The overall goal of this proposal is to understand the role of AML1 in myeloid development. AML1 expression initiates from the commitment of hematopoietic stem cells and continues during the maturation of myeloid cells. It regulates the promoter activity of several important myeloid specific genes, such as IL-3, GM-CSF, and M-CSF receptor. The analysis of AML1 deficient mice demonstrates that AML1 plays the most fundamental role for definitive hematopoiesis. AML1 gene is originally cloned from human acute myeloid leukemia cells. Further analysis indicate that AML1 is the most common transcription factor involved in various forms of chromosomal translocations associated with either acute myeloid leukemia or acute lymphoid leukemia. We have demonstrated the synergistic effect of AML1 with C/EBP and PU.1 on the activation of myeloid specific gene expression, localized the critical domains of AML1 for the synergy, analyzed the differential gene expression in AML1 fusion gene knock-in mice and in a myeloid cell line. Furthermore, we have demonstrated using transgenic mouse models that AML1 fusion protein directly contributes to the development of acute myeloid leukemia but itself is not sufficient to cause leukemia. In this funding period, we propose to study the role of AML1 in myeloid development by analyzing the regulation of its function by phosphorylation and critical additional mutations associated with AML1 fusion protein in myeloid leukemia development. The results of these studies will significantly improve our understanding of the molecular mechanisms by which the myeloid lineage undergoes commitment and differentiation. The specific aims of this proposal are 1) define the function of AML1 phosphorylation, 2) study the mechanism of AML1 - C/EBP synergy during myeloid leukemogenesis, 3) identify additional mutations and signaling pathways cooperated with the AML1 fusion protein in myeloid leukemia.