CFTR and chronic bronchitis: Smokers with or without COPD had reduced CFTR activity measured by nasal potential difference compared to age-matched non-smokers (−6.3±1.4 and −8.0±2.0 mV, respectively vs. −15.2±2.7 mV control, each P<0.005, n = 12–14/group); this CFTR decrement was associated with symptoms of chronic bronchitis as measured by the Breathlessness Cough and Sputum Score (r = 0.30, P<0.05) despite controlling for smoking (r = 0.31, P<0.05).