MUC1 and COVID-19: Elevated serum levels of KL-6 have been identified as a biomarker for alveolar epithelial proliferation and injury in various respiratory conditions [10], including acute respiratory distress syndrome [11], hypersensitivity pneumonia [12], idiopathic interstitial pneumonia [13], pulmonary sarcoidosis [14], coronavirus disease 2019 (COVID-19) [15], and CTD-ILD [16].