Abstract AF9/MLLT3, located on chromosome band 9p22, was first identified as a fusion partner of the Mixed Lineage Leukemia (MLL) gene in acute myeloid leukemia. The AF9 protein has been shown to play an important role in erythrocyte/ megakaryocyte development by regulating the expression of genes encoding hematopoietic transcription factors. Moreover, in hematopoietic precursor cells, expression of a chimeric protein comprised of the C-terminus of AF9 fused to the N-terminus of MLL leads to leukemic transformation. Thus AF9 contributes both to normal as well as neoplastic blood cell development. Our lab and others have shown the direct or indirect association of the C-terminus of AF9 with several distinct proteins including activators and repressors of transcription (AF4, pTEFb, DOT1L, CBX8, RING1b, and BCoR), thus suggesting that the function of AF9 can be substantially changed depending upon the nature of molecules that specifically associate with it. We hypothesize that AF9 is an important component of several distinct macromolecular complexes, each of which has a unique function. Specifically, DOT1L is a chromatin modifying enzyme that methylates histone H3 at lysine 79. CBX8 is component of the repressive Polycomb PRC1 complexes. Here, we report that the association of AF9 with CBX8 competes with DOT1L for AF9 binding and alters expression of genes regulated by DOT1L. In this context, CBX8 enhances rather than represses gene expression. Citation Format: Bhavna Malik, Charles S. Hemenway. Importance of protein interactions in mediating AF9/MLLT3 function. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 761. doi:10.1158/1538-7445.AM2013-761 Note: This abstract was not presented at the AACR Annual Meeting 2013 because the presenter was unable to attend.