So far, only few reference genes have been validated for cells from respiratory compartment; specifically GNB2L1 was validated for bronchoalveolar macrophages in patients with chronic obstructive pulmonary disease (COPD) [10] and GAPDH (glyceraldehyde-3-phosphate dehydrogenase) for non-small cell lung cancer [11]. Here, GAPDH is linked to chronic obstructive pulmonary disease.