CLDN4 and chronic obstructive pulmonary disease: Of the COPD-specific genes we studied, CEACAM1, COL6A3, NOL3, COL1A2, MLPH, MUC1, P8, UNQ473, CLDN4, RNASE1, H19, DEFA1 and LOC653600 were upregulated more than tenfold, mainly related to nuclear proteins, collagens or molecular structure.