MUC5B and chronic obstructive pulmonary disease: We found that mucus plugs from patients with asthma were rich in MUC5AC (Figure 2, G and H), whereas those from patients with COPD were rich in MUC5B (Figure 2, I and J), and the ratio of MUC5AC to MUC5B was significantly higher in asthma than in COPD (Figure 2K).