MBTPS1 and idiopathic pulmonary fibrosis: Taken together, our results showed that increased serum S1P was correlated with pulmonary functional depression CTD-UIP patients rather than IPF patients, and it might be a clinically useful biomarker in evaluating the severity of CTD-UIP patients due to the involvements of S1P both in immune-inflammatory response and fibrogenesis, both of which are the primary pathogenic mechanism of CTD-UIP.