CBS and chronic obstructive pulmonary disease: Photomicrographs showing representative photomicrographs of cystathionine-γ-lysase (CSE), cystathionine-β-synthase (CBS) and 3-mercaptopyruvate sulphur transferase (MPST) staining in the bronchial mucosa from control non-smokers, control smokers with normal lung function and mild/moderate COPD patients.