Mucociliary dysfunction in CF likely involves multiple mechanisms, including deficient mucus hydration that leads to mucus hyperconcentration because of impaired CFTR-mediated chloride and fluid secretion, as well as abnormal mucin polymer formation in the acidic airway surface pH milieu because of impaired CFTR-mediated bicarbonate secretion (3, 48, 49). The gene discussed is MUC5AC; the disease is cystic fibrosis.