Chromosomal translocations involving the mixed-lineage leukemia (MLL) gene lead to acute myeloid or lymphoid leukemias that are often associated with poor prognosis, particularly in infants. The translocations result in in-frame fusion between MLL and one of the over sixty known partner genes that are heterogeneous in nature. AF4 and AF9 are two of the most common MLL fusion partners. Importantly, they also interact specifically with each other in subnuclear foci. An AF4-mimetic peptide which disrupts the AF4-AF9 interaction is able to cause necrotic cell death in leukemic cell lines harboring MLL-AF4 or MLL-AF9 fusion oncogenes, emphasizing the importance of the AF4-AF9 interaction in leukemias caused by these fusions. AF4 and AF9 have been isolated as components of multiple transcription elongation complexes whose other members include the RA polymerase II regulator pTEFb and the chromatin modifying enzyme Dot1. Dot1 and its unique histone H3 lysine 79 methyltransferase activity are implicated in leukemias associated with MLL chimeric ncogenes. Data presented in this dissertation indicate that both AF4 and Dot1 are critical co-factors for MLL-AF9 associated leukemias. Interruption of the AF4---MLL-AF9 interaction, either by mutations or by overexpression of a dominant negative AF4 fragment, abolishes MLL-AF9---mediated hematopoietic cell immortalization as well as MLL target gene activation. In addition, loss of Dot1 function compromises cell viability by triggering apoptosis in hematopoietic cells immortalized by MLL-fusion oncoproteins. These findings poses AF4 and Dot1 as potential therapeutic targets for treatment of acute leukemias associated with MLL fusio oncogenes