Based on evaluations of CFTR channel activity in air-liquid interface cultures seeded with mixtures of CF and non-CF airway epithelial cells, the frequency of basal cells requiring gene insertion (for either ex vivo editing followed by transplantation or direct in vivo editing) to restore CFTR activity to the CF airway is estimated to be on the order of 15–30% (Farmen et al., 2005) or perhaps even less (Lee et al., 2020). This evidence concerns the gene CFTR and cystic fibrosis.