MUC5B and idiopathic pulmonary fibrosis: One club cell subpopulation highly expressed MUC5B (MUC5B+ club cells), a known genetic risk gene for IPF [16], while the other club cell subpopulation highly expressed SCGB3A2 (SCGB3A2high club cells), a marker heterogeneously expressed in the club cells [14, 15] (Fig 2C).