CLDN1 and chronic obstructive pulmonary disease: We did not find correlations between claudin-1 expression and smoking status (never-smokers vs. smokers) (p = 0.32), COPD severity (GOLD stages) (rs = −0.2977, p = 0.167), and pack-years (rs = −0.3349, p = 0.128) (see supplementary file: Figure S1).