MUC1 and connective tissue disorder: KL-6 is a high-molecular-weight mucin-like glycoprotein produced by type II pneumocytes and bronchial epithelial cells.[33] KL-6 serves as a sensitive marker of interstitial lung diseases (ILDs) such as pulmonary fibrosis, connective tissue disease-associated ILD, hypersensitivity pneumonitis, and pulmonary sarcoidosis.[34] Increased levels of KL-6 reflect greater lung damage and regeneration of type II pneumocytes.