CFTR and cystic fibrosis: Regarding the constitutive activation of F508del-mediated chloride secretion that we observed with chronic ivacaftor treatment in CF nasal epithelial cultures grown in PneumaCult medium, previous studies on the molecular mechanism of ivacaftor potentiation revealed that it directly binds to F508del-CFTR and induces a phosphorylation-dependent increase in open probability (11, 45–47).