MUC1 and hypersensitivity pneumonitis: Increasing attention has been paid to a serologic marker of pulmonary disease, KL-6 (Krebs von den Lungen-6, a high molecular weight glycoprotein expressed on proliferated type 2 alveolar pneumocytes and epithelial cells), that is known to occur in patients with interstitial pneumonia, hypersensitivity pneumonitis, tuberculosis, sarcoidosis, and pulmonary alveolar proteinosis.