MUC5AC and idiopathic pulmonary fibrosis: IPF biomarkers can be categorized into three groups, namely biomarkers associated with alveolar epithelial cell dysfunction (such as the Krebs von den Lungen-6 (KL-6) antigen, carbohydrate antigen (CA)15-3, CA125, the mucin MUC5B and surfactant proteins A and D), biomarkers associated with extracellular matrix remodelling and fibroproliferation (such as matrix metalloproteinase-1 (MMP-1) and -7 (MMP-7), lysyl oxidase-like 2 and periostin), and biomarkers related to immune dysfunction (such as CC chemokine ligand-18 and YKL-40) [5,6,7,8,9] (Table 1).