MUC1 and idiopathic pulmonary fibrosis: KL-6 is a sensitive serum marker for various interstitial lung diseases including idiopathic pulmonary fibrosis, radiation pneumonitis, drug-induced pneumonitis, collagen vascular disease-associated interstitial pneumonitis, extrinsic allergic alveolitis, pulmonary sarcoidosis, pulmonary alveolar proteinosis and cystic fibrosis [5-7].