About the cover: Cover is based on an image of a fluorescent micrograph of an antigen-challenged mouse lung demonstrating Muc5ac mRNA expression in the bronchial airway epithelium. This model was used to demonstrate that the selective induction of Muc5ac expression in the bronchial epithelium is the central and defining event in mucous metaplasia in allergic mice. For more information, see article by Young and colleagues beginning on page 273.