All cells were negative for the tight junction protein ZO-1 after 7 days (supplementary Fig. S1E), but weak ZO-1 expression was observed apically after 14 days and at later time points the staining intensity had increased, demonstrating the ability of the cells to form tight junctions on both normal and COPD scaffolds. This evidence concerns the gene TJP1 and chronic obstructive pulmonary disease.