CFTR and cystic fibrosis: None of the gene therapy trials reported so far has shown evidence for correction of the sodium transport defect, and in vitro cell-mixing experiments implied that the correction of sodium hyperabsorption requires high numbers (close to 100%) of non-CF cells.32,33 We are currently not aware of studies that have addressed how much CFTR expression in how many cells may be required to correct the bicarbonate transport defect and associated alterations in mucus unfolding34 and airway surface liquid pH.8