MUC1 and pulmonary fibrosis: Comparison of PGRN and activin A with other possible markers of disease activity in IPF, e.g., KL-6 [21], Matrix metalloproteinases 1 and 7, and Surfactant Proteins A and D, chemokines, and cytokines such as chemokine ligand 18 and interleukin 8 [22] are urgently needed to assist diagnosis and treatment of acute exacerbations of idiopathic pulmonary fibrosis.