Richard Moriggl contributed to a recent publication in oncotarget demonstrating that sustained activation of STAT5 induced by Bcr-Abl in chronic myeloid leukemia (CML) cells promotes production of reactive oxygen species (ROS) by repressing expression of two antioxidant enzymes, catalase and glutaredoxin-1(Glrx1). Downregulation of catalase and Glrx1 expression was also observed in primary cells from CML patients. Catalase was shown not only to reduce ROS levels but also, to induce quiescence in Bcr-Abl-positive leukemia cells. These data support a role of persistent STAT5 signaling in the regulation of ROS production in myeloid leukemias and highlight the repression of antioxidant defenses as an important regulatory mechanism