The importance of this mobile barrier is emphasized by the lung inflammation and infection that develop in mice and humans with deficiency of mucus production or ciliary beating (Roy et al., 2014; Horani and Ferkol, 2021); the positive selection in humans of an overexpressing allele of the constitutively expressed mucin MUC5B, which presumably helps protect against infection even though it results in pulmonary fibrosis late in life (Evans et al., 2016); and constraint against monoallelic loss-of-function of MUC5B in humans (Lek et al., 2016). This evidence concerns the gene MUC5AC and infection.