One study found higher airway and alveolar epithelial TGF-β1 in COPD subjects without chronic bronchitis compared to controls [31], while another compared current or ex-smokers with COPD to current or ex-smokers without COPD and found higher TGF-β1 in bronchiolar and alveolar epithelium in the COPD groups [13]. This evidence concerns the gene TGFB1 and chronic bronchitis.