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The AML1/Runx1 protein is required for definitive hematopoiesis and for the maturation of adult megakaryocytic cells. Alterations of the Runx1 gene by mutations, deletions and chromosome translocations are associated with several types of acute leukemia. Runx1 functions as both an activator and repressor of gene transcription with promoter and cell type context dependency. Likely, this relates to the ability of Runx1 to interact with a variety of transcription factors such as MEF, C/EBPa, Ets-1 and GATA-1 and also with repressor proteins such as Groucho, mSin3 and HDACs. Runx1 is post-translationally modified through acetylation and phosphorylation, and the acetylable and phosphorylable forms of Runx1 can activate transcription to higher level in Runx1 dependent reporter assays when HATs or Erk2 are coexpressed. Runx1 has also been shown to be methylated on lysine residues by SUV39H1 methyltransferase in fibroblasts. Based on the presence of a SGRGK motif in the runt domain of Runx1, we have been examining whether Runx1 is methylated on arginine residues by the protein arginine methyltransferases (PRMT). We have found that PRMT1 and PRMT5 are associated with Runx1 in AML cells by co-immunoprecipitation assays and using in vitro by GST-pulldown assays with in vitro translated PRMT(s) have shown that the interactions are direct. Using a luciferase gene reporter assay, we show that PRMT1 acts synergistically with p300 to activate Runx1 mediated transcription in response to cell proliferation signals. We have mapped the arginine methylation sites in Runx1 using GST-Runx1 fusion proteins, site-specific mutagenesis and mass spectrometry analysis. We have found three potential arginine methylation sites, one in the Runt domain, and two in the Runx1 carboxy-terminal region. Interestingly, one of these sites is in the region shown to interact with both the mSIN3A transcriptional repression complex and with p300. This suggests that arginine methylation of Runx1 may affects its transcriptional activating and repressing functions. Chromatin immunoprecipitation assays are underway to show how arginine methylation of Runx1 affects its activities in hematopoietic cells. Additional studies examining the effects of cross-talk between arginine methylation, lysine acetylation and serine phosphorylation has on Runx1 functions (biological and biochemical) will be presented.