The secretory or gel-forming mucin, Mucin 5AC (MUC5AC) is one of the major components of the mucus in the airways, and hyper- or hyposecretion of this component is a hallmark of a number of chronic obstructive pulmonary diseases (COPD) (Rose and Voynow, 2006). The gene discussed is MUC5AC; the disease is chronic obstructive pulmonary disease.