Although these lines of evidence suggest that modest endogenous CFTR expression may suffice to improve CF lung disease, complementation of the defect in lung function by ectopic CFTR expression in CF airways is complicated because CFTR expression is highly regulated at different levels in the airways and in different epithelial cell types (Jiang and Engelhardt, 1998; Davis and Wypych, 2021). Here, CFTR is linked to cystic fibrosis.