CFTR and cystic fibrosis: To determine the in vivo relevance of ETI-induced constitutive CFTR activity that we observed in patient-derived airway cultures, we assessed CFTR-mediated chloride secretion in the absence and presence of cAMP-dependent stimulation with isoproterenol as determined by nasal potential difference measurements in healthy individuals and in patients with CF carrying at least 1 F508del allele before and after initiation of ETI therapy (17, 38).