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In this study, we analyzed the roles for AML1/RUNX1 in the regulation of the c-mpl promoter. Wild-type AML1 activated the c-mpl promoter through the proximal AML-binding site in luciferase assays using 293T and HeLa cells. In accord with this result, electrophoretic mobility shift assay and chromatin immunoprecipitation assays demonstrated that AML1 bound to this site. Next, we analyzed the function of AML1 using a mutant of AML1 lacking the C terminus (AML1dC), which was originally found in a patient with myelodysplastic syndromes. AML1dC dominant-negatively suppressed transcriptional activity of wild-type AML1. However, unexpectedly, AML1dC-transduced murine c-Kit AML1 (RUNX1) is a family member of heterodimeric transcription factors named core binding factors. AML1 was originally identified at a breakpoint on human chromosome 21 in the t(8;21) translocation and known as the most common targets of chromosomal translocations in human leukemia (1, 2). In addition to chromosomal translocations, recent reports have shown the importance of point mutations of AML1 in hematological malignancies, such as acute myelogenous leukemia (AML) 2 and myelodysplastic syndromes (MDS) (3). The Runt domain of AML1 is utilized for DNA binding and heterodimerization with a partner PEBP2␤/CBF␤. Although PEBP2␤ by itself does not bind to the DNA, the association with PEBP2␤ is necessary for AML1 to elicit its biologic activity (4 -6). AML1 can regulate the transcription of the target gene both positively and negatively through the binding to the consensus DNA sequence, TGT/cGGT, possibly dependent on the cellular context and/or its target gene. For example, it positively regulates the expression cytokines and their receptors in myeloid and lymphoid lineage cells (7-12), whereas it negatively regulates CD4 transcription in immature thymocytes (13). Several experiments using conventional and conditional gene targeting in mice demonstrated that AML1 is essential for the early step in definitive hematopoiesis (14). North et al. (15) revealed that AML1 is required for the generation of hematopoietic stem cells (HSCs) from the vitelline and umbilical arteries and from the aorta-gonad-mesonephros (AGM) region. In addition, AML1 is necessary for the transitions from the stage of doublenegative (DN)2 to DN3 and DN3 to DN4 in the T-lymphocyte development (9, 17). Furthermore, AML1 plays an important role in the maturation of megakaryocytes and platelet production. AML1 deletion in adult mice led to the impaired polyploidization of megakaryocytes and low platelet production (17, 18), whereas the number of megakaryocyte progenitors was not altered in these mice, suggesting that AML1 is indispensable for the terminal maturation of megakaryocytes. Also, the hereditary loss-of-function mutation of AML1 or

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