This study used TPH1 knockout (deficiency of peripheral 5-HT) and wild-type C57BL/6 (sufficiency of peripheral 5-HT) mice to investigate the role of 5-HT in bleomycin-induced pulmonary fibrosis, which may allow 5-HT as a biomarker of early detection of pulmonary fibrosis and a potential therapy target. The gene discussed is TPH1; the disease is pulmonary fibrosis.