that describes mutations of AML1 in patients with MDS and the mechanism whereby the mutant acts as a dominant negative inhibitor of wild-type AML1.