Quantitative real-time PCR (qRT-PCR) analysis of samples from smokers with or without COPD confirmed that relative to normal smokers without COPD, the expression of TBX3, TBX5, HDAC6, SIRT1, SIRT5, is suppressed in severe cases of COPD (Figure 2). Here, SIRT5 is linked to chronic obstructive pulmonary disease.