MUC5B and idiopathic pulmonary fibrosis: This “distal airway outgrowth model” accommodates our identification of an IPF/UIP risk variant in the airway gene MUC5B, gene expression data that implicate the distal airways in the pathogenesis of IPF/UIP, previously published radiological evidence of connectivity between the airway and the HC, histological analyses of progressive heterogeneous parenchymal destruction, and the present DIF identification of airway cell types in the HC.