Normal development necessitates that cells establish specific gene expression patterns and transmit them stably through cell division. During mitosis, transcription ceases as most transcriptional regulators are removed from their target genes. The mechanism by which newly formed daughter cells reassemble the transcriptional apparatus to reactivate appropriate gene expression programs is thought to involve nuclear factors that remain bound to mitotic chromosomes, termed mitotic bookmarks. However, an understanding of what enables mitotic retention of these factors, as well as their specific role(s) in lineage commitment and/or lineage stability remains incomplete. The goal of this proposal is to define the mechanism by which the bookmarking factor MLL1 (mixed lineage leukemia 1) occupies its mitotic targets, and to determine whether mutations in MLL1 alter mitotic functions in such a way as to perturb hematopoiesis. Recent studies from our lab demonstrate that MLL1, a histone methyltransferase that is required for normal embryonic and adult hematopoiesis, is globally retained on mitotic chromosomes. MLL1 chromatin occupancy patterns are reorganized in mitosis, shifting towards the most highly expressed genes. Yet the physical interactions that control MLL1 mitotic redistribution, as well as its relevance to normal hematopoiesis are unknown. MLL1 is frequently rearranged via chromosomal translocation to generate leukemogenic fusion proteins that sustain aberrant gene expression programs and block hematopoietic differentiation. Preliminary results suggest that the MLL1 fusion protein, MLL-AF9, remains bound to mitotic chromosomes, however, whether this is required for MLL-AF9 to sustain inappropriate transcriptional patterns that result in leukemia is not known. In this proposal I will investigate the interactions that direct MLL1 mitotic occupancy, and examine the mitotic functions of MLL1 and MLL-AF9 in normal and malignant hematopoiesis. In Aim 1, I will systematically define the minimal region(s) of MLL1 that are both necessary and sufficient to bind mitotic chromatin, and assess mitosis-specific functions of MLL1 in hematopoietic progenitor cells. In Aim 2, I will examine the global occupancy patterns of MLL-AF9 and wild-type MLL1 in interphase and mitosis in leukemia cells, and define relationships to both normal and leukemia-specific transcriptional programs. I will determine if mitosis-specific MLL-AF9 function is required for leukemic transformation by engineering MLL-AF9 fusion proteins that are degraded selectively during mitosis. Together these experiments are expected to provide insight into the mechanisms by which MLL1 carries out its mitotic functions, and whether oncogenic MLL fusions exhibit perturbed mitotic bookmarking properties that impede normal programs of differentiation. More generally, the proposed studies are, to our knowledge, the first to directly test a role for epigenetic mitotic memory in cellular transformation.