CFTR and cystic fibrosis: The F508C variant (c.1523T>G; p.Phe508Cys) does not cause CF when paired with a CF-causing CFTR variant and generates currents similar to WT-CFTR (https://cftr2.org), whereas the G551D variant (c.1652G>A; p.Gly551Asp) is a CF-causing variant with function of approximately 1% and known to respond well to ivacaftor.