We hypothesized that bronchiectasis in severe asthma is caused by disruption of the airway epithelium, relating to active eosinophilic airway inflammation with degranulation of toxic proteins, as well as increased mucus production, relating to the transition of the epithelium to mucus cell hyperplasia caused by IL-5– and IL-13–driven pathways, respectively. This evidence concerns the gene IL5 and asthma.