The successful knockout of Muc5ac has been achieved and was verified by genomic Southern and PCR analysis. The homozygous Muc5ac-/- mice had significantly reduced levels of mucus on the surface of the gastric mucosa and in the gastric pit compared to wild type mice. Furthermore, immunohistochemical results and dot-blot data confirmed the absence of Muc5ac in the stomachs of Muc5ac-/- mice. In addition, the middle ear of the Muc5ac-/- animals contains smaller goblet cells with decreased mucous content compared to wild type mice. In addition, mononuclear inflammatory cell infiltration was observed in the lamina propria of the middle ear of the Muc5ac-/- mice, which would make the epithelium more susceptible to bacterial invasion. The conjunctiva and nasopharynx of the Muc5ac-/- mice also have significantly fewer and smaller goblet cells which contain decreased amount of mucous. The protective role of Muc5ac in the stomach was evaluated by the oral dosing of Muc5ac wildtype and deficient mice with indomethacin (indo), a non-steroidal anti-inflammatory drug known to produce gastric ulcers. At various times after acute dosing the stomachs were examined for gastritis and ulceration. The results indicated that Muc5ac deficiency increased the susceptibility to indo-induced gastritis and ulcer formation as indicated by both the number and size of the gastric lesions. Furthermore, histological examination also showed that the severity of gastric lesion erosion was greater in Muc5ac-/- mice. However, the mice used in these studies were infected with Helicobacter hepaticus and Helicobacter typhlonius. When we repeated these studies using mice free of all these pathogens we found that mice lacking Muc5ac were not more vulnerable to the indomethacin treatment. We further evaluated the role of Muc5ac in protecting the gastric epithelium by treating WT and Muc5ac -/- mice with hydrochloric acid and surprisingly found that the WT mice developed more lesions.