Reactive oxygen species (ROS) are oxygen-containing side products of normal aerobic metabolism. Although it was initially believed that ROS constitute only damaging by-products, recent evidence suggest that ROS actively participate in signal transduction pathways and regulate cell survival and proliferation in a dose-depended manner [1,2]. Endogenous intracellular redox state is tightly controlled by free radical scavenging systems and the appropriate ROS levels differ among cell types and differentiation status . Hematopoietic stem cells (HSCs) in mice reside mainly in the ROSlow subset of hematopoietic progenitors , whereas forced increase of ROS in human Lin-CD34+CD38cord blood cells leads to loss of stem cell activity [5-7]. Similarly, though increased ROS levels in tumor cells have been implicated in cancer initiation and progression.