MUC5AC and chronic obstructive pulmonary disease: To gain more direct insight into mucin-related inflammatory pathways during exacerbations, in a subset for which sufficient sample amounts were available (n = 11 patients with COPD, n = 10 healthy individuals, Supplemental Figure 2A), we performed stable isotope–labeled mass spectrometry to quantify the changes in MUC5AC and MUC5B concentrations during COPD exacerbations and to calculate the ratios of MUC5AC to MUC5B over time.