Richard C. Boucher and his team from the University of North Carolina—Chapel Hill put forward the term “muco-obstructive lung diseases” to encompass lung diseases with the following common pathophysiological features: small airway involvement with mucus plugging, dysregulation of both mucin homeostasis (hyperconcentration) and mucin biophysical properties (increased mucus viscosity and elasticity), and airflow obstruction [18]. The gene discussed is MUC5AC; the disease is lung disorder.