RACK1 and chronic obstructive pulmonary disease: 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].