However, the mass spectrometric analyses, coupled with other assays, confirmed that MUC5AC to MUC5B ratios were increased (~1.2) at baseline in patients with COPD compared with healthy controls (~0.2) and remained at similar ratios during exacerbations, consistent with coordinate upregulation of MUC5AC and MUC5B in individuals with resting or exacerbating COPD. Here, MUC5B is linked to chronic obstructive pulmonary disease.