CFTR and cystic fibrosis: While it is well established that impaired CFTR-mediated Cl− secretion and airway surface dehydration due to mutations in the CFTR gene cause cystic fibrosis (CF) with early onset and severe chronic obstructive airways disease [37]–[39], little is known about the relationship between endogenous variability of WT CFTR function and susceptibility for lung disease in humans.