To determine the effect of the ivacaftor-dependent constitutive activation of F508del-CFTR–mediated chloride secretion on the viscoelastic properties of the mucus, we treated CF nasal epithelial cultures grown in PneumaCult medium with ET, ETI, or vehicle control; labeled the mucus layer with fluorescent dye; and measured fluorescent recovery after photobleaching. The gene discussed is CFTR; the disease is cystic fibrosis.