The hematopoietic system is a regenerative tissue with high cell turnover. Normal homeostasis of the hematopoietic system is insured by hematopoietic stem cells (HSCs). The important features of HSCs are the capacity to self-renew and multipotency. Multipotent HSCs perpetuate through life and are able to form all differentiated cell lineages of the blood. Most HSCs with long-term self-renewal capacity are quiescent and divide infrequently. However quiescent HSCs can be activated in response to injury. The signals responsible for the activation of quiescent HSCs are poorly investigated. The Trumpp laboratory has recently identified a role for IFNα in activation of quiescent HSCs in vivo. Activation of HSCs by IFNα involves increased phosphorylation of STAT1. Therefore the role of STAT1 in the homeostasis of the hematopoietic system was investigated by characterization of the hematopoietic system in STAT1-/- mice. A 2-fold decrease in short-term HSC (ST-HSCs) and multipotent progenitors (MPPs) was observed in the bone marrow of STAT1-/- mice compared to C57Bl/6 (wild type) mice. Analysis of progenitors of HSCs also revealed a small increase in common myeloid progenitors (CMPs) and a small decrease in common lymphoid progenitors (CLPs) in the bone marrow of STAT1-/- mice. These results indicate that STAT1 play a role in the homeostasis of ST-HSCs and MPPs in bone marrow. Functional analysis of STAT1-/- LT-HSCs using in vivo long-term reconstitution assay suggests that STAT1-/- LT-HSCs can reconstitute lethally irradiated mice. Nevertheless further investigations are necessary to examine whether STAT1 plays a role in HSCs self-renewal. The unexpected role of IFNα in activation of quiescent HSCs leads to the hypothesis that endogenously produced IFNα might be important for activation of HSCs in response to injury. To address this we generated the mouse model MxCre;R26REYFP to monitor endogenous IFNα/β production in the bone marrow after injury. Our results suggest that IFNα/β is produced in response to 5-FU induced myeloid suppression and G-CSF treatment in vivo. The role of IFNα/β during the recovery phase after 5-FU induced myeloid suppression was investigated using IFNAR-/- mice. These results suggest that endogenous IFNα/β might be important for the mobilization of HSCs into the spleen induced in response to 5-FU