Chromosomal rearrangements resulting in alteration of gene expression are a major cause of hematological malignancies. Our goal is to advance the understanding of the biochemical and molecular mechanisms of rearrangement-based leukemia, and to provide insights into how translocations affect cellular division by altering gene expression. The proposed research explores the regulation of gene expression via the MLL gene and its translocation partners found in human leukemia. Our studies during the current funding cycle have considerably expanded our molecular understanding of the role of MLL and one of its translocation partners, the ELL protein. We demonstrated that translocations of MLL into the ELL gene result in leukemic pathogenesis. We also identified the domain of the ELL protein required for this process. Further, we have identified and purified to homogeneity the yeast homologue of MLL, the Set1 protein in a complex we call COMPASS. Our studies demonstrated that this complex is a histone methyltransferase required for the regulation of gene expression by RNA polymerase II. A recently-identified human MLL complex, which is the functional homologue of COMPASS, also functions as a histone methyltransferase. Our studies further demonstrated that histone ubiquitination is required for histone methylation by COMPASS. These studies helped create the paradigm that chromatin modifications participate in the etiology of leukemia. Building on these discoveries, the goal of this proposal is to characterize molecularly the roles of MLL and its translocation partners and their homologues in the etiology of leukemia. This goal will be aggressively pursued via three specific aims. Aim 1 will determine the molecular composition of the MLL complex and how translocations alter its biochemical function and integrity, resulting in leukemic pathogenesis. Aim 2 will define the mechanism of targeting of the MLL complex and its histone methyltransferase activity to chromatin. Aim 3 will define the biochemical roles of MLL partners in the pathogenesis of leukemia. Results obtained from these proposed studies will be instrumental in designing targeted therapies for the treatment of such hematological malignancies.