Of interest, plasma and sputum levels of KL-6, a glycoprotein classified as a human MUC1 mucin, are higher in COPD patients than healthy smokers and non-smokers [18], which is in line with our findings as we found MUC1 to be upregulated in nasal epithelium of COPD patients. Here, MUC5AC is linked to chronic obstructive pulmonary disease.