Previous studies in our lab identified a novel gene, latexin (Lxn), that regulates murine hematopoietic stem cells through balancing apoptosis, self-renewal and proliferation. In these dissertation studies, I performed a series of experiments to examine the function of Lxn using a Lxn conventional knockout mouse, and characterize Lxn’s role in the presence of hematopoietic stresses such as ionizing radiation, cytokines induced-mobilization, and hematopoietic malignancy. The first series of experiments was designed to determine the role of Lxn in hematopoiesis under homeostatic conditions. I found that Lxn-/- mice exhibited hyperproliferative hematopoiesis, a repopulation advantage and elevated self-renewal capacity which was intrinsic to the Lxn-/- hematopoietic cells. Furthermore, I identified a reduction in apoptotic frequency in Lxn-/- hematopoietic progenitors, which may account for the expansion seen in the progenitor population. In a second series of experiments, I discovered a role of Lxn in the radio-sensitivity of hematopoietic cells. I found that loss of Lxn in mice confers resistance to ionizing radiation. Lxn-/- mice showed rapid hematological recoveries after radiation exposure at the stem and progenitor cell (HSPC) level. The ablation of Lxn hindered irradiation-induced apoptosis which may underlie the radiation resistance through regulating hematopoietic recovery. In a third series of experiments, I studied the interaction of Lxn-/- stem and progenitor cells with their microenvironment. Using a granulocyte colony-stimulating factor-induced mobilization model, I determined that the ability of HSPCs to mobilize into the bloodstream was significantly increased in Lxn-/- mice. The adhesive properties of hematopoietic cells were compromised in Lxn-/- animals. Gene expression studies on progenitor cells identified cell-to-ECM interactions were down-regulated upon Lxn deletion, implying the enhanced mobilization efficiency of hematopoietic cells from Lxn-/- mice correlated with reduced adhesion of hematopoietic progenitor cells to stroma. Last, but not least, I performed a series of experiments to study the putative tumor suppressor role of Lxn in hematological malignancy. I found that Lxn expression was down-regulated in primary tumor and tumor cell lines by promoter methylation. Overexpression of Lxn inhibited lymphoma cell growth both in vitro and in vivo. Overexpressed Lxn increased apoptosis frequency by suppressing the expression of several anti-apoptotic genes, and therefore reduced the tumor growth.