Kehlet SN, et al. – The current study's goal was to develop a competitive enzyme–linked immunosorbent assay (ELISA) targeting the specific degraded fragment of decorin, identify the protease generating this fragment, and to examine the biomarker potential of this fragment in serum from patients with various lung pathologies. Using the decorin degraded by cathepsin–S (DCN–CS) competitive ELISA, cathepsin–S degraded decorin could be quantified in serum. The findings suggested that degradation of decorin by cathepsin–S is an important part of the pathology of lung cancer and idiopathic pulmonary fibrosis (IPF).