Tightly compressed mucin packets in those vesicles are thought to expand rapidly upon reaching the luminal environment due to changes in pH and salt conditions Birchenough et al., 2015; the CFTR ion channel, which is mutated in cystic fibrosis patients, is expressed on adjacent cell types (Kreda et al., 2012) and is important for establishing the proper luminal environment for mucin expansion to occur (Kreda et al., 2012). The gene discussed is MUC5AC; the disease is cystic fibrosis.