Somatically acquired point mutations of AML1/RUNX1 gene have been recently identified in rare cases of acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS). Moreover, germ line mutations of AML1 were found in an autosomal dominant disease, familial platelet disorder with predisposition to AML (FPD/AML), suggesting that AML1 mutants, as well as AML1 chimeras, contribute to the transformation of hematopoietic progenitors. In this report, we showed that AML1 point mutations were found in 6 (46%) of 13 MDS patients among atomic bomb (A-bomb) survivors in Hiroshima. Unlike acute or chronic leukemia patients among A-bomb survivors, MDS patients exposed relatively low-dose radiation and developed the disease after a long latency period. AML1 mutations also were found in 5 (38%) of 13 therapy-related AML/MDS patients who were treated with alkylating agents with or without local radiation therapy. In contrast, frequency of AML1 mutation in sporadic MDS patients was 2.7% (2 of 74). Among AML1 mutations identified in this study, truncated-type mutants lost DNA binding potential and trans-activation activity. All missense mutations with one exception (Gly42Arg) lacked DNA binding ability and down-regulated the trans-activation potential of wild-type AML1 in a dominant-negative fashion. The Gly42Arg mutation that was shared by 2 patients bound DNA even more avidly than wild-type AML1 and enhanced the trans-activation potential of normal AML1. These results suggest that AML1 point mutations are related to low-dose radiation or alkylating agents and play a role distinct from that of leukemogenic chimeras as a result of chromosomal translocations caused by sublethal radiation or topoisomerase II inhibitors.