During hematopoiesis, descendants of hematopoietic stem cells (HSCs) become committed to differentiate along specific cell lineages, eventually acquiring the characteristics of mature terminally differentiated cells. The identification and isolation of HSCs has advanced substantially over the past few decades, however, the mechanisms that regulate stem cell lineage commitment and differentiation are still obscure. Programmed cell death plays a crucial role in the regulation of normal hematopoietic differentiation, and this cellular process is frequently altered in hematologic malignancies. In Philadelphia chromosome negative myeloproliferative neoplasms (Ph- MPN), dysregulation of cell death pathways is frequently observed, implicating cell death proteins as potential factors inthe pathogenesis of disease and targets for therapeutic intervention. Currently the molecular pathogenesis of Ph- MPNs is largely unknown. Recent studies have highlighted the importance of the anti-apoptotic protein apoptosis repressor with caspase recruitment domain (ARC) in human malignancies, however, the function of ARC in normal and malignant hematopoiesis has not been thoroughly investigated. Our work aims to identify the role of ARC in mouse and human hematopoietic stem and progenitor cells (HSPCs), and in the Ph- MPN primary myelofibrosis (PMF). Utilizing an ARC knockout (ARC [-/-]) mouse model for our investigation, we have found that aged ARC [-/-] mice exhibit significantly altered blood cell counts and splenomegaly with an expansion of myeloid and immature cells consistent with extramedullary hematopoiesis. ARC [-/-] bone marrow revealed reduced blood sinusoids and increased bone marrow fibrosis according to reticulin staining. Further analysis showed expansion of the bone marrow HSC compartment and in vitro methylcellulose colony assays revealed altered HSC differentiation capacity. The hematologic findings in ARC [-/-] mice are consistent with a novel mouse model of PMF. Additionally, the ARC [-/-] phenotype is transplantable into lethally irradiated congenic recipients. We hypothesize that ARC plays a critical role in mouse and human HSPC function and that loss of ARC may contribute to the pathogenesis of PMF. To characterize ARC in lineage commitment and differentiation of mouse and human HSPCs, we will utilize genetic murine models, stem cell transplantation, in vitro stem cell functional assaysand in vivo xenotransplantation experiments. To elucidate the mechanism of ARC in HSCs, we will use transcriptional profiling to guide our investigation and identify molecular pathways critical to ARC function. Furthermore, we will quantitate ARC transcript levels in HSCs from patients with PMF to determine if ARC expression is relevant to the disease pathogenesis. PMF has the most severe morbidity and greatest mortality of the Ph-MPNs and the identification of novel mouse models and potential therapeutic targets is urgently required.