[en] Ionizing radiation can elicit an activated phenotype that promotes rapid and persistent remodeling of the extracellular matrix (ECM) through the induction of proteases and growth factors, as well as in response to chronic production of reactive oxygen species (ROS). In addition, the results of previously conducted cDNA microarrays and real-time RT-PCR analysis (unpublished) suggest that radiation-induced mammary tumors were specifically induced by cathepsin S (CTSS), but that dimethylbenz(a)anthracene (DMBA)-induced mammary tumors were not. CTSS is a lysosomal cystein protease that is synthesized as an inactive precursor (36kDa) and activated in the acidic environment of lysosomes by proteolytic cleavage of its propeptide. In this study, we further investigate the mechanism by which CTSS is induced by radiation as well as its function