Studies also found that the expression of UPR signaling molecules including GRP78, XBP1, CHOP and other proteins were not found in every COPD patient, suggesting that the UPR pathway was activated only in a part of COPD population (Min et al., 2011; van 't Wout et al., 2015). This evidence concerns the gene XBP1 and chronic obstructive pulmonary disease.