MUC5B and pulmonary fibrosis: Prior to 2018 the focus was primarily on exploring the basic pathophysiological mechanisms of pulmonary fibrosis and identifying biomarkers associated with the onset and progression of the disease such as alveolar macrophages SP-A/SP-D angiotensin-converting enzyme circulating fibroblasts gene expression MUC5B promoter polymorphisms and KL-6.