MUC1 and interstitial lung disease: Serum KL-6 is elevated in a variety of different ILDs, including the idiopathic interstitial pneumonias, hypersensitivity pneumonitis, drug-induced pneumonitis, sarcoidosis, and connective tissue disease-associated ILD (CTD-ILD), reflecting the prominent role of alveolar epithelial injury and proliferation across ILDs [29].