MUC1 and acute respiratory distress syndrome: Based on these concepts, we can speculate that in the nonsurvivors, the alveolar type II cells could initially proliferate during the early stages of ARDS, leading to elevated KL-6 pulmonary ELF concentrations; however, after development of severe alveolar epithelial damage, the type II cells died or disorganized epithelial repair occurred, leading to decrease in level of KL-6 in ELF.