MUC5AC and chronic obstructive pulmonary disease: Using a metric of mucus tethering (percentage tethering, Supplemental Figure 2A) to quantify the degree of continuity between the mucus plugs and the epithelium, we found that percentage of tethering correlated positively and significantly with MUC5AC immunostaining in asthma but not in COPD (Figure 2M and Supplemental Figure 1C).