Based on these cross-species anatomical differences, one could hypothesize that MUC5B may perform baseline barrier and clearance functions in human distal airways; in support of this, MUC5B overexpression and protein accumulation was observed in the distal lungs in idiopathic pulmonary fibrosis, indicating impaired clearance [50,51]. The gene discussed is MUC5B; the disease is idiopathic pulmonary fibrosis.