S100A9 and idiopathic pulmonary fibrosis: Thus, in this study, we focused on common ILDs, including IPF, HP, CTD-ILD, and SAR which are difficult to diagnose and differentiate from progressive-fibrosing phenotype, and we aimed to investigate whether S100A9 and/or KL-6 could serve as a biomarker for differentiating and/or indicating the progression for these common ILDs.