MUC1 and pulmonary alveolar proteinosis: SP-D and KL-6/MUC1 are useful biomarkers in the diagnosis of various ILDs, such as IPF, collagen vascular disease-associated interstitial pneumonitis, radiation pneumonitis, hypersensitivity pneumonitis, pulmonary alveolar proteinosis, and drug-induced pneumonitis [7, 8].